Carbonyl. carbonyl interactions involving Tr-C=O moieties (Tr = first-row transition element) have been studied using crystal structure data retrieved from the Cambridge Structural Database and by use of DFT calculations. By comparison with organic ketones, Tr-C=O systems show an increased tendency to form close CO center dot center dot center dot CO interactions, with 45% of these forming pairwise interactions in a sheared antiparallel dimer motif and 55% having a perpendicular ( single interaction) geometry. The bulky Tr and steric hindrance arising from other ligands at Tr play a significant role in the formation and geometry of the interactions. DFT calculations for the antiparallel dimer indicate that interaction energies for Tr-C=O systems are slightly stronger than for organic ketones, and there is evidence of a stronger C=O bond dipole in Tr-C=O systems. With interaction energies comparable to those for medium strength hydrogen bonds, we conclude that CO center dot center dot center dot CO interactions in Tr-C=O species may have a role to play in the design of novel carbonyl-containing inorganic and metal-organic structures.