Open chains versus closed rings: Comparison of binuclear butadiene cobalt carbonyls with cyclic hydrocarbon analogs

The reaction of Co2(CO)8 with butadiene has long been known to give the binuclear (C4H6)2Co2(CO)2(μ-CO)2. In addition, other (diene)2Co2(CO)2(μ-CO)2 derivatives have been synthesized. In this connection the binuclear butadiene cobalt carbonyls (C4H6)2Co2(CO)n (n = 4, 3, 2, 1) have now been investigated by density functional theory. For (C4H6)2Co2(CO)4 several doubly carbonyl bridged stereoisomers were found differing only in the arrangements of the butadiene ligands and with predicted Co–Co single bond distances in the range 2.53–2.61 Å. A similar multiplicity of stereoisomers is not possible for the corresponding cyclobutadiene derivative (C4H4)2Co2(CO)4. The lone unbridged (C4H6)2Co2(CO)4 structure lies ∼13 kcal/mol above the lowest energy doubly bridged isomer with a significantly longer Co–Co distance of 2.74 Å. For (C4H6)2Co2(CO)3 the lowest energy structures are doubly carbonyl bridged structures with CoCo double bond distances in the range 2.45–2.51 Å. However, triply carbonyl bridged (C4H6)2Co2(μ-CO)3 structures with significantly shorter CoCo distances of 2.24–2.30 Å are also found. For (C4H6)2Co2(CO)2 several structures with bridging butadiene ligands and terminal carbonyl groups are energetically competitive with those with bridging carbonyl groups and terminal butadiene ligands, unlike the corresponding cyclobutadiene complexes. The lowest energy singlet (C4H6)2Co2(CO) has a two-electron donor bridging carbonyl group and an extremely short Co–Co distance of 2.13 Å suggesting the formal quadruple bond required to give each cobalt atom the favored 18-electron configuration. This differs from the cyclobutadiene analog (C4H4)2Co2(CO) in which the bridging carbonyl group is a four-electron donor η2-μ-CO group and the cobalt-cobalt distance is much longer at ∼2.5 Å.