The electronic spectroscopy of transition metal di-hydrides H2M(CO)4 (M = Fe,Os): a theoretical study based on CASSCF/MS-CASPT2 and TD-DFT

Abstract

Transition energies to the low-lying singlet electronic excited states of H₂M(CO)₄ (M = Fe,Os) are calculated at the CASSCF/MS-CASPT2 level of theory using relativistic effective core potentials in the ab initio model potential (AIMP) approach. The main features of the absorption spectra of both molecules differ significantly. The spectrum of H₂Fe(CO)₄ is dominated by low-lying excited states corresponding mainly to 3d_Fe → σ^*_u and 3d_Fe → σ^*_g excitations calculated between 39 310 cm⁻¹ (4.9 eV) and 43 210 cm⁻¹ (5.4 eV). These allowed transitions contribute to the shoulder observed in the experimental spectrum around 270 nm (37 040 cm⁻¹) and are responsible for the photoreactivity of H₂Fe(CO)₄ at 254 nm (39 370 cm⁻¹). In contrast the lowest part of the absorption spectrum of the osmium analog H₂Os(CO)₄ is characterized by a high density of metal–ligand-charge-transfer (MLCT) states between 47 220 cm⁻¹ (5.9 eV) and 52 430 cm⁻¹ (6.55 eV) and corresponding to 5d_Os → π^*_CO excitations. The transitions corresponding to 5d_Os → σ^*_u and 5d_Os → σ^*_g excitations are displaced to the upper part of the absorption spectrum of H₂Os(CO)₄ (beyond 60 000 cm⁻¹ or 166 nm) and cannot induce H₂ elimination under irradiation at 254 nm like in the iron complex. A series of sigma-bond–ligand-charge-transfer (SBLCT) states corresponding to σ_g → π^*_CO and σ_u → π^*_CO excitations not present in H₂Fe(CO)₄ is found exclusively in the spectrum of H₂Os(CO)₄ beyond 55 000 cm⁻¹. The upper part of the spectrum of H₂Fe(CO)₄ (below 220 nm) is assigned to intense MLCT transitions corresponding to 3d_Fe → π^*_CO excitations. The MS-CASPT2 and TD-DFT methods agree qualitatively as far as the assignment and transition energies of the low-lying states are concerned in both molecules whereas a comparison of the calculated oscillator strengths is more problematic. The transitions are generally underestimated at the TD-DFT level, an effect that is even more pronounced for the charge transfer excitations to the carbonyl ligands. The high density of singlet electronic states in these simple transition metal hydrides carbonyls illustrates the complexity of the electronic spectroscopy in this class of molecules. This explains the poor resolution of these overcrowded spectra and the difficulty in understanding the photoreactivity of these molecules.