One of the most fundamental topics in modern organogermanium chemistry is the study of low-coordinate species, and within this realm, the field of multiply bonded compounds is now one of the mainstreams. It therefore comes as no surprise that the literature covering the latter field is vast. Heteronuclear multiply bonded combinations of germanium with the Main Group elements of groups 13, 15, and 16, >Ge=E¹³–, >Ge=E¹⁵–, and >Ge=E¹⁶, respectively, are excluded from the consideration.  Moreover, numerous compounds, in which the low-coordinate Ge center is intramolecularly (by n-donor substituents) or intermolecularly (through external donor ligands) coordinated, thus experiencing remarkable electronic perturbation, are also not considered, except for silagermenylidenes >Si=Ge(NHC): and digermanium(0) complexes :Ge⁰(NHC/or NHSi)=Ge⁰(NHC/or NHSi):, which otherwise cannot be stabilized for their isolation.

The very first isolable digermene, namely tetra(alkyl)digermene Dis₂Ge=GeDis₂ [Dis = CH(SiMe₃)₂] 1, reported by Lappert and coworkers in 1976, was synthesized by the reaction of bis(amino)germylene [(Me₃Si)₂N]₂Ge: with DisLi [11], and its crystal structure was reported ten years later (Scheme 1) [12]. Dimeric was in the solid state (as was confirmed by X-ray diffraction study), and digermene 1 dissociated in solution into a pair of monomeric germylenes Dis₂Ge:, thus implying the easy breaking of the weak Ge=Ge double bond (Scheme 1). Remarkably, crystallographic studies revealed that the Ge=Ge bond in 1 was notably short (r_Ge=Ge = 2.347(2) Å) and not twisted, the geometry around both Ge centers was pyramidal (Σ_Ge = 348.5°), and the Dis-substituents at the Ge=Ge double bond were arranged in a trans-bent fashion (θ = 32°).

Since 1976, numerous digermenes have been isolated and, in the majority of cases, structurally characterized, with most of these cases being reported after 2000. The synthetic strategies toward stable digermenes can be categorized into the four main approaches: (1) photolysis of cyclotrigermanes (route A); (2) reduction of dihalo- or monohalogermylenes with Grignard or organolithium reagents (route B); (3) reductive dehalogenation of 1,1-dihalogermanes (route C); and (4) 1,2-addition or cycloaddition to digermynes (route D) (Scheme 2).

Route A (photolysis of hexaarylcyclotrigermanes) is mostly of a historical importance as the method employed by Masamune and coworkers for preparation of the first stable tetra(aryl)digermenes Ar₂Ge=GeAr₂ [13]. However, because of the synthetic limitations of this approach, which requires cyclotrogermane precursors that are not readily available, currently this method is not commonly used. Since 2000, there was only one report from the Baines group on an improved synthetic procedure for the tetra(mesityl)digermene Mes₂Ge=GeMes₂ 2 that was generated by photolysis of hexa(mesityl)cyclotrigermane precursor in THF at −70 °C [14].

According to approach B, the reduction of isolable di(halo)germylenes X₂Ge:/(X₂Ge:–dioxane complex) or mono(halo)germylenes X(R)Ge: with Grignard RMgX or organolithium RLi reagents generates at first transient germylenes R₂Ge: that dimerize forming digermenes R₂Ge=GeR₂, and this method was used for the preparation of Lappert’s germylene Dis₂Ge=GeDis₂ 1 [11,12]. Since 2000, a few other isolable digermenes were prepared by method B: Ar(R)Ge=Ge(Ar)R [R = Me, Et, Ph; Ar = 2,6-(2,4,6-ⁱPr₃-C₆H₂)₂-C₆H₃] 3a–c [15], [Me₃Si–C≡C](Ar′)Ge=Ge[C≡C–SiMe₃]Ar′ [Ar′ = 2,6-(2,6-ⁱPr₂-C₆H₃)₂-C₆H₃] 4 [16], Ar*Ge=GeAr* [Ar* = 2,5-^tBu₂-C₆H₃] 5 [17], Bbt(Br)Ge=Ge(Bbt)Br [Bbt = 2,6-[(Me₃Si)₂CH]₂-4-[(Me₃Si)₃C]-C₆H₂] 6 (equilibrating in solution with the monomeric germylene Bbt(Br)Ge:) [18], and Tbb(Br)Ge=Ge(Tbb)Br [Tbb = 2,6-[(Me₃Si)₂CH]₂-4-^tBu-C₆H₂] 7 [19]. A donor-acceptor Lewis base–Lewis acid digermene complex {[(OC)₅W]←:GeH₂–H₂Ge←[:IPr]} [IPr = 1,3-bis(2,6-diisopropylphenyl)-2H-imidazol-2-ylidene] 8 was also prepared, featuring however single (instead of double) germanium–germanium bond, being therefore not classified as a true digermene [20].

Reductive dehalogenation of 1,1-di(halo)germanes (method C) is by far the most popular protocol for synthesis of stable digermenes, due to the ready availability of the starting R₂GeX₂. Using this approach, the synthesis and crystal structure of the following digermenes were reported: (R₃Si)₂Ge=Ge(SiR₃)₂ [R₃Si = SiMe₂^tBu] 9 [21,22], Tbt(Mes)Ge=Ge(Tbt)Mes [Tbt = 2,4,6-[(Me₃Si)₂CH]₃-C₆H₂] 10 [23], Fc(Tip)Ge=Ge(Fc)Tip [Tip = 2,4,6-ⁱPr₃-C₆H₂, Fc = ferrocenyl] 11 [20]. In tetra(aryl)digermene 10, the strong repulsive interaction of the bulky aryl substituents caused the room-temperature dissociation of the Ge=Ge bond into the corresponding germylenes Tbt(Mes)Ge:, as was monitored by UV–Vis spectroscopy (ΔH = 14.7 ± 0.2 kcal/mol, ΔS = 42.4 ± 0.8 cal/mol·deg) [24].

Method D, namely 1,2-addition or cycloaddition across the Ge≡Ge triple bond of digermynes, is the latest approach toward digermenes that was enabled by the recent availability of the stable digermynes. This approach is exemplified by the preparation of Ar′(H)Ge=Ge(Ar′)H [Ar′ = 2,6-(2,6-ⁱPr₂-C₆H₃)₂-C₆H₃] 12 [25] (which is also available by method B [26]) and Ar′(H)Ge=Ge(Ar′)R [Ar′ = 2,6-(2,6-ⁱPr₂-C₆H₃)₂-C₆H₃, R = cyclopentyl] 13 [27]. Using method D, several compounds with a Ge–Ge bond were classified as digermenes [28,29,30]. However, given their remarkably long (even longer than many Ge–Ge single bonds) and accordingly quite weak Ge=Ge bonds, this classification is somewhat doubtful.

Below, some recently published representative examples of the stable digermenes are described.

Thus, employing method C, Lee, Sekiguchi and coworkers reported the tetra(silyl)digermene (^tBu₂MeSi)₂Ge=Ge(SiMe^tBu₂)₂ 14, featuring very bulky substituents, that was readily available in large-scale by the reductive dehalogenation of the (^tBu₂MeSi)₂GeCl₂ precursor with potassium graphite [31,32,33]. In the solid state, digermene 14 manifested a quite unusual combination of the structural features, namely a very long [r_Ge=Ge = 2.346(2) Å] and exceptionally twisted (τ = 52.8°) Ge=Ge double bond, nevertheless featuring practically planar geometry at its sp²-Ge centers (Σ_Ge = 358.8 and 359.2°) [31,32]. In solution, digermene 14 has a very distinct deep sapphire-blue color, in contrast to all other isolable digermenes that are yellow, orange, or red. Accordingly, its longest wavelength UV band was observed at 618 nm [π(HOMO)–π*(LUMO)], a value that was extraordinarily red-shifted compared to those of other stable digermenes [31,32]. This was caused by the extreme twisting of the Ge=Ge double bond due to the severe steric repulsion of bulky silyl substituents, resulting in rather poor 4p_π(Ge)–4p_π(Ge) orbital overlap, destabilization of the HOMO, and accordingly to the overall decrease in the HOMO–LUMO energy gap. Moreover, exceptional twisting of the Ge=Ge bond in 14 results in its partial breaking and progressively increasing biradical contribution. Digermene 14 does not dissociate in solution into the germylenes (^tBu₂MeSi)₂Ge:, maintaining its structural integrity up to 80 °C, as confirmed by Raman and UV–Vis spectroscopy measurements, as well as trapping reactions study [31,32,33]. Nevertheless, 14 can behave as a germylene source when strongly nucleophilic Lewis bases (isocyanide or ortho-benzoquinone) are applied, forming germylene reactivity products although not involving “free” germylenes into the reaction process [31,32]. CV measurement of digermene 14 in ortho-dichlorobenzene in the presence of [ⁿBu₄N][B(C₆F₅)₄] inert electrolyte afforded two reversible redox couples, for both oxidation and reduction processes, with E_1/2(ox) = 0.38 V and E_1/2(red) = −1.50 V [32]. This implies that both cation-radical [14]^+∙ and anion-radical [14]^−∙ are persistent under the CV measurement conditions.

