Unusual support effect on metal-CO interaction is shown occurring when an electron occupies an anti-bonding level in the metal-CO system. Then, the presence of a reducible support increases the interaction since the unpaired electron is formally transferred towards the support. Such effect is manifest for metal elements from columns VIB and IB. A reducible support allows a formal electronic transfer, oxidizing the metal adsorbed. Accordingly, the metal-CO interaction should be considered as the interaction between CO and a metal cation (M+) which is much stronger than the interaction with the neutral atom. Spin localization confirms our analysis. This effect has consequences on the geometry of adsorption, decreasing the M-C distances. (C) 2009 Elsevier B.V. All rights reserved.