Binuclear cyclopentadienylmetal cyclooctatetraene derivatives of the first row transition metals: effects of ring conformation on the bonding of an eight-membered carbocyclic ring to a pair of metal atoms

Binuclear Cp(2)M(2)(μ-C(8)H(8)) derivatives have been synthesized for M = V, Cr, Co, and Ni and have now been studied theoretically for the entire first row of transition metals from Ti to Ni. The early transition metal derivatives Cp(2)M(2)(μ-C(8)H(8)) (M = Ti, V, Cr. Mn) are predicted to form low-energy cis-Cp(2)M(2)(μ-C(8)H(8)) structures with a folded C(8)H(8) ring (dihedral angle ～130°) and short metal-metal distances suggesting multiple bonding. These predicted structures are close to the experimental structures for M = V, Cr with V≡V and Cr≡Cr bond lengths of ～2.48 and ～2.36 ?, respectively. The middle to late transition metals form trans-Cp(2)M(2)(μ-C(8)H(8)) structures (M = Mn, Fe, Co, Ni) with a twisted μ-C(8)H(8) ring and no metal-metal bonding. The hapticity of the central μ-C(8)H(8) ring in such structures ranges from five for Mn and Fe to four for Co and three for Ni and thus depend on the electronic requirements of the central metal atom. This leads to the favored 18-electron configuration for both metal atoms in the singlet Fe, Co, and Ni structures but only 17-electron metal configurations in the triplet Mn structure. In addition, the late transition metals form trans-Cp(2)M(2)(μ-C(8)H(8)) structures (M = Fe, Co, Ni), with the tub conformation of the μ-C(8)H(8) ring functioning as a tetrahapto (M = Fe, Co) or trihapto (M = Ni) ligand to each CpM group. A μ-C(8)H(8) ring in the tub conformation also bonds to two CpFe units as a bis(tetrahapto) ligand in both singlet and triplet cis-Cp(2)Fe(2)(μ-C(8)H(8)) structures.