The photochemistry of the phosphine-substituted transition metal carbonyl complexes Cr(CO)5PH3 and ax-Fe(CO)4PH3 is studied with time-dependent DFT theory to explore the propensity of the excited molecules to expel their ligands. The influence of the PH3 ligand on the properties of these complexes is compared with the photodissociation behavior of the binary carbonyl complexes Cr(CO)6 and Fe(CO)5. The lowest excited states of Cr(CO)5PH3 are metal-to-ligand charge transfer (MLCT) states, of which the first three are repulsive for PH3 but modestly bonding for the axial and equatorial CO ligands. The repulsive nature is due to mixing of the initial MLCT state with a ligand field (LF) state. A barrier is encountered along the dissociation coordinate if the avoided crossing between these states occurs beyond the equilibrium distance. This is the case for expulsion of CO but not for the PH3 group as the avoided state crossing occurs within the equilibrium Cr−P distance. The lowest excited state of ax-Fe(CO)4PH3 is a LF state that is repulsive for both PH3 and the axial CO. Excited-state quantum dynamics calculations for this state show a branching ratio of 99 to 1 for expulsion of the axial phosphine ligand over an axial CO ligand. The nature of the phosphorus ligand in these Cr and Fe complexes is only of modest importance. Complexes containing the three-membered phosphirane or unsaturated phosphirene rings have dissociation curves for their lowest excited states that are similar to those having a PH3 ligand. Analysis of their ground-state Cr−P bond properties in conjunction with frontier orbital arguments indicate these small heterocyclic groups to differ from the PH3 group mainly by their enhanced σ-donating ability. All calculations indicate that the excited Cr(CO)5L and Fe(CO)4L molecules (L = PH3, PC2H5, and PC2H3) prefer dissociation of their phosphorus substituent over that of an CO ligand. This suggests that the photochemical approach may be a viable complement to the ligand exchange and redox methods that are currently employed to demetalate transition metal complexed organophosphorus compounds.
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