ETS-NOCV calculations were performed on 3-Co, 3-Rh and 3-Ir to investigate the interaction of the formyl moiety with the transition metal centre. As described previously by us in a related system, the principal contribution (∆ρ1) to the bonding is the -donation of the HOMO of the {CHO} -ligand to a transition-metal d-orbital (66 to 75 % of EORB). The secondary (∆ρ2) and tertiary (∆ρ3) interactions involve π back-donation from a metal-based orbital to the orthogonal π*-orbitals of the {CHO} -ligand and are largely negligible in comparison to ∆ρ1 (9 to 12 % of EORB).