Scheschkewitz and coworkers synthesized tetra(silyl)digermene 15 by dimerization of a cyclic germylene–NHC complex (Scheme 3) [34]. The exocyclic Ge=Ge bond in 15 revealed structural features that are typical for digermenes: short bond [r_Ge=Ge = 2.2944(4) Å], strongly pyramidalized Ge centers [Σ_Ge = 334.5°], and trans-bending of the silyl-substituents [θ = 37.7°]. Accordingly, digermene 15 was stable both in the solid state and in solution.

The same group also recently reported the first isolable digermenide Tip₂Ge=Ge(Tip)Li 16 made by the reduction of Tip₂GeCl₂ with metallic lithium [35]. In this anionic digermene, the Ge=Ge double bond is short [r_Ge=Ge = 2.284(6) Å], unremarkably trans-bent (θ = 7.1/12.8°), and twisted (τ = 19.9°). Digermenide 16 can be further functionalized at the anionic Ge site forming novel silyl-substituted digermenes Tip₂Ge=Ge(Tip)R (R = SiMe₃ [35], SiPhMe₂ [35], SiPh₃ [36], SiMe₂Cl [36], SiMePhCl [36], and SiPh₂Cl [36]) and even persistent (acyl)digermenes Tip₂Ge=Ge(Tip)[C(O)R] (R = ^tBu [36], 2-methylbutan-2-yl [36], 1-adamantyl [36]).

Matsuo, Sasamori and coworkers prepared 1,2-di(halo)digermenes Eind(X)Ge=Ge(X)Eind (X = Cl, Br) 17a,b (a: X = Cl, b: X = Br) by the redistribution of stable di(aryl)germylene Eind₂Ge: and GeX₂∙dioxane complex [37]. Bearing electronegative substituents, 17a,b expectedly revealed strong structural deformation at the Ge=Ge double bond: trans-bending of substituents (θ = 44.3/43.3°) and pyramidalization at the Ge centers (Σ_Ge = 335.9 and 337.1°). In line with this, the Ge=Ge bond in 17a,b is rather long [r_Ge=Ge = 2.4119(5) and 2.4145(3) Å], as a manifestation of the weak bonding between Ge atoms, which was translated into the ready dissociation of this bond. Accordingly, in solution, di(halo)digermenes Eind(X)Ge=Ge(X)Eind dissociate to (halo)germylenes Eind(X)Ge:.

Dicationic derivative 18, which can be viewed as a digermene with cationic imidazolium substituents, as reported by Aldridge and coworkers, was synthesized by the reaction of a (chloro)germylene–NHC complex with either Na{B[3,5-(CF₃)₂-C₆H₃]₄} or Li{Al[OC(CF₃)₃]₄} (Scheme 4) [38]. In 18, the Ge=Ge bond is short [r_Ge=Ge = 2.300(2) Å], and Ge centers are only insignificantly pyramidalized (Σ_Ge = 353.1 and 353.6°). Exchanging in 18 NHC substituents for Me₄-NHC and boryl substituents for Ar-groups [Ar = 2,6-Mes₂-C₆H₃ (Mes = 2,4,6-Me₃-C₆H₂)], the same group synthesized another dicationic digermene 19 manifesting longer Ge=Ge distance of 2.380(1) Å [39].

Very recently, Scheschkewitz and coworkers found that the thermolysis (benzene, 65 °C, 18 h) of the unsymmetrically substituted digermene Tip₂Ge=Ge(Tip)[SiR₂Dma] (R = Me, Ph; Dma = 2-Me₂N-C₆H₄) produced a mixture of redistribution products, Tip₂Ge=GeTip₂ and trans-[DmaR₂Si](Tip)Ge=Ge(SiR₂Dma)Tip 20 [Ge=Ge bond (for R = Me): r_Ge=Ge = 2.2576(5) Å, θ = 21.5°, τ = 0°] [40]. This approach was then applied toward the development of ADMET (acyclic diene metathesis) polymerization of digermenes. Accordingly, thermolysis (benzene, 65 °C, 48 h) of a diene with terminal digermene fragments linked by a p-phenylene spacer, Tip₂Ge=Ge(Tip)–SiMe₂–1,4-[2,5-(Me₂N)₂-C₆H₂]–SiMe₂–Ge(Tip)=GeTip₂ 21 [Ge=Ge bonds: r_Ge=Ge = 2.3038(4) Å, θ = 24.9/31.9°, τ = 18.0°], formed ADMET-polyene Tip₂Ge{=Ge(Tip)–SiMe₂–1,4-[2,5-(Me₂N)₂-C₆H₂]–SiMe₂–Ge(Tip)=}_nGeTip₂, with the number-average degree of polymerization being 23, mass-average degree of polymerization being 45, and dispersity index being 1.95.

In comparison to organic alkenes >C=C<, structural deformations of the double bond in digermenes >Ge=Ge< (stretching, trans-bending, and twisting) are even more pronounced compared to those of the corresponding disilenes >Si=Si<. The extent of these structural distortions in digermenes follows some general tendencies: electronegative substituents provoke notable elongation and weakening of the Ge=Ge bond, as well as trans-bending at the doubly bonded Ge centers, whereas electropositive substituents cause shortening and strengthening of the Ge=Ge bond and planarization at the doubly bonded Ge centers. Accordingly, the shortest Ge=Ge bond [r_Ge=Ge = 2.2576(5) Å] was found in the di(aryl)di(silyl)digermene 20, whereas the longest one [r_Ge=Ge = 2.5087(7) Å] was detected in the di(aryl)di(bromo)digermene 6, with the exceptionally bulky Bbt substituents. Moreover, in line with what was mentioned above, tetra(silyl)digermene 9 has planar (the least trans-bent) geometry at its sp²-Ge centers (θ = 0.3°), whereas the greatest trans-bending was observed in the di(aryl)digermene 12 (θ = 45.0°). Twisting in digermenes is controlled by the substituent effect, to range from non-twisted Ge=Ge double bonds (τ = 0.0° in tetra(alkyl)digermene 1 and di(aryl)di(silyl)digermene 20) to extraordinarily twisted (τ = 52.8° in tetra(silyl)digermene 14 with very bulky silyl substituents).

To date, 22 neutral organogermanium compounds featuring an endocyclic Ge=Ge double bond, 14 three-membered rings, 4 four-membered rings, 3 five-membered rings, and 1 six-membered ring, are reported.

Three-Membered Ring Compounds

There are currently 14 heavy cyclopropene analogues incorporating a Ge=Ge double bond into the three-membered ring: 12 homonuclear cyclotrigermenes cyclo-[Ge₃] and 2 heteronuclear 1H-siladigermirenes cyclo-[Si–Ge=Ge]. Unlike their acyclic congeners, all of these cyclic digermenes are synthesized by special methods which are not outlined in Scheme 2.

The first isolable cyclotrigermenes 22a,b were synthesized by Sekiguchi and coworkers by the reaction of ^tBu₃EM (a: E = Si, M = Na; b: E = Ge, M = Li) with GeCl₂•diox (Scheme 5) [41]. Remarkably, one-electron oxidation of 22a with [Ph₃C]⁺[BPh₄]⁻ produced tris(tri-tert-butylsilyl)cyclotrigermenylium tetraphenylborate [(^tBu₃Si)₃Ge₃]⁺[BPh₄]⁻, as the germanium analogue of aromatic 2π-electron cyclopropenylium ion [42].

Unsymmetrically substituted cyclotrigermenes 23a–e (a: R = Si^tBu₃; b: R = Ge^tBu₃; c: R = Si(SiMe₃)₃; d: R = Ge(SiMe₃)₃; e: R = Mes) were later reported by the same authors prepared by the reaction of tris(tri-tert-butylsilyl)cyclotrigermenylium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate with the alkali metal salts RM (R = silyl, germyl, aryl; M = alkali metal) (Scheme 6 and Table 2) [43]. Likewise, treatment of (tri-tert-butylsilyl)cyclotrigermenylium tetrakis(2,3,5,6-tetrafluorophenyl)borate with potassium halides KX (X = Cl, Br, I) formed halogen-substituted cyclotrigermenes 24a–c (a: X = Cl, b: X = Br; c: X = I) (Scheme 7 and Table 2) [44].

Lee, Sekiguchi, and coworkers reported novel three-membered ring cyclic digermenes, 1H-siladigermirene 25a and 1H-trigermirene (cyclotrigermene) 25b, synthesized by the reaction of 1,1,2,2-tetra(chloro)digermane R–GeCl₂–GeCl₂–R (R = SiMe^tBu₂) with 1,1-di(lithio)silane R₂SiLi₂ or 1,1-di(lithio)germane R₂GeLi₂, respectively (Scheme 8 and Table 2) [45,46].

In due course, the solid-state thermolysis of the tetra(silyl)digermene (^tBu₂MeSi)₂Ge=Ge(SiMe^tBu₂)₂ 14²⁷ (170 °C, 1 h, evacuated sealed tube) was found to be an attractive alternative to the above-described preparation of 1H-trigermirene 25b, improving the isolated yield of the latter up to 48% [47].

