A systematic density functional theory study on the structures and binding energies of triplatinum carbonyls Pt-3(CO)(x)(q), with x = 1-6 and q = 0, - 1, reveals an anomalous trend in binding energies of the CO ligands to the triangular platinum core, with the first three ligands strongly bound at terminal Pt sites, followed by weaker bound ligands at Pt-Pt bridge sites for x = 4-6. This work provides a novel explanation to the anomalous CO binding trend reported in mid-1990s from collision-induced dissociation and photodissociation experiments. (c) 2005 Elsevier B.V. All rights reserved.