A (Diborole)cobalt Complex with a C–H···B Bridge, CpCo(1,3‐C 3 B 2 Me 5 H), and Its Thallium Derivative: Synthesis, Structure, and Bonding

The reaction of CpCo(1,3‐C 3 B 2 Me 5 H) ( 1 ) with CpTl affords the thallium derivative CpCo(1,3‐C 3 B 2 Me 5 )Tl ( 2 ). The structures of 1 and 2 were determined by X‐ray diffraction at 100 K. An “extra” hydrogen atom in 1 occupies a C–H ··· B bridging position. According to DFT calculations, 1 exists as a mixture of two enantiomers with an enantiomerization barrier of only 0.5 kcal mol –1 . The transition state has C s symmetry with an endo ‐CH hydrogen atom. The isomeric iso ‐ 1 with the Co–H ··· B bridge is less stable than 1 by 10 kcal mol –1 .The formation of 1 and iso ‐ 1 from C 3 B 2 Me 5 H and CpCo(C 2 H 4 ) 2 have almost equal activation energies. The isomerization of iso ‐ 1 to 1 was shown to proceed as a two‐step hydrogen transfer. The bonding of the “extra” hydrogen atom in the related CHB‐bridged carborane nido ‐2,3,5‐C 3 B 3 R 5 H 2 and metallacarboranes M(C 5 R 5 )(C 3 B 2 R′ 5 H) (R, R′ = H, Me; M = Co, Rh, Ir) as well as their hydridic isomers MH(C 5 R 5 )(C 3 B 2 R′ 5 ) was compared. According to energy decomposition analysis, the bonding of the parent anion [CpCo(1,3‐C 3 B 2 H 5 )] – with metal cations becomes stronger in the following order: K + < Na + < Tl + < Li + < [RuCp] + . The attractive interactions between the [CpCo(1,3‐C 3 B 2 H 5 )] – and Tl + fragments are 68 % electrostatic and 32 % covalent.