Synthesis and Properties of Cobalt Complexes with [B 11 H 11 ] 4- Ligands.

The unusually high oxidation state + IV of cobalt is stabilized by ligands based on [B 11 H 11 ] 4- in dark blue colored Cs 4 [Co(B 11 H 10.11 (OH) 0.75 ) 2 ]·4.56H 2 O, K 4 [Co(B 11 H 9.19 (OH) 1.81 ) 2 ]·2H 2 O, Cs 8 [Co{(B 11 H 6 ) 2 (O)(O 2 BOH) 4 }] 2 ·4H 2 O and K 4 [Co{(B 11 H 6 ) 2 (O 2 BOH) 5 }]·7H 2 O. These compounds were obtained by reacting Co 2+ salts with [B 11 H 14 ] - under alkaline conditions. In the absence of oxygen, Co(+III) compounds such as the light brownish K 4 [Co(B 11 H 11 )(CN) 3 ]·KCl·2.5H 2 O are formed. The title compounds were characterized by X-ray crystallography. Cs 8 [Co{(B 11 H 6 ) 2 (O)(O 2 BOH) 4 }] 2 ·4H 2 O and K 4 [Co(B 11 H 11 )(CN) 3 ]·KCl·2.5H 2 O were also characterized using IR-, UV-vis and cyclovoltammetry. Magnetic measurements of Cs 4 [Co(B 11 H 10.11 (OH) 0.75 ) 2 ]·4.56H 2 O and ESR measurements of Cs 8 [Co{(B 11 H 6 ) 2 (O)(O 2 BOH) 4 }] 2 ·4H 2 O show that in these Co(+IV) low-spin d 5 complexes the unpaired electron is on the d x 2- y 2 , d xy ( E 2g ) orbitals.