Synthesis and Properties of Cobalt Complexes with [B11H11]4- Ligands.

The unusually high oxidation state + IV of cobalt is stabilized by ligands based on [B11H11]4- in dark blue colored Cs4[Co(B11H10.11(OH)0.75)2]·4.56H2O, K4[Co(B11H9.19(OH)1.81)2]·2H2O, Cs8[Co{(B11H6)2(O)(O2BOH)4}]2·4H2O and K4[Co{(B11H6)2(O2BOH)5}]·7H2O. These compounds were obtained by reacting Co2+ salts with [B11H14]- under alkaline conditions. In the absence of oxygen, Co(+III) compounds such as the light brownish K4[Co(B11H11)(CN)3]·KCl·2.5H2O are formed. The title compounds were characterized by X-ray crystallography. Cs8[Co{(B11H6)2(O)(O2BOH)4}]2·4H2O and K4[Co(B11H11)(CN)3]·KCl·2.5H2O were also characterized using IR-, UV-vis and cyclovoltammetry. Magnetic measurements of Cs4[Co(B11H10.11(OH)0.75)2]·4.56H2O and ESR measurements of Cs8[Co{(B11H6)2(O)(O2BOH)4}]2·4H2O show that in these Co(+IV) low-spin d5 complexes the unpaired electron is on the dx2-y2, dxy (E2g) orbitals.