The character of low-lying excited states of mixed-ligand metal carbonyls.: TD-DFT and CASSCF/CASPT2 study of [W(CO)4L] (L = ethylenediamine, N,N′-dialkyl-1,4-diazabutadiene) and [W(CO)5L] (L = pyridine, 4-cyanopyridine)

This contribution presents the results of the TD-DFT and CASSCF/CASPT2 calculations on [W(CO