The alkyl-substituted heavy cyclopropene analogues 26a,b (a: E = Si; b: E = Ge), representing the nearest homologues of the above-described 1H-siladigermirene 25a and 1H-trigermirene 25b, being distinguished from them by only one CH₂-unit, were prepared by Lee, Sekiguchi, and coworkers by the reductive dehalogenation of 1,3-di(chloro)cyclobutane derivatives (Scheme 9) [48]. The overall process was proposed to proceed via the transient bicyclo[1.1.0]butane derivatives, rapidly isomerizing at room temperature to the more stable heavy cyclopropenes 26a,b.

In cyclotrigermenes, the Ge=Ge double bonds [r_Ge=Ge = 2.239(4)–2.2743(8) Å] are typically shorter than those of acyclic digermenes [r_Ge=Ge = 2.2576(5)–2.5087(7) Å] (Table 1 and Table 2). The longest Ge=Ge bonds were detected in halogen-substituted cyclotrigermenes 24a–c [r_Ge=Ge = 2.2721(6)–2.2743(8) Å], and they were realized in terms of π_Ge=Ge–σ*_Ge–X orbitals mixing facilitated by the electronegative halogen atoms X at the sp³-Ge [44]. Usually, cyclotrigermenes exhibited significant twisting about their Ge=Ge double bonds: τ = 8.1–51.0° (Table 2). Cyclotrigermene 25b with very bulky ^tBu₂MeSi substituents showed a record twisting for the endocyclic Ge=Ge bond in heavy cyclopropenes (τ = 60.5°) [46].

Four neutral cyclobutene derivatives containing heavy group 14 elements and incorporating Ge=Ge double bond are currently known: one homonuclear (tetragermetene cyclo-[Ge₄]) and three heteronuclear (disiladigermetene cyclo-[Si₂Ge₂], trigermetene cyclo-[Ge₃C], and digermetene cyclo-[Ge₂C₂]).

The very first compound of this type, namely disiladigermetene 27, was reported by Lee, Sekiguchi, and coworkers, formed by the unexpected ring expansion of either 3H- or 1H-disilagermirenes [49] with GeCl₂•diox (Scheme 10 and Table 2) [50]. The Ge=Ge double bond in 27 [r_Ge=Ge = 2.2911(4) Å] is one of the longest reported for the cyclic digermenes, whereas the endocyclic Si–Ge bonds are sizably shortened, and the exocyclic Si–Cl bonds are elongated. This was explained by the important extent of π_Ge=Ge–σ*_Si–Cl negative hyperconjugation promoted by the presence of electronegative chlorine atoms and folding of the Si₂Ge₂-ring (folding angle = 28.3°). The geometry around the sp²-Ge atoms in 27 is only slightly pyramidal: Σ_Ge = 357.2°/358.6°.

Likewise, Lee, Sekiguchi, and coworkers found that the similar reaction of 1H-trigermirene 25b with the GeCl₂•diox yielded the first (and still the only known) homonuclear heavy cyclobutene analogue, tetragermetene 28 (Scheme 11 and Table 2) [51]. Structurally, tetragermetene 28 is similar to the above-described disiladigermetene 27: r_Ge=Ge = 2.2993(5) Å (28) vs. 2.2911(4) (27), and Σ_Ge = 354.4°/356.4° (28) vs. 357.2°/358.6° (27).

Weidenbruch and coworkers developed an alternative approach toward heavy cyclobutene analogues by reacting tetra(germa)buta-1,3-diene Tip₂Ge=Ge(Tip)–Ge(Tip)=GeTip₂ with 2-methoxyphenyl isocyanide to produce 1,2,3-trigermet-1-ene 29, featuring a germanium–germanium double bond within the Ge₃C-skeleton (Scheme 12 and Table 2) [52]. The Ge=Ge bond in 29 was marginally shorter that those in disiladigermetene 27 and tetragermetene 28: 2.2808(7) Å vs. 2.2911(4) and 2.2993(5) Å, respectively.

The four-membered ring 1,2-digermet-1-ene 30 with an endocyclic Ge=Ge double bond was prepared by Sasamori, Tokitoh, and coworkers by [2 + 2] cycloaddition of the stable digermyne BbtGe≡GeBbt (Bbt = 2,6-[(Me₃Si)₂CH]₂-4-[(Me₃Si)₃C]-C₆H₂) and ethylene, (Scheme 13) [53]. In 30, its remarkable ring strain and trans-bending of the substituents (θ = 39.5 and 39.7°) caused elongation and weakening of the Ge=Ge double bond [r_Ge=Ge = 2.4132(5) Å].

Except for the above-described neutral heavy cyclobutene analogues 27–30, there are several four-membered ring compounds where the Ge=Ge double bond is a part of the tri(germa)allylic system (cationic, radical, or anionic). Thus, Weidenbruch and coworkers reported the tetra(germa)cyclobutenyl anion 31 unexpectedly produced by exhaustive reduction of Tip₂Ge=GeTip₂ with excess of lithium (Scheme 14) [54]. In 31, the Ge₄-ring is practically planar (sum of the internal bond angles = 360°), and within the tri(germa)allylic system, the Ge–Ge bonds length of 2.3679(6) Å is just in-between those of the typical Ge–Ge single and Ge=Ge double bonds.

Cyclic compound 31 can be directly compared with acyclic tri(germa)allyl anion derivative 32, reported by Power and coworkers and prepared by the reductive ring opening of a cyclotrigermenyl radical 33 with KC₈ (Scheme 15) [55]. The germanium–germanium bond length [2.422(2) Å] in 32 is also intermediate between those of the Ge–Ge single and Ge=Ge double bonds, and the Ge₃-array is characterized by a wide Ge–Ge–Ge bond angle [159.19(10)°].

Most recently, Lee, Sekiguchi, and coworkers synthesized silatri(germa)cyclobutenylium ion derivative 34 by the oxidative demethylation of the cyclotrigermene 25b [45] with [Et₃Si]⁺∙[B(C₆F₅)₄]⁻ (Scheme 16) [56]. In 34, the SiGe₃ four-membered ring is strongly folded (folding angle 40.4°), thus enabling Ge2∙∙∙Ge3 through-space orbital interaction manifested in the short transannular Ge2–Ge3 distance of only 2.9346(3) Å. Overall, the structural peculiarities of 34 (ring folding and short transannular distance) testify to the important extent of its homoaromaticity. Accordingly, 34 is to be classified as a germanium analogue of the cyclobutenylium ion, i.e., a homo-tri(germa)cyclopropenylium ion. The homoaromaticity of 34 was further confirmed by the calculation of the nucleus-independent chemical shift (NICS) at 1 Å above the Ge₃-ring center, which was diagnostically negative (−17.3) as a manifestation of the diatropic ring current. The “homoaromatization energy” of 34, calculated as the barrier to inversion of the Ge₃Si-ring (through the planar allylic-type cationic transition state lacking homoaromatic stabilization), was exceedingly low, i.e., only 3.7 kcal/mol [56]. In accordance with its homoaromaticity, 34 showed practically no alteration in the Ge–Ge bond lengths of its Ge₃-fragment [r_Ge=Ge = 2.3327(3) Å (Ge1–Ge2) and 2.3400(3) Å (Ge1–Ge3)], and it showed essentially planar geometry at all skeletal Ge atoms [Σ_Ge = 360.0° (Ge1), 358.7° (Ge2), and 357.5° (Ge3)].

Upon the one-electron reduction of 34 with KC₈, a free-radical species, namely silatri(germa)cyclobutenyl radical 35, was cleanly formed (Scheme 16) [56]. The homoaromaticity of 34 is completely lost upon its reduction, which was seen in the remarkable flattening of the SiGe₃-ring in 35 (folding angle was reduced from 40.4° in the starting 34 to only 6.9° in the resulting 35) and great elongation of the Ge2∙∙∙Ge3 transannular distance [3.3315(4) Å in 35, or 14% elongation compared with 34]. Thus, 35 is to be classified as the allylic free radical, featuring the unpaired electron delocalized over the two terminal Ge atoms, [∙Ge2–Ge1=Ge3 ↔ Ge2=Ge1–Ge3∙]. Accordingly, the lengths of both Ge–Ge bonds in 35 are intermediate between those for the single and double bonds [2.3458(3) Å (Ge1–Ge2) and 2.3206(3) Å (Ge1–Ge3)], and all Ge atoms maintain essential planarity of their geometry upon reduction [Σ_Ge = 358.6° (Ge1), 355.2° (Ge2), and 359.9° (Ge3)]. The EPR resonance g-value of 35 (2.0227) is in the range typical for the silyl-substituted Ge-centered free radicals. In the Ge₃-unit of 35, the hyperfine coupling constants (hfcc) for the terminal Ge nuclei are markedly greater than that for the central Ge nucleus: a(⁷³Ge2) [or a(⁷³Ge3)] = 1.54 mT [or 1.44 mT] vs. a(⁷³Ge1) = 0.59 mT. This observation agrees well with the allylic radical formulation of 35, in which the odd electron is mostly localized at the Ge2 and Ge3 termini. Given that the small values of the ⁷³Ge hfcc in 35 imply the location of its unpaired electron in the orbital of π-symmetry, 35 should be categorized as a π-radical. The allylic free radical 35 can be compared with the cyclotrigermenyl radical 33 prepared by Power and coworkers by the stoichiometric reduction of the aryl(chloro)germylene Ar(Cl)Ge: [Ar = 2,6-(2,4,6-Me₃-C₆H₂)₂-C₆H₃] with KC₈ (Scheme 15) [55]. In the cyclotrigermenyl radical 33, the average Ge–Ge bond distance within the Ge₃-ring of 2.35(7) Å is comparable with those of the silatri(germa)cyclobutenyl radical 35 [2.3458(3) Å and 2.3206(3) Å]. The EPR characteristics of 33 are also comparable to those of 35: g = 2.0069 [2.0227 in 35], and a(⁷³Ge) = 1.6 mT [a(⁷³Ge2) or a(⁷³Ge3) = 1.54 or 1.44 mT in 35].

