In pursuit of understanding metavalency

The term metavalent bonding was introduced recently to describe bonds that are in between metallic and covalent. They are characterized by nearly half bond order, large Born Effective Charge (BEC), low band gap, moderate electrical conductivity and large optical dielectric constant. While the presence of large BEC, half-bond order and low band gap are often highlighted as their striking features, chemists are familiar with half-bond order systems, and hence there is a reluctance to consider metavalency as a new bonding mechanism. Here we use molecular orbital theory and DFT calculations to identify the uniqueness in the bonding in known metavalent materials and compare that with hypervalent half-bond order systems that are not classified as metavalent. We show that the metavalent materials are compounds with partial σ bonds that are formed mainly by p-p overlap and has low ionicity. If such systems have low band gaps either due to low bonding-antibonding splitting or due to delocalization, that result in large BEC. This suggests that metavalency is not a new bonding mechanism.