Variable hapticity of the cyclooctatetraene ring in sandwich compounds of the first row transition metals

Density functional theory methods (B3LYP and BP86) indicate that the preferred structures for such early transition metal derivatives are (η⁸-C₈H₈)M(η⁴-C₈H₈) (M = Ti, V, Cr) with one octahapto η⁸-C₈H₈ ring and one tetrahapto η⁴-C₈H₈ ring. In such structures only 12 of the 16 carbon atoms of the two C₈H₈ rings are bonded to the metal, leading to 16-, 17-, and 18-electron complexes, respectively, in accord with the experimentally known structures for the Ti and V derivatives. The preferred structures for the Mn and Fe derivatives are (η⁶-C₈H₈)M(η⁴-C₈H₈) (M = Mn, Fe) with one hexahapto and one tetrahapto C₈H₈ ring and thus having 17- and 18-electron configurations, respectively, in accord with experimental data on the iron complex. The lowest energy structure for the cobalt complex is (η⁴-C₈H₈)Co(η^2,2-C₈H₈) with two different types of tetrahapto C₈H₈ rings and a 17-electron metal configuration. The nickel complex (C₈H₈)₂Ni appears to prefer a structure with a 16-electron configuration and two trihapto C₈H₈ rings, similar to the known (η³-C₃H₅)₂Ni rather than a bis(tetrahapto) structure with the favored 18-electron configuration. These theoretical studies indicate that in (C₈H₈)₂M derivatives of the first row transition metals, the number of carbon atoms in the pair of C₈H₈ rings involved in the bonding to the central metal atom gives the metal atoms 16-, 17-, or 18-electron configurations.