The bond between molybdenum and substituted 1,10-phenanthroline ligands in a series of [Mo(CO)4(phen*)]\ud complexes has been studied by combining experimental data (νCO) with DFT calculations. First, natural orbitals for chemical valence (NOCV) were calculated: The resulting charge-transfer magnitudes (Δqi) associated with the deformation density channels (Δρi) were related to σ-donation and π-back-donation.\ud Then, energy decomposition analysis was performed by applying the extended transition state (ETS) scheme. The outcomes of the ETS-NOCV approach has allowed us to quantify the energetic contribution of both ligand-to-metal (Eσ) and metal-to ligand (Eπ) interactions. A new parameter (Tphen) has been introduced comprising both Eσ and Eπ and thus providing a descriptor for the overall electronic contribution given by phenanthrolines\ud to the metal–ligand bond. This was corroborated by the linear correlation found between Tphen and the νCO vibration\ud modes of [Mo(CO)4(phen*)] complexes, at least for those containing a 2,9-unsubstituted phenanthroline. The case of\ud [Mo(CO)4(phen*)] derivatives with a 2,9-substituted phen* is also discussed