Understanding the fundamental properties of metal-metal interactions is crucial to the application of multimetallic systems in catalysis, materials science, and magnetism. The polarity of heterometallic metal-metal bonds is one of the key parameters that can be used to describe multimetallic systems and rationalize their experimental properties and electronic structure. Metal-metal bond polarity is a function of the donor-acceptor interactions between the metal atoms involved in the interaction, with electropositive early transition metals acting as acceptors and electronegative late transition metals acting as donors. This work describes how this expected trend in metal-metal bond polarity can be reversed through the preparation and systematic study of a novel series of Mo2M(dpa)4(OTf)2 (M = Cr, Mn, Fe, Co, and Ni) heterotrimetallic extended metal atom chain (HEMAC) complexes. Structural and physical characterization as well as electronic structure calculations support the presence of a σ symmetry 3c/3e– bond that is delocalized across the entire trimetallic chain. The covalency of the Mo2…M interaction may be modulated by the choice of the heterometal ion, with electropositive heterometals resulting in polar, donor-acceptor interactions and more electronegative heterometals resulting in covalent Mo2…M interactions. Notably, we find that this trend in Mo2…M interaction polarity is in contradiction to the trend expected if the electronegativities of the metal atoms in the trimetallic chain are considered independently from the Mo2 quadruple bond. We attribute the observed trends in the Mo2…M interaction polarity by the decreasing energetic differential between the energetically stable σ bonding molecular orbital of the Mo2 quadruple bond and the heterometal acceptor d orbitals with increasing electronegativity of the heterometal ion (Cr &lt; Mn &lt; Fe &lt; Co &lt; Ni). These results demonstrate that the low-energy, filled σ component of the metal-metal quadruple bond acts as a dative “ligand” to a heterometal ion, which may be used in tandem with the heterometal ion electronegativity to systematically tune the polarity of metal-metal bonds within multimetallic systems.

Tuning the polarity of metal-metal bonds with heterotrimetallic extended metal atom chain (HEMAC) complexes

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