Photodissociation of the Phosphine-Substituted Transition Metal Carbonyl Complexes Cr(CO)₅L and Fe(CO)₄L:  A Theoretical Study

Abstract

The photochemistry of the phosphine-substituted transition metal carbonyl complexes Cr(CO)(5)PH(3) and ax-Fe(CO)(4)PH(3) is studied with time-dependent DFT theory to explore the propensity of the excited molecules to expel their ligands. The influence of the PH(3) 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)(5)PH(3) are metal-to-ligand charge transfer (MLCT) states, of which the first three are repulsive for PH(3) 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 PH(3) group as the avoided state crossing occurs within the equilibrium Cr-P distance. The lowest excited state of ax-Fe(CO)(4)PH(3) is a LF state that is repulsive for both PH(3) 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 PH(3) 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 PH(3) group mainly by their enhanced sigma-donating ability. All calculations indicate that the excited Cr(CO)(5)L and Fe(CO)(4)L molecules (L = PH(3), PC(2)H(5), and PC(2)H(3)) 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.

The chemistry of group-VIb metal carbonyls

“…All calculations indicate that the excited Cr(CO)5L molecules (L = PH3, PC2H5, and PCH3) prefer dissociation of their phosphorus substituent over that of an CO ligand ( Figure 7). 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 [87]. …”

The photochemistry of transition metal complexes using density functional theory

“…In [Cr(CO) 5 (PH 3 )], PECs calculated with TD-DFT showed that avoided crossings with higherlying 1 LF states render low-lying 1 MLCT states dissociative along the Cr−PH 3 coordinate, whereas energy barriers arise along the Cr−CO coordinates [14].…”

“…For example, MLCT states were identified as the lowest states in [Cr(CO) 5 (PH 3 )] [14], [W(CO) 5 (4-cyanopyridine)] and [W(CO) 5 (piperidine)] [15].…”

Photoinduced bond oscillations in ironpentacarbonyl(FeCO)5 give delayed, synchronous bursts of carbonmonoxide(CO) release

“…Wernet et al [22], vibrational modes were obtained at the DFT/B3LYP/cc-pVDZ level of theory using the MOLPRO package, interfaced to SHARC has been used [32]. We also checked the variation of geometry at DFT level of theory and found that cam-B3LYP which is used for ESMD in this study and TPSSH which is often the best functional for transition metal system predicts very similar geometry to the CASPT2 (12,12) geometry.…”