The Dichotomy of Dimetallocenes: Coaxial versus Perpendicular Dimetal Units in Sandwich Compounds

Abstract
Theoretical studies of the dimetallocene (η5-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 Å, corresponding to a Zn−Zn single bond. (η5-C5H5)2Ni2 (singlet state) and (η5-C5H5)2Cu2 (triplet) have similar structures with a Ni⋮Ni triple bond (2.06 Å) and a CuCu double bond (2.22 Å). 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.

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