The present contribution reports investigations on the metal-ligand bond lengths and interaction energies in selected carbon monoxide complexes of metal cations sharing the ns(2) np(0) valence configuration. 1- and 4-component DFT geometry optimizations have been performed for cations ranging from Ge2+ to Uuq(2+), the dication of element 114 (Ununquadium). The nature of the bonding has been studied by means of energy decomposition analysis. The magnitude of the relativistic effects is shown to evolve slowly and to become predominant for Uuq for the molecular properties investigated. (C) 2008 Elsevier B. V. All rights reserved.