Considering a simplified molecular orbital picture of selected donor-acceptor interactions, it becomes clear that both formyl ligands of 3 should be interacting with the same metal dorbitals (Figure ). A consequence of this effect is apparent on comparing the electronic structures of 3 with the monomeric unit of 2, labelled 2'. The carbonyl ligand is expected to be a better -acceptor than the formyl and compete for electron-density from the d-orbitals. As such, the back-donation component of the bonding to the mono(formyl) ligand of 2' is expected to be weaker than that to bis(formyl) ligands of 3. The impact of the competition for electron density is apparent in the M-C WBIs to the formyl group which take values of 0.81 -0.97 in 2' but 0.94 -1.08 in 3.