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 Ni![[triple bond, length as m-dash]](http://www.rsc.org/images/entities/char_e002.gif)
Ni 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 Ni![[double bond, length as m-dash]](http://www.rsc.org/images/entities/char_e001.gif)
Ni 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).
