Principles of electronic structure in transition metal complexes. Additive ligand electronic effects and core-valence ionization correlations for Mo(CO)_6-n(PMe₃)_n where n=0, 1, 2, 3

Gas phase core photoelectron spectroscopic (XPS) results are reported for a series of trimethylphosphine substituted molybdenum carbonyls: Mo(CO) ₆, Mo(CO)₅(PMe₃), cis-Mo(CO) ₄(PMe₃)₂, trans-Mo(CO)₄(PMe ₃)₂, and fac-Mo(CO)₃(PMe₃) ₃. Core ligand additivity, defined as a constant shift in core ionizations with each successive step of ligand substitution, is indicated by these data. The shift per phosphine substitution is -0.65±0.10 eV for the molybdenum 3d_5/2 ionization, -0.75±0.11 eV for the carbon (carbonyl) 1s ionization, and -0.78±0.09 eV for the oxygen 1s ionization. Comparison of core and valence data sets for these complexes illustrates a second principle, core-valence ionization correlation. The ratio of the Coulombic valence metal d level shifts to the core metal shifts is 0.74±0.06. This trend, in a system with extensively delocalized metal orbitals, shows that core and valence photoelectron spectroscopies are intimately related and that key additional understanding of electron distributions and bonding can be obtained from correlating the information of these techniques. Simple models for both the ligand additivity and core-valence ionization correlation principles are presented to demonstrate the fundamental features and possible limitations of these principles.