Five-electron donor versus seven-electron donor bridging phospholyl rings in binuclear cobalt carbonyl derivatives

The lowest energy (C4H4P)2Co2(CO)n (n = 3, 2, 1) structures have exclusively terminal phospholyl ligands and correspond closely to those of the analogous cyclopentadienyl derivatives (C5H5)2Co2(CO)n. Thus the lowest energy (C4H4P)2Co2(CO)n structures are the singly bridged trans-(η5-C4H4P)2Co2(CO)2(μ-CO) with a formal Co–Co single bond, the doubly bridged (η5-C4H4P)2Co2(μ-CO)2 with a formal Codouble bond; length as m-dashCo double bond, and either singlet or triplet singly bridged (η5-C4H4P)2Co2(μ-CO). Higher energy triplet (C4H4P)2Co2(CO)n structures are found with a five-electron donor η5,η1-C4H4P ligand bridging a cobalt–cobalt bond. None of the (C4H4P)2Co2(CO)n (n = 3, 2, 1) structures has a seven-electron donor η5,η1-C4H4P ligand without a metal–metal bond in contrast to the lowest energy (C4H4P)2Mn2(CO)n (n = 5, 4) structures.