Three compounds of this type with the Ge=Ge double bond within the five-membered ring skeleton have been reported to date. Weidenbruch and coworkers synthesized the first two representatives by [1 + 4] cycloaddition of sulfur/or selenium to the tetra(germa)buta-1,3-diene Tip₂Ge=Ge(Tip)–Ge(Tip)=GeTip₂, forming thia- or selena-tetra(germa)cyclopentenes 36a,b (Scheme 17 and Table 2) [52,57]. The structural features of heavy cyclopentenes 36a,b are typical for the cyclic digermenes: r_Ge=Ge = 2.2841(5) Å (36a) and 2.2975(5) Å (36b), and Σ_Ge = 348.4°/351.4° (36a) and 349.2°/349.9° (36b); and Ge₄E rings are practically planar (sums of the internal bond angles are 539.5° (36a) and 537.7° (36b)).

The latest example of the five-membered ring cyclic digermene, namely bicyclic digermene 37 with a bridging Ge=Ge double bond, was prepared by Marschner and coworkers; it was unexpectedly formed by the reaction of 1,3-di(potassio)trisilane with GeBr₂∙dioxane and PEt₃ (Scheme 18) [58]. In 37, the Ge=Ge double bond revealed the structural parameters that are characteristic for the cyclic digermenes: shortened bond [r_Ge=Ge = 2.2663(9) Å], negligible trans-bending of substituents [θ = 2.5°], and moderate twisting [τ = 16.2°]. Upon its electrochemical (CV) reduction, digermene 37 showed two reversible reduction waves corresponding to generation of anion-radical and dianion.

Only one “heavy cyclohexene” derivative has been reported to date. Marschner and coworkers prepared this tetracyclic digermene 38 with an endocyclic Ge=Ge double bond bridging polycyclic scaffold (Scheme 19) [58]. Digermene 38 was available via the synthetic strategy applied for the preparation of the above-described bicyclic digermene 37, namely by the reaction of 1,4-di(potassio)cyclohexasilane with GeBr₂∙dioxane in the presence of PEt₃. The metric parameters of 38 are comparable to those of 37: r_Ge=Ge = 2.2896(6) Å, θ = 2.1/8.3°, and τ = 5.2°.

The first isolable germenes were independently synthesized in 1987 by the groups of Berndt and Escudié. Berndt and coworkers prepared germenes 39a,b by the coupling of electrophilic “cryptocarbene” and bis(amino)germylenes (Scheme 20) [59]. The formulation of 39a,b as germenes Ge=C was supported by the observation of low-field resonance for their doubly bonded C atoms [δ(¹³C) = 115.0 ppm (39a) and 93.2 ppm (39b)] and rather short Ge=C bond [r_Ge=C = 1.827(4) Å (39a)]. The Ge=C double bond in 39a was significantly twisted [τ = 36°], although the sp²-Ge center featured a planar geometry [Σ_Ge = 359.9°]. The spectroscopic and crystallographic data of 39 are indicative of the substantial contribution of the ylide resonance structure with a positive charge on the Ge atom and a negative charge delocalized over the B–C–B allylic fragment of the B₂C₂ four-membered ring (Scheme 20) [59]. This was exemplified by the shielding of the B atoms [δ(¹¹B) = 66.0 ppm (39a) and 65.0 ppm (39b)] and shortening of the cyclic sp²-C–B bonds in 39a [r_C–B = 1.534(7)/1.523(6) Å].

Escudié and coworkers applied another approach toward their stable dimesityl(fluorenylidene)germene 40: dehydrofluorination of the (fluorenyl)fluorogermane percursor by its lithiation with ^tBuLi with subsequent elimination of LiF from the intermediate lithium salt (Scheme 21, R = R′ = Mes) [60]. In 40, the Ge=C double bond of 1.803(4) Å (mean value for the two crystallographically independent molecules) was notably shorter than the typical Ge–C single bonds and even shorter than that in 39a, being practically undistorted [Σ_Ge = Σ_C = 360.0°, τ = 6°] [61]. Except for the steric protection of the Ge=C bond by the bulky Mes groups, extra stabilization in 40 results from the contribution of the charge-separated Ge^δ+=C^δ− resonance form enabled by the effective aromatic delocalization of the negative charge over the cyclopentadiene fragment of the fluorenylidene group.

Applying the same synthetic protocol, Escudié and coworkers prepared other stable germenes 40, as outlined in Scheme 21: R = R′ =Dis [62]; R = Dis, R′ = Mes [62]; R = R′ = fluorenyl [63]; and R = t-Bu, R′ = fluorenyl [63]. They also reported generation of the halogen-substituted germenes (Me₅C₅)(X)Ge=CR₂ [CR₂ = fluorenylidene] 40 (Scheme 21: R = C₅Me₅; R′ = F, Cl) by β-elimination of Me₃SiF from the (Me₅C₅)(X)FGe–(Me₃Si)CR₂ precursors [64]. Lacking resonance stabilization, dimesityl(neopentyl)germene Mes₂Ge=CH–CH₂^tBu 41 was synthesized by Couret and coworkers via the addition of ^tBuLi to Mes₂(F)Ge–CH=CH₂, followed by β-elimination of LiF from the intermediate lithium salt Mes₂(F)Ge–CH(Li)–CH₂^tBu [65].

A series of stable germenes 42–44, prepared from a germylene Ar₂Ge: (Ar = 2-^tBu-4,5,6-Me₃-C₆H), were reported by Weidenbruch and coworkers (Scheme 22) [17,66,67,68,69]. The germene 42, similar to Berndt’s germene 39a, disclosed a short [r_Ge=C = 1.845(10) Å] and remarkably twisted [τ = 33°] Ge=C bond, and planar geometry at both sp²-Ge and sp²-C atoms [Σ_Ge = Σ_C = 359.9°] [66]. The reaction of two equivalents of germylene Ar₂Ge: with two equivalents of phosphaalkyne ^tBu–C≡P unexpectedly produced a germene 43 with an exocyclic Ge=C double bond [r_Ge=C = 1.833(4) Å] (Scheme 22) [67]. A series of bis(germenes) connected via acetylene spacer, 44a–c, were synthesized by the double [1 + 2] cycloaddition of germylenes Ar₂Ge: to the C≡C bonds of bis(alkynes), followed by the ring-opening isomerization of the transient bis(germirenes) (Scheme 22) [17,68,69]. The length of the Ge=C double bond in 44a–c was in the range expected for germenes: 1.819(6) Å in 44a [17], 1.819(2) Å in 44b [68], and 1. 840(4) Å in 44c [69]. The observable conjugation between the terminal Ge=C bonds and the central C≡C bond in 44a–c was seen in the diagnostic bathochromic shift of their longest wavelength UV absorptions [500 nm (44a) [17], 518 nm (44b) [68], and 595 nm (44c) [69], as well as in the shortening of the central C–C bonds in the Ge=C–C≡C fragments.

Two stable germenes, exemplifying the shortest and the longest extremes for the range of Ge=C bond lengths, are of particular interest. The germene with the shortest Ge=C double bond, 45, was prepared by Okazaki and coworkers by the reaction of di(aryl)germylene Tbt(Tip)Ge: (generated by the reduction of di(bromo)germane Tbt(Tip)GeBr₂ with lithium naphthalenide) with CS₂ (Scheme 23) [70]. The length of the Ge=C double bond in 45 was only 1.771(16) Å, a value which was substantially smaller than those of Berndt’s germene 39a (1.827(4) Å) [59] and Escudié’s germene 40 (1.803(4) Å) [61]. Furthermore, the Ge=C bond in 45 showed no signs of structural distortion, exhibiting a trigonal-planar geometry at both sp²-Ge and sp²-C centers [Σ_Ge = 359.7°, Σ_C = 360.0°] and a negligible twisting [τ = 4°].

