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(η2,2-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.