The dichotomy of dimetallocenes: coaxial versus perpendicular dimetal units in sandwich compounds.

Theoretical studies of the dimetallocene (eta5-C5H5)2Zn2 lead to optimized D5h or D5d structures in which the Zn-Zn bond is coaxial with the C5 axes of the two Cp rings, with a Zn-Zn distance of 2.33 A, corresponding to a Zn-Zn single bond. (eta5-C5H5)2Ni2 (singlet state) and (eta5-C5H5)2Cu2 (triplet) have similar structures with a NiNi triple bond (2.06 A) and a Cu=Cu double bond (2.22 A). However, DFT computations on (C5H5)2Ni2 and (C5H5)2Cu2 (both singlet states) lead to a totally different type of optimized structure (Ci symmetry) lying at significantly lower energies, with the metal-metal bonds perpendicular to the C5 axes of the Cp rings.