The germene 46 with the longest ever reported Ge=C bond [r_Ge=C = 1.895(3) Å] was reported by Lee, Sekiguchi, and coworkers in 2002 [71]. This endocyclic Ge=C bond was embedded into the norbornene bicyclic skeleton of 46, which was unexpectedly formed by the reaction of 2,4-disila-1-germatricyclo[2.1.0.0^2,5]pentane cage 47 with benzaldehyde (Scheme 24). The major factor responsible for such extraordinary stretching of the germanium–carbon bond is most likely a steric repulsion between the bulky substituents. Nevertheless, despite this exceptional steric crowding around the double bond, the Ge=C bond in 46 revealed practically no structural deformations, neither pyramidalization at the doubly bonded Ge and C atoms nor twisting of the Ge=C bond: Σ_Ge = 359.8°/Σ_C = 358.9°, τ = 7.1°.

The latest representative of isolable germenes, namely the first stable Brook-type germene 48 with an exocyclic Ge=C double bond, was prepared by Haas and coworkers [72]. This O-silylated germene 48 was prepared by the reaction of Me₃SiCl with the stable germenolate 49, predominantly exhibiting acyl germyl anion character with a negatively charged Ge, Ge–C single bond, and C=O double bond (Scheme 25). Spectroscopic and structural characteristics of 48 are typical for the isolable germenes: strongly deshielded sp²-C atom [δ(¹³C) = 210.0 ppm]; short [r_Ge=C = 1.835(2) Å], pyramidalized at Ge [Σ_Ge = 351.7° (cf.: Σ_C = 360.0°)], and twisted (torsional angles: O–C=Ge–Si = 10.0° and C_Mes–C=Ge–Si = 18.1°) Ge=C double bond.

The first ever reported silagermene, metastable Mes₂Si=GeMes₂, was thermally or photochemically generated from hexa(mesityl)siladigermirane precursor, cyclo-(Mes₂Si–GeMes₂–GeMes₂), by Baines and coworker [73]. Stable in solution only below −70 °C, Mes₂Si=GeMes₂ was identified by low-temperature NMR and UV measurements (δ(²⁹Si) = 80.6 ppm and 414 nm (π_Si=Ge–π*_Si=Ge), respectively), as well as by the quenching with methanol forming 1,2-addition product Mes₂Ge(H)–Si(OMe)Mes₂. Above −70 °C, Mes₂Si=GeMes₂ underwent irreversible 1,2-migration of Mes-group from Ge to Si generating transient mesityl(silyl)germylene Mes(Mes₃Si)Ge:.

The first isolable silagermene, 1H-disilagermirene 50, with a cyclic Si=Ge double bond within the three-membered ring GeSi₂-skeleton, was reported by Lee, Sekiguchi, and coworkers in 2000 [49]. Moreover, 50 was quantitatively available from 3H-disilagermirene precursor 51 by either thermal or photochemical isomerization, driven by the thermodynamic preference for the silagermene >Si=Ge< over the disilene >Si=Si< (Scheme 26). In 50, the sp²-Si center was expectedly deshielded [δ(²⁹Si) = 100.7 ppm], and the Si=Ge double bond was notably twisted (torsional angle R–Si=Ge–R = 40.3°).

Lee, Sekiguchi, and coworkers later developed another approach toward isolable silagermenes by the thermal isomerization of disilagermabicyclo[1.1.0]butane 52, forming novel 1H-disilagermirene 53, as the nearest homologue of 50, distinguished from the latter by only a CH₂ unit (Scheme 27) [48]. Compared to 50, 53 revealed a more deshielded sp²-Si center: δ(²⁹Si) = 100.7 ppm and 126.6 ppm, respectively. Bearing RCH₂-substituent at the sp³-Si atom, 1H-disilagermirene 53 is the first representative of the alkyl-substituted cyclopropene analogues of the heavy group 14 elements.

The other three silagermenes, namely (^tBu₂MeSi)₂Si=GeMes₂ 54 [74], Mes₂Si=Ge(SiMe^tBu₂)₂ 55 [75], and (^tBu₃Si)₂Si=GeMes₂ 56 [76], were available by the reaction of 1,1-di(lithio)derivatives (^tBu₂MeSi)₂SiLi₂, (^tBu₂MeSi)₂GeLi₂, and (^tBu₃Si)₂SiLi₂ with di(aryl)dichlorides Mes₂GeCl₂, Mes₂SiCl₂, and Mes₂GeCl₂, respectively. The silagermene 55 showed a characteristic low-field signal of its sp²-Si atom [δ(²⁹Si) = 146.9 ppm], whereas silagermenes 54 and 56 exhibited unusually shielded sp²-Si centers [δ(²⁹Si) = 22.4 ppm and 18.7 ppm, respectively]. This striking spectroscopic distinction resulted from the differing substitution pattern in 54 and 56 (donating groups on Si and withdrawing groups on Ge), which alters the natural Si^δ+=Ge^δ− bond polarity (like in 55) into a reversed one Si^δ−=Ge^δ+ (like in 54 and 56). Only silagermene 56 was crystallographically identified as featuring peculiar properties caused by the exceptional steric crowding of its ^tBu₃Si-substituents: rather long [r_Si=Ge = 2.2769(8) Å] and markedly twisted [τ = 24.7°] Si=Ge double bond with planar geometry at the sp²-Si and sp²-Ge atoms [Σ_Si = Σ_Ge = 360°]. Upon thermolysis at 100 °C, 56 quantitatively isomerized to a symmetrically substituted silagermene (E)-[^tBu₃Si(Mes)Si=Ge(Mes)Si^tBu₃] [76].

Iwamoto, Kira, and coworkers reported identically substituted silagermene, (^tBuMe₂Si)₂Si=Ge(SiMe₂^tBu)₂ 57, prepared by the reductive debromination of 1,2-di(bromo)silagermane Br(^tBuMe₂Si)₂Si–Ge(SiMe₂^tBu)₂Br with sodium in toluene [22]. As with other stable silagermenes, 57 manifested a low-field resonance for its sp²-Si center [δ(²⁹Si) = 144.0 ppm], a short (compared to other silagermenes) [r_Si=Ge = 2.2208(4) Å] and almost undistorted [τ = 7.5°] Si=Ge double bond, and practically planar geometry at the doubly bonded Si and Ge atoms [θ = 0.6°].

The ”inorganic ethylene” donor–acceptor push–pull complex, {[IPr:]→SiH₂–H₂Ge:→[W(CO)₅]} 58, in which the “SiH₂–H₂Ge” fragment is complexed with both Lewis base [IPr (IPr = 1,3-bis(2,6-diisopropylphenyl)-2H-imidazol-2-ylidene)] and Lewis acid [W(CO)₅], was reported by Rivard and coworkers [77]. Moreover, 58 was synthesized by the reaction of the di(chloro)silylene complex [IPr∙SiCl₂] [78] and [Cl₂Ge∙W(CO)₅], giving at first [IPr∙SiCl₂–Cl₂Ge∙W(CO)₅] a complex that was subsequently reduced with LiAlH₄ to form the final complex, {[IPr:]→SiH₂–H₂Ge:→[W(CO)₅]} 58. Given the strongly shielded Si nucleus [δ(²⁹Si) = –71.9 ppm (triplet, ¹J_Si–H = 192.2 Hz)] and very long Si–Ge bond of 2.3717(14) Å, the central Si–Ge interaction in 58 is best classified as a single, rather than a double, bond. This was supported by the computations on the model compound (Me in place of the real 2,6-ⁱPr₂-C₆H₃ substituents on N atoms in IPr ligand; B3LYP/cc-pVDZ-pp), which yielded WBI_Si–Ge value of only 0.88, thus suggesting Si–Ge single-type bonding.

The latest example of the stable silagermenes was very recently reported by Scheschkewitz and coworkers. Their cyclic potassium silagermenide 59, representing a Si=Ge analogue of a vinyl anion, was synthesized by the reduction of a germylene–NHC complex with KC₈ (Scheme 28) [79]. The spectroscopic and structural features of 59 conform well to those of other stable silagermenes: characteristically deshielded sp²-Si center [δ(²⁹Si) = 142.9 ppm (in C₆D₆) and 138.5 ppm (in thf-d₈)] and short Si=Ge bond [r_Si=Ge = 2.2590(3) Å]. Moreover, the GeSi₂C-ring in 59 is nearly planar, with a negligible folding of 1.9°. According to the crystallographic and UV-spectroscopic data, the π-conjugative interaction between the endocyclic Si=Ge double bond and exocyclic C=N double bond in 59 is notable with the calculated value for the π_Si=Ge–π*_C=N interaction energy of 23.6 kcal mol⁻¹. Silagermenide 59 can be functionalized at the anionic Ge with the appropriate electrophiles to form novel neutral Ge-substituted silagermenes 60a,b (Scheme 28) [79]. Similar to the starting silagermene 59, both (silyl)silagermene 60a [E = SiPh₃] and (phosphanyl)silagermene 60b [E = P(NⁱPr₂)₂] showed low-field resonances for their sp²-Si atoms [δ(²⁹Si) = 136.6 ppm (singlet) and 104.5 ppm (doublet, ²J_Si–P = 9.8 Hz), respectively], and short Si=Ge bonds [r_Si=Ge = 2.2020(2) and 2.2252(4) Å, respectively]. The GeSi₂C-ring in 60a is practically planar (folding angle = 0.2°), and the geometry around the Ge atom is slightly pyramidal (Σ_Ge = 357.3°).

