 A pair of π bonds arises from overlap of filled d-orbitals on the metal with a pair of π-antibonding orbitals projecting from the carbon atom of the CO.The latter kind of binding requires that the metal have d-electrons, and that the metal is in a relatively low oxidation state (2-CO or often just μ-CO, νCO, νCO is usually shifted by 100–200 cm−1 to lower energy compared to the signatures of terminal CO, i.e.in the region 1800 cm−1.