The synthesis of the first polyhedral boranes, carboranes, and metallacarboranes in the early 1960s was one of the major highlights in the development of inorganic chemistry over the last century [1]. The first reports on the synthesis of icosahedral carboranes appeared almost sixty years ago, at the end of 1963 when both the United States and the Soviet Union almost simultaneously declassified documents about their boron fuel projects ^{[2][3][4][5][6]}[2,3,4,5,6]. A few months later, the nucleophile-promoted removal of one boron atom from the icosahedral ortho-carborane cage to form the 11-vertex nido-carborane cage species (Figure 1) was reported ^[7][8][7,8]. It was one of the most significant findings in the early years of the development of carborane chemistry, and now, more than five decades later, it remains indispensable for the synthesis of numerous metallacarboranes ^{[9][10][11][12][13][14][15][16][17]}[9,10,11,12,13,14,15,16,17] and hydrophilic functionalized carboranes for medical ^{[18][19][20][21][22][23][24][25][26][27]}[18,19,20,21,22,23,24,25,26,27] and other ^{[28][29][30][31][32][33][34][35][36][37][38][39][40]}[28,29,30,31,32,33,34,35,36,37,38,39,40] applications.

Metallacarboranes based on the dicarbollide ligand [7,8-C₂B₉H₁₁]²⁻, which is formed upon the deprotonation of nido-carborane with strong bases, resemble the well-known transition metal cyclopentadienyl complexes. However, the dicarbollide ligand differs from the cyclopentadienyl ligand in a number of ways. In addition to its 3D character, the dicarbollide ligand is a significantly stronger donor than the cyclopentadienyl one and has a double charge. The donor nature of the dicarbollide ligand can be largely tuned via the introduction of substituents of various natures. At the same time, the charge of the ligand can be partially compensated by introducing into the dicarbollide ligand the so-called charge-compensating substituents of an onium nature (ammonium, phosphonium, sulfonium, etc.). This significantly brings the properties of the dicarbollide and cyclopentadienyl complexes closer together and causes a high interest in metallacarboranes based on charge-compensated dicarbollide ligands ^{[41][42][43][44][45][46][47][48][49][50]}[41,42,43,44,45,46,47,48,49,50].

2. Charge-Compensated Derivatives of Nido-Carborane with Boron–Nitrogen Bond

Due to the great diversity of nitrogen chemistry, the charge-compensated derivatives of nido-carborane with the B-N bond are characterized by the greatest variety of forms. The first example of the synthesis of charge-compensated derivatives of nido-carborane with a B-N bond was the reaction of the parent nido-carborane with pyridine in benzene in the presence of anhydrous FeCl₃, leading to the asymmetrically substituted pyridinium derivative 9-Py-7,8-C₂B₉H₁₁ (Scheme 1) ^[51][73], the structure of which was later supported via a single-crystal X-ray diffraction study (Figure 2) ^[52][74]. When FeCl₃·6H₂O was used instead of anhydrous FeCl₃, the by-product of the reaction was the disubstituted pyridinium derivative 9,11-Py₂-7,8-C₂B₉H₉ ^[52][74]. The reaction with 7,8-dimethyl-nido-carborane proceeds in a similar way, giving 9-Py-7,8-Me₂-7,8-C₂B₉H₉ ^[51][73]. In a similar way, the reaction of nido-carborane with methyl isonicotinate in the presence of FeCl₃ in refluxing benzene results in 9-(4′-MeO(O)CC₅H₃N)-7,8-C₂B₉H₁₁ ^[53][75].