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 Å 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 Å). 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).