Silagermenes, in which the Si=Ge double bond is a part of the 1,3-diene (>Si=Ge–C=C<), allene (>Si=Ge=Si</>Ge=Si=Ge<), or vinylidene (>Si=Ge:) system, are discussed separately in Section 4.2.1, Section 5.1, and Section 6.2.1, respectively.

The first reported transient germastannene, [Mes₂Ge=SnTip₂], was generated via the dehydrofluorination of Mes₂(H)Ge–Sn(F)Tip₂ by ^tBuLi by Escudié and coworkers [80]. Decomposing at room temperature, this germastannene was proved to be as such on the basis of its low-field ¹¹⁹Sn NMR resonance [360.0 ppm, −20 °C] and trapping reactions with MeOH and PhCHO.

As for the stable germastannenes, four of them were reported in 2003/2004. The first three were synthesized by the research groups of Weidenbruch and Sekiguchi in 2003. Thus, Weidenbruch and coworkers prepared the first structurally authenticated germastannene, Tip₂Ge=SnTip₂ 61, by a one-pot low temperature reaction of TipMgBr, GeCl₂•diox and SnCl₂ [81]. Expectedly, germastannene 61 revealed low-field resonance of its sp²-Sn center [δ(¹¹⁹Sn) = 268.0 ppm], short Ge=Sn bond [r_Ge=Sn = 2.5065(5) Å] that was substantially shorter than the typical Ge–Sn single bonds, and remarkable trans-bending of the substituents [θ = 30.2° (at Ge) and 43.3° (at Sn)]. Being stable in the solid form, 61 decomposed in solution via dissociation of the Ge=Sn double bond to give a cyclotristannane cyclo-(Tip₂Sn)₃ and a digermene Tip₂Ge=GeTip₂.

Another stable germastannene, unsymmetrically substituted (^tBu₂MeSi)₂Ge=SnTip₂ 62, was synthesized in 2003 by Sekiguchi and coworkers by the coupling of 1,1-di(lithio)germane (^tBu₂MeSi)₂GeLi₂ and di(aryl)di(chloro)stannane Tip₂SnCl₂ (Scheme 29) [82]. In the absence of X-ray data, 62 was identified by its characteristic low-field tin resonance [δ(¹¹⁹Sn) = 525.1 ppm]. Upon thermolysis at 50 °C, 62 quantitatively isomerized to a symmetrically substituted germastannene, (E)-[Tip(^tBu₂MeSi)Ge=Sn(SiMe^tBu₂)Tip] 63 (Scheme 29) [82]. A concerted isomerization pathway involving simultaneous 1,2-migration of the silyl and aryl groups in the starting germastannene 62 was proposed, based on the negative value of the activation entropy (ΔS^‡ = –12.0 cal/K·mol). The doubly bonded Sn atom in isomeric 63 was more shielded than that in the starting 62, 373.4 ppm vs. 525.1 ppm, as a result of their different substitution patterns. The preliminary crystallographic data of 63 revealed the trans-bending of substituents at the Ge=Sn bond with θ angles of ca. 28.0°.

The only currently known cyclic germastannene, ³Δ-1,2,3,4-disilagermastannetene 64, featuring an endocyclic Ge=Sn double bond incorporated into the four-membered ring skeleton, was synthesized by Lee, Sekiguchi and coworkers in 2004 [83]. Moreover, 64 was readily available by the ring expansion of either 3H- or 1H-disilagermirenes [49] with SnCl₂•diox (Scheme 30). As is typical for germastannenes, the doubly bonded Sn atom was diagnostically deshielded [δ(¹¹⁹Sn) = 439.3 ppm]. In a sharp contrast to the acyclic tetra(aryl)germastannenes Mes₂Ge=SnTip₂ [80] and Tip₂Ge=SnTip₂ 61 [81], cyclic tetra(silyl)germastannene 64 was indefinitely stable both in the solid state and in solution, showing no signs of dissociation of its >Ge=Sn< double bond into the germylene >Ge: and stannylene >Sn:. The unexpected high thermal stability of 64 was assigned to the influence of its σ-donating silyl substituents, further enhanced by the proposed π_Ge=Sn→σ*_Si–Cl orbital mixing lowering the π_Ge=Sn-orbital energy level and thus stabilizing the HOMO of the molecule [83].

Since 2004, no stable germastannenes >Ge=Sn< were reported.

The first digermyne Ar′–Ge≡Ge–Ar′ [Ar′ = 2,6-(2,6-ⁱPr₂-C₆H₃)₂-C₆H₃] 65 was synthesized and structurally characterized in 2002 by Power and coworkers via the reduction of the (chloro)germylene Ar′(Cl)Ge: with potassium [84]. The C_Ar′–Ge–Ge–C_Ar′ core in digermyne 65 was planar and trans-bent with the Ge–Ge–C_Ar′ bond angle of 128.7°. The Ge–Ge bond was rather short [r_Ge≡Ge = 2.2850(6) Å], which is indicative of its considerable multiply bonded character. On the other hand, accumulation of the lone pair electron density at the Ge atoms results in the trans-bending of their substituents, and consequently in the decrease (compared to the ideal triple bonding) of the Ge–Ge bond order and bond strength.

Following the report, Power’s group prepared a series of stable terphenyl-substituted digermynes, Ar*–Ge≡Ge–Ar* [Ar* = 2,6-(2,4,6-ⁱPr₃-C₆H₂)₂-C₆H₃] 66 [85,86], Ar″–Ge≡Ge–Ar″ [Ar″ = 4-Cl-{2,6-(2,6-ⁱPr₂-C₆H₃)₂}-C₆H₂] 67 [87], Ar‴–Ge≡Ge–Ar‴ [Ar‴ = 4-Me₃Si-{2,6-(2,6-ⁱPr₂-C₆H₃)₂}-C₆H₂] 68 [87], and Ar**–Ge≡Ge–Ar** [Ar** = 3,5-ⁱPr₂-{2,6-(2,4,6-ⁱPr₃-C₆H₂)₂}-C₆H] 69 [87], which were uniformly prepared by the reduction of the corresponding aryl(chloro)germylenes with K or KC₈. The digermyne 66 was characterized by NMR spectroscopy, elemental analysis, and its reaction with 2,3-dimethyl-1,3-butadiene [85,86]. All structurally characterized digermynes 67–69 displayed features similar to those of the first digermyne 65, namely planar [C_Ar–Ge–Ge–C_Ar torsional angles = 180° (for 67 and 68) and 165.8° (for 69)] and trans-bent [trans-bent Ge–Ge–C_Ar angles = 124.2° (for 67), 128.4° (for 68), and 136.1° (for very crowded 69)] C_Ar–Ge–Ge–C_Ar core arrays [84,87]. The Ge–Ge bonds in digermynes 67–69 were short [r_Ge≡Ge = 2.3071(3) Å (for 67), 2.2438(8) Å (for 68), 2.2125(13) Å (for 69)], being comparable to those in digermenes >Ge=Ge<. The different Ge–Ge bond lengths in digermynes 65 and 67–69 can be understood in terms of the substituents’ electronic effects. Thus, digermynes 68 and 69 with the shortest Ge–Ge bonds feature electron donating alkyl or silyl substituents on the central aryl rings, whereas digermyne 65 (which has no such electron donating groups) and digermyne 67 (with electron withdrawing Cl substituents) have notably longer Ge–Ge bonds. Given that the electron donating groups typically reduce the extent of the trans-bending, whereas electron withdrawing groups having the opposite effect, it is reasonable that a smaller bending leads to a higher bond order and a shorter Ge–Ge bond, and vice versa.

Jones and coworkers reported bis(amido)digermyne R–Ge≡Ge–R {R = N(SiⁱPr₃)[2,6-(CHPh₂)₂-4-ⁱPr-C₆H₂]} 70 prepared by the reduction of the germylene R(Cl)Ge: with [(^MesNacnac)Mg]₂ [28]. However, given the Ge–Ge bond distance of 2.3568(3) Å in 70 that is markedly longer than those of Power’s di(aryl)digermynes [84,87], it is better formulated as a digermene (WBI = 1.75) [28], rather than a digermyne.

By contrast, the constitution of the stable di(aryl)digermyne Bbt–Ge≡Ge–Bbt (Bbt = 2,6-[(Me₃Si)₂CH]₂-4-[(Me₃Si)₃C]-C₆H₂) 71, as the triply bonded derivative, was firmly established by Tokitoh and coworkers [88]. 71 was readily prepared by the reduction of the 1,2-di(bromo)digermene precursor trans-[Bbt(Br)Ge=Ge(Bbt)Br] with KC₈. The two crystallographically independent molecules in the unit cell of 71 showed slightly twisted [C_Ar–Ge–Ge–C_Ar torsional angles = 160.2° and 168.9°] and very short [r_Ge≡Ge = 2.2060(7) Å and 2.2260(7) Å] Ge–Ge bonds (markedly shorter that those in Power’s digermynes 65 and 67–69).

