Cationic beryllium–group 13 heterobimetallic dimetallocenes with a donor–acceptor bond

A comprehensive geometrical and bonding analysis of heterobimetallic dimetallocenes of the form [Cp-Be-M-Cp]⁺, where M represents group 13 elements (B, Al, Ga, In, Tl), is presented. The equilibrium geometry of dimetallocenes indicates a collinear geometry along the centre of the two Cp rings, the Be centre and the group 13 element, except for the Tl complex. Optimised geometries reveal that Be-C bond distances decrease when the group 13 element changes from B to Tl. The molecular orbital and NBO analyses show a Be-M σ-bond formed by the overlap of the 2s orbital of Be with the sp^<i>n</i> hybrid orbital of the group 13 element. The percentage of s character in the sp^<i>n</i> hybrid orbital increases as the group 13 element changes from B to Tl. However, the natural population analysis (NPA) indicates a significant charge polarisation with a low value of the Wiberg bond index. Energy decomposition analysis with natural orbitals for chemical valence (EDA-NOCV) reveals that the Be-M σ bond can be represented by a donor-acceptor interaction (Cp-Be⁺ ← M-Cp). Apart from the <i>σ</i> bonding interaction, there exists a slight π bonding interaction arising from hyperconjugative donation from the [BeCp]⁺ bonding molecular orbital to the [BCp] antibonding molecular orbital. Even though the strength of the Be-M bond decreases, the percentage of covalent interaction increases when the group 13 element changes from B to Tl.