Terminal versus bridging cyclobutadiene rings in binuclear nickel carbonyl derivatives: A cube-antiprism twist of the cyclobutadiene rings in the perpendicular structures

Abstract

Cyclobutadiene–nickel derivatives are accessible from reactions of dichlorocyclobutenes with nickel carbonyl. In this connection the binuclear cyclobutadiene–nickel carbonyls (C4H4)2Ni2(CO)n (n = 3, 2, 1) have been examined theoretically as possible sources of new dimetallocene structures and new structures with metal–metal multiple bonds. The lowest energy (C4H4)2Ni2(CO) isomer is predicted to have a perpendicular structure with two bridging C4H4 rings forming an elongated square antiprism enclosing the pair of nickel atoms. The NiNi distance of ∼2.25 A corresponds to the formal triple bond required to give both nickel atoms the favored 18-electron configuration, with all eight carbon atoms of the two C4H4 rings involved in M–C bonding. A related low energy structure is found for (C4H4)2Ni2(CO)2 except that the two bridging C4H4 rings form an elongated cube enclosing a pair of nickel atoms at a somewhat longer bonding distance (∼2.33 A). In this case only a NiNi double bond is required to give both nickel atoms the 18-electron configuration. Other low energy (C4H4)2Ni2(CO)n (n = 1, 2, 3) structures include a monocarbonyl (η4-C4H4)2Ni2(η2-μ-CO) with a four-electron donor bridging carbonyl group, a coaxial doubly bridged dicarbonyl (η4-C4H4)2Ni2(μ-CO)2, and cis and trans isomers of a bent singly bridged tricarbonyl (η4-C4H4)2Ni2(CO)2(μ-CO).