Reducing 1,2-di(bromo)digermene (E)-Tbb(Br)Ge=Ge(Br)Tbb (Tbb = 2,6-[(Me₃Si)₂CH]₂-4-^tBu-C₆H₂) with KC₈, Sasamori, Tokitoh and coworkers synthesized novel di(aryl)digermyne Tbb–Ge≡Ge–Tbb 72 [19]. As other digermynes, 72 displayed short Ge≡Ge bond [r_Ge≡Ge = 2.2410(9)/2.2221(9) Å] and trans-bent C_aryl–Ge–Ge angle (130.5/130.7°) (for two independent molecules in the unit cell of 72).

Heteronuclear alkyne analogues of the type –Ge≡E– (E = C, Si, Sn, Pb), featuring a triple bond between germanium and different group 14 element, were always among the top challenges for experimental pursuits. However, despite numerous research efforts, such compounds (as stable derivatives) have not been isolated to date, although germynes R–Ge≡C–R′ were most closely approached, both computationally and experimentally.

Theoretical studies by Su and a coworker predicted that the germynes can be synthetically accessible, if appropriately substituted with bulky groups to prevent isomerization and oligomerization of highly reactive germynes [89].

The first chemical evidence for the transient germyne ArGe≡CSiMe₃ [Ar = 2,6-(ⁱPr₂NCH₂)₂-C₆H₃], generated by the low-temperature photolysis (λ = 300 nm, −50 C°) of (diazo)germylene precursor ArGe[C(N₂)SiMe₃] and trapped with ^tBuOH forming ArGe(O^tBu)₂CH₂SiMe₃, was reported by Couret and coworkers [90]. The transient germyne ArGe≡CSiMe₃ [Ar = 2,4-^tBu-6-(ⁱPr₂NCH₂)-C₆H₂] was similarly generated and trapped with water and ^tBuOH by Mazières and coworkers [91].

Likewise, photolyzing stable (diazo)germylene, the Kato and Baceiredo group generated metastable (stable below −30 °C) phosphine-stabilized germyne [92]. This compound, however, featured a very long Ge–C separation, [r_Ge–C = 1.887(5) Å], a value that was intermediate between those of typical Ge–Ge single and Ge=Ge double bonds. Accordingly, it is better described as a singly bonded bis(carbenoid) [:Ge–C:] rather than a triply bonded germyne [Ge≡C].

The very first compound of this type, Tip₂Ge=Ge(Tip)–Ge(Tip)=GeTip₂ (Tip = 2,4,6-ⁱPr₃-C₆H₂) 73, was synthesized in 2000 by Weidenbruch and coworkers via the reduction of the digermene Tip₂Ge=GeTip₂ with lithium, followed by the reaction of the resulting digermenyllithium [Tip₂Ge=Ge(Tip)Li] with TipBr (Scheme 31) [54]. The Ge–Ge bonds in 73 are in accord with the formulation of the 1,3-diene system >Ge1=Ge2–Ge3=Ge4<, being 2.3568(6) Å (for Ge1=Ge2), 2.4581(5) Å (for Ge2–Ge3), and 2.3439(5) Å (for Ge3=Ge4), respectively. Both Ge=Ge bonds in 73 are trans-bent [θ = 35.4°/31.1° (for Ge1=Ge2) and 33.3°/31.1° (for Ge3=Ge4)] and twisted [τ = 22.4° (Ge1=Ge2) and 21.3° (for Ge3=Ge4)], as is typical for the digermenes. The longest wavelength absorption of 73 in hexane is 560 nm, which remarkably exceeded those of yellow or orange digermenes by 140 nm [and even those of tetra(silyl)digermenes by ca. 100 nm], thus testifying to the presence of conjugation between the two Ge=Ge double bonds.

The same authors later reported improved synthesis of 73 by the one-pot reaction of GeCl₂∙dioxane complex, TipMgBr, and Mg in THF, allowing for an increase of the isolated yield of 73 to 31% (cf.: 11% [50]) [57]. In o-dichlorobenzene, tetra(germa)buta-1,3-diene 73 revealed a reversible reduction [E_1/2(red) = –0.46 V] and irreversible oxidation [E_p(ox) = 0.15 V], whereas in THF both reduction and oxidation processes were irreversible [E_p(red) = –1.75/–1.3 V and E_p(ox) = 0.6 V] [93].

Apart from 73, there is only one tetra(germa)butadiene derivative reported by Matsuo and coworkers, tetra(germa)cyclobutadiene Ge₄[EMind]₄ (EMind = 1,1,7,7-tetraethyl-3,3,5,5-tetramethyl-s-hydrindacen-4-yl) 74 (as the germanium analogue of the isostructural tetra(sila)cyclobutadiene [94]) [95]. Moreover, 74 was available by the reduction of di(chloro)digermene [EMind](Cl)Ge=Ge(Cl)[EMind] with lithium naphthalenide (Scheme 32). In contrast to tetra(sila)cyclobutadiene, tetra(germa)cyclobutadiene 74 was thermally stable, with the UV-absorptions observed at 458 (ε = 15000), 510 (ε = 7400), and 836 nm (ε = 150). The Ge₄-ring is rhombic-planar [Σ_{internal bond angles} = 360°] with the cyclic Ge–Ge bonds of 2.430 Å (av.). Two germanium atoms feature sp²-like trigonal-planar geometry (Σ_Ge = 360.0°), whereas the other two germanium atoms have sp³-like pyramidal configuration (Σ_Ge = 334.0 and 327.7°) with the trans-bending of substituents (θ = 37.9 and 40.6°). Theoretical studies revealed that the Ge–Ge bond orders in 74 = 1.08–1.09 (WBI). Within the Ge₄-ring, there is a clear charge separation: if the two diagonal Ge atoms are slightly positively charged (+0.146 and +0.151, NPA charges), the other two Ge atoms are strongly positively charged (+0.602 and +0.573, NPA charges). All of these data are indicative of the crucial contribution of the charge-separated resonance form cyclo-[Ge⁺–Ge⁻–Ge⁺–Ge⁻], which was also the case for the previously reported tetra(sila)cyclobutadiene. The polar Jahn–Teller distortion results in the relaxation of the inherent 4π-antiaromaticity of 74, thus forming a charge-separated rhombic–planar singlet structure, which is nonaromatic (based on the NICS values).

Lee, Sekiguchi, and coworkers developed a different approach toward 1,3-dienes containing Ge atoms. They synthesized five-membered ring derivatives including the >Si=Ge–C=C< 1,3-diene fragment, starting from the [2 + 2] cycloaddition of the heavy cyclopropene (1H-disilagermirene 50 [49]) with terminal alkynes to yield bicyclo[2.1.0]pentenes 75, which underwent valence isomerization, finally forming 1,2-disila-3-germacyclopenta-2,4-dienes 76 [²⁹Si NMR (sp²-Si): +124.2 ppm (76a), +123.6 ppm (76b), and +127.3 ppm (76c)] (Scheme 33) [96,97,98]. Such heavy cyclopentadienes 76 constitute rather unusual cyclic systems with two formally conjugated double bonds, Si=Ge and C=C, which are actually isolated, despite the planarity of the five-membered ring. This conclusion was based on the crystallographic and UV spectroscopic data. Thus, in the Si=Ge–C=C fragment, all bonds are within the standard ranges (for example, for 76a, 2.250(1) Å (Si=Ge), 1.972(3) Å (Ge–C), and 1.343(5) Å (C=C)); that is, no elongation of the double bonds and shortening of the single bonds expected for the conjugated system were observed. Likewise, there was no notable red-shift in the longest wavelength UV absorption (π–π*) of 76a compared to that of the starting 50 with isolated Si=Ge bond, 472 vs. 467 nm, respectively. Most likely, the lack of 1,3-conjugation between the Si=Ge and C=C bonds in 1,2-disila-3-germacyclopenta-2,4-dienes 76 results from the substantial spatial and energetic mismatch of the Si=Ge and C=C bonds molecular orbitals, because effective overlap of the latter is a prerequisite for the π-electrons delocalization.

This synthetic strategy toward Ge=Ge–C=C conjugated systems was pioneered by Power’s group and then developed by Sasamori and coworkers, who prepared and isolated stable 1,2-di(germa)cyclobutadienes 77a,b via the reaction of their respective digermynes with diphenylacetylene (Scheme 34) [99,100,101]. In both 77a and 77b, the Ge₂C₂-ring is practically planar (Σ_{internal bond angles} = 359.8° (77a) [99] and 359.9° (77b) [101]), albeit the Ge centers are strongly pyramidalized [Σ_Ge = 318.8° and 317.3° (77a [99])]. Within the Ge=Ge–C=C fragment, the bonds germanium–germanium/germanium–carbon/carbon–carbon are classified as double/single/double bonds, respectively: 2.4708(9)/[2.022(5) and 2.027(5)]/1.365(7) Å (77a) [99], and 2.4160(5)/2.022(2)/1.362(5) Å (77b) [101]. Accordingly, the structure of 77a,b is best described by the resonance form cyclo-[Ge=Ge–C=C] with the weak Ge=Ge double bond, as depicted in Scheme 34.

