Terminal versus bridging methylborole ligands and agostic hydrogen atoms in binuclear cobalt carbonyl derivatives

Density functional theory predicts an (η5-C4H4BCH3)2Co2(CO)4 structure having terminal methylborole ligands and two bridging CO groups similar to the experimental structure of the product synthesized from Co2(CO)8 and suitable borole derivatives as well as analogous to experimentally known (η4-C4H4)2Co2(CO)2(μ-CO)2 and (η5-C5H5)2Fe2(CO)2(μ-CO)2. However, the unsaturated tricarbonyl (η5-C4H4BCH3)2Co2(CO)3 has a more complicated potential energy surface with low energy singlet and triplet structures having one, two, or three bridging CO groups. The lowest energy structure for the dicarbonyl (η5-C4H4BCH3)2Co2(CO)2 is a singlet with terminal methylborole ligands and bridging CO groups as well as a short Co≡Co distance of ∼2.2 Å suggesting a formal triple bond. However, another low energy (η5-C4H4BCH3)2Co2(CO)2 structure has a bridging methylborole ligand and two terminal CO groups. In the lowest energy (C4H4BCH3)2Co2(CO) structure the pendent methyl groups of the methylborole ligands participate in the ligand-metal bonding through agostic hydrogen atoms forming C–H–Co three-center bonds. A bridging methylborole ligand with such an agostic hydrogen atom is a six-electron donor to the Co2 system.