A Quantitative Description of the σ-Donor and π-Acceptor Properties of Substituted Phenanthrolines

The bond between molybdenum and substituted 1,10-phenanthroline ligands in a series of [Mo(CO)(4)(phen*)] complexes has been studied by combining experimental data (nu(CO)) with DFT calculations. First, natural orbitals for chemical valence (NOCV) were calculated: The resulting charge-transfer magnitudes (Delta q(i)) associated with the deformation density channels (Delta rho(i)) were related to sigma-donation and pi-back-donation. 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 (E-sigma) and metal-to-ligand (E-pi) interactions. A new parameter (T-phen) has been introduced comprising both E-sigma and E-pi and thus providing a descriptor for the overall electronic contribution given by phenanthrolines to the metal-ligand bond. This was corroborated by the linear correlation found between T-phen and the nu(CO) vibration modes of [Mo(CO)(4)(phen*)] complexes, at least for those containing a 2,9-unsubstituted phenanthroline. The case of [Mo(CO)(4)(phen*)] derivatives with a 2,9-substituted phen* is also discussed.