By reacting 1,2-di(germa)cyclobutadiene 77b [101] with (Me₂N)₃P=Se, Sasamori and coworkers synthesized 2,5-di(germa)selenophene 78, formed through the transient “housene”-type bicyclic selenadigermirane with its subsequent isomerization to 78 (Scheme 35) [102]. The selenium nucleus in 78 resonated in the low-field region at 481.8 ppm (⁷⁷Se NMR). On the basis of its structural data [r_Ge–C = 1.921(3)/1.922(3) Å, r_C–C = 1.375(4) Å, θ_Ge = 7.8°(trans), nonplanar SeGe₂C₂-ring] and computational studies, 78 is better formulated as a singlet (digerma)biradicaloid 78B, while classical Lewis representation 78A is a minor contributor (Scheme 35). Nevertheless, 78 exhibited some degree of aromaticity, as its NICS(1) of –8.0 was comparable with that of the organic selenophene.

Following their original report on the synthesis of the stable tri(sila)allene >Si=Si=Si< [103], Kira and coworkers subsequently prepared tri(germa)allene 79a [104], 1,3-di(germa)silaallene 79b [104], and 2-germadi(sila)allene 79c [105]. Of them, 79a and 79b were prepared by the co-reduction of tetra(chloro)digermane 80a and chloro(trichlorosilyl)germane 80b with KC₈, whereas 79c was synthesized by the co-reduction of di(alkyl)silylene 81 and GeCl₂∙diox with KC₈ (Scheme 36). All sp²-Si atoms were diagnostically deshielded, with the central Si atom in 79b being more strongly deshielded than the peripheral Si atoms in 79c: +236.6 ppm vs. +219.4 ppm. The longest wavelength UV absorptions in 79a, 79b, and 79c were found at 630, 612, and 599 nm, respectively, thus pointing to the significant conjugation between the cumulated double bonds. The allenic bonds in 79 were all in the range of the typical double bonds: r_Ge=Ge = 2.321(2) Å and 2.330(3) Å in 79a, r_Ge=Si = 2.2697(8) Å in 79b, and r_Si=Ge = 2.2366(7) Å and 2.2373(7) Å in 79c. The allenic E=E′=E framework in 79 was notably bent [bending angle = 122.61(6)° (in 79a), 125.71(7)° (in 79b), and 132.38(2)° (in 79c)], and the terminal atoms E were strongly pyramidalized [θ_E = 348.5°/348.6° (in 79a), 349.3° (in 79b), and 353.9°/354.0° (in 79c)].

Apart from Kira’s heavy allenes described above, there was only one recent report on the title compounds: Sasamori, Tokitoh, and coworkers prepared cyclic 1,3-di(germa)-2-silaallene 82 via the reductive dechlorination of tetra(chloro)-2,5-digerma-1-silacyclopentane with KC₈ (Scheme 37) [106]. In 82, the SiGe₂C₂-ring is planar, and the germanium–silicon bonds are typical double bonds [r_Si=Ge = 2.2681(18) and 2.2900(18) Å]. However, the central allenic Si in 82 was extraordinarily shielded [δ(²⁹Si) = –16.5 ppm] [106] compared to that of Kira’s 1,3-di(germa)-2-silallene (236.6 ppm) [104]. This was explained by the contribution of the “silylone” resonance structure [>Ge:→Si⁰←:Ge<] of 82, resulting from the acute Ge–Si–Ge bond angle (80.1°) due to incorporation of the Ge–Si–Ge unit into the five-membered ring skeleton. This was supported by calculations which showed that the germanium–silicon bonds in 82 are highly polarized as Ge^δ+–Si^δ−, –0.27 for Si and +0.90/+0.91 for Ge (NPA charges), and this is indicative of an important Ge-to-Si σ-donation.

Only a couple of isolable 1-germaallenes >Ge=C=C< are currently known. The first one, Tip₂Ge=C=C(^tBu)Ph 83, was reported by West and coworkers synthesized by the reaction of Tip₂Ge(F)–C≡C–Ph with ^tBuLi via the transient [Tip₂Ge(F)–C(Li)=C(^tBu)Ph], which eliminated LiF to finally yield 5 [107]. In solution, 83 decomposed at room temperature within 15 h. In 83, as in other heteroallenes, the Ge=C and C=C bonds were short [r_Ge=C = 1.783(2) Å and r_C=C = 1.314(2) Å], the Ge=C=C fragment was bent (159.2°), the geometry at the Ge atom was pyramidal (Σ_Ge = 348.4°), and the central C atom was greatly deshielded [δ(¹³C) = +235.1 ppm].

The second 1-germaallene, Tbt(Mes)Ge=C=CR₂ (CR₂ = fluorenylidene) 84, was prepared by the reductive dechlorination of Tbt(Mes)ClGe–C(Cl)=CR₂ with ^tBuLi [108]. Like in 83, the central allenic carbon of 84 resonated in the diagnostic low-field at +243.6 ppm. Without trapping reagents, 84 underwent slow intramolecular cyclization via C–H activation of one of the (Me₃Si)₂CH groups by the Ge=C bond to form a benzogermacyclobutene derivative.

Vinylidenes R₂C=C:, with substituent-free terminal carbon atoms, are the valence isomers of alkynes RC≡CR. They are exceptionally reactive, and for their stabilization, external bases, such as NHC, are particularly effective. Accordingly, all—except for Aldridge’s di(germa)vinylidene (vide infra)—isolable heavy group 14 analogues of vinylidenes R₂E=E′: (E, E′ = heavy group 14 elements) were stabilized by NHC-coordination.

The first di(germa)vinylidene 85, synthesized by Aldridge and coworkers, was free from external NHC-coordination, in contrast to the NHC-supported silagermenylidenes (vide infra) (Scheme 38) [109].

85 was prepared by the reduction of the boryl(chloro)germylene–NHC complex with K (or KC₈), followed by the oxidation of the intermediate dianion by [Ph₃C]⁺[B(C₆F₅)₄]⁻ (Scheme 38). The germanium–germanium bond in 85 is planar and short [r_Ge=Ge = 2.312(1) Å] in order to formulate it as a Ge=Ge double bond. In the crystal, weakly stabilizing interactions were found between one of the aryl π-systems of each of the boryl substituents and the vacant p-orbital at the monocoordinated Ge atom. The HOMO of 85 is the Ge=Ge π-bond, whereas the HOMO–1 is mostly the naked Ge lone pair. The longest wavelength UV-absorption in 85, observed at 460 nm, is due to the Ge=Ge bond π–π* electronic transition.

Another stable di(germa)vinylidene 86 was reported by Wesemann and coworkers, synthesized by the reduction of intramolecularly phosphine-stabilized chloro(germyl)germylene with [(^MesNacnac)Mg]₂ (^MesNacnac = {[(2,4,6-Me₃-C₆H₂)N–(Me)C]₂CH}⁻) or metallic sodium (Scheme 39) [110]. The germanium–germanium bond in 86 [r_Ge=Ge = 2.3060(2) Å] is within the range of the Ge=Ge double bonds, but it is slightly shorter than that in 85 [r_Ge=Ge = 2.312(1) Å]; the bicyclic framework is planar, and the tricoordinate sp²-Ge disclosed nearly planar geometry (Σ_Ge = 358.8°). As the manifestation of the Ge=Ge double bonding in 86, π_Ge=Ge and π*_Ge=Ge orbitals represent the HOMO and LUMO, respectively. The bonding situation in 86 can be alternatively viewed as a double Lewis base-stabilized Ge atom in the formal oxidation state 0. Thus, di(germa)vinylidene 86 can potentially serve as a source for Ge⁰-single atom transfer by the germanium abstraction reactions.

The first isolable heavy Group 14 elements analogue of the vinylidene, NHC-stabilized silagermenylidene Tip₂Si=Ge:(⟵NHC) (NHC = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) 87, was prepared by the co-reduction of Tip₂SiCl₂ and [NHC→GeCl₂] complex with lithium naphthalenide, as reported by Scheschkewitz and coworkers [111]. As is typical for the Si=Ge doubly bonded system, the Si atom in 87 is markedly deshielded [δ(²⁹Si) = 158.9 ppm] and has a planar configuration (Σ_Si = 359.8°), and the Si=Ge bond is short [r_Si=Ge = 2.2521(5) Å]. Moreover, the Si=Ge bond in 87 is practically untwisted (τ = 3.1°), which favors an effective p_π–p_π orbital overlap, and the NHC-ligand coordinates to the Ge nearly orthogonally (C_NHC–Ge–Si bond angle = 98.9°), thus maximizing the [n_{NHC lone pair} → p_Ge] orbital interaction. Subsequently, the same group prepared another NHC-stabilized silagermenylidene, [Tip₂(Cl)Si](Tip)Si=Ge:(⟵NHC) (NHC = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) 88, whose spectral and structural characteristics are similar to those of 87: δ (²⁹Si) = 162.5 ppm, r_Si=Ge = 2.2757(10) Å, C_NHC–Ge–Si bond angle = 101.9° [112].