Complexes with a Single Metal–Metal Bond as a Sensitive Probe of Quality of Exchange-Correlation Functionals

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Abstract
The electronic structure of the vanadium dimer complex [V(C5H5)]2Pn with a single metal–metal bond was characterized, and the energies of higher spin states were evaluated. To simplify evaluation of orbital contributions to bonding between atoms and fragments, occupancy-perturbed bond orders were introduced. The structure and experimentally determined singlet–triplet gap in this complex can be used to test the quality of modern exchange-correlation functionals. Most generalized gradient approximation (GGA) functionals were determined to be quite suitable to reproduce the metal–metal distance and the single–triplet energy gap in [V(C5H5)]2Pn. Further accuracy improvement can be achieved by using empirical dispersion corrections. Hybrid exchange-correlation functionals, including the B3LYP functional, performed poorly for both structural and energy predictions. The hybrid functionals significantly overestimate the stability of the singlet state with the antiferromagnetically coupled high-spin metal ions relative to the lowest-energy triplet state and the singlet state with stronger metal–metal interactions. Thus, these XC functionals are not quite suitable for computational studies of multinuclear 3d transition metal complexes with weak-to-intermediate metal–metal bonding.

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