Chemical bonding in simple transition metal carbonyls is examined under the interacting quantum atoms approach (IQA), which provides an energetic viewpoint within the quantum theory of atoms in molecules (QTAIM). We have studied both classical and nonclassical isoelectronic series of complexes, with different coordinations and geometries and studied the evolution of the IQA interatomic interactions, using several levels of theory. Our results in classical carbonyls are compatible with the standard Dewar-Chatt-Duncanson model, although multi-center bonding may have an important role in some complexes. The increase (decrease) in the CO distance upon bonding is faithfully coupled to a decrease (increase) in the CO covalent energy, although the main energetic change in the CO moiety is electrostatic and due to charge transfer and/or polarization of its electron density. The metal-ligand interaction energy is dominated by covalent effects and depends strongly on the total net charge of the complex, being larger for negatively charged molecules, where pi-back-donation is very important. The electrostatic (ionic-like) metal ligand interaction energy is small in general, although it becomes more and more stabilizing with increasing coordination number.