The electronic description of octahedral (fac-[M(CO)(3)L-3](n), with M = Re, Ru, and Mn, and [Cr(CO)(5)L](n)), square-planar (cis-[Pt(CO)(2)L-2](n)), and tetrahedral ([Ni(CO)(3)L](n)) carbonyl complexes (where L=monodentate ligand) was obtained via density functional theory and natural population analyses in order to understand what effects are probed n these species by vibrational spectroscopy and electrochemistry as a function of the ligand electronic parameter of the associated L. The analysis indicates that while ligand electronic parameters may be considered as a measure of the net donor power of the ligand, the net transfer of the electron density (or charge) does not occur from the ligand to the metal ion. In [M(CO)(x)L-y](n) carbonyl species, the charge transfer occurs from the ligand L to the oxygen atom of the bound carbon monoxides. This charge transfer translates into changes of the polarization (or permanent dipole) and the covalency of the C O bonds, and it is this effect that is probed in IR spectroscopy. As the analysis shifts from IR radiations to electrochemical potentials, the parameters best describe the relative thermodynamic stability of the oxidized and reduced [M(CO)(x)L-y](n/n+1) species. No relationship is found between the metal natural charge of the [M(CO)(x)L-y](n) fragments analyzed and the parameters. Brief considerations are given on the possible design of CO-releasing molecules.