Agostic hydrogen atoms versus cobalt-cobalt multiple bonding in binuclear borole cobalt carbonyls.

Graphical abstract The lowest energy structures for the tetracarbonyl (C 4 H 4 BH) 2 Co 2 (CO) 4 are doubly bridged structures similar to (C 4 H 4 BCH 3) 2 Co 2 (CO) 4. The lowest energy structures for the unsaturated derivatives (C 4 H 4 BH) 2 Co 2 (CO) n (n = 3, 2, 1) have bridging borole ligands with agostic hydrogen atoms from their B–H groups to a cobalt atom. Highlights • The (C 4 H 4 BH) 2 Co 2 (CO) n (n = 4, 3, 2, 1) systems have been examined by density functional theory. • Low-energy doubly bridged (C 4 H 4 BH) 2 Co 2 (CO) 2 (µ-CO) 2 structures are found similar to (C 4 H 4 BCH 3)Co 2 (CO) 2 (µ-CO) 2. • The lowest energy structures for (C 4 H 4 BH) 2 Co 2 (CO) n (n = 3, 2, 1) have agostic B–H hydrogens from bridging borole ligands. Abstract The (C 4 H 4 BH) 2 Co 2 (CO) n (n = 4, 3, 2, 1) systems have been examined by density functional theory for comparison with the analogous (C 4 H 4 BCH 3) 2 Co 2 (CO) n systems. For the tetracarbonyl (C 4 H 4 BH) 2 Co 2 (CO) 4 a higher energy unbridged structure is found at ∼7 kcal/mol above the lowest energy doubly bridged structure similar to (C 4 H 4 BCH 3) 2 Co 2 (CO) 4. The lowest energy structures for the unsaturated derivatives (C 4 H 4 BH) 2 Co 2 (CO) n (n = 3, 2, 1) have bridging borole ligands with agostic hydrogen atoms from their B–H groups to a cobalt atom. Structures without agostic hydrogens analogous to the low-energy structures for the corresponding methylborole derivatives (C 4 H 4 BCH 3) 2 Co 2 (CO) n are higher energy (C 4 H 4 BH) 2 Co 2 (CO) n structures. [ABSTRACT FROM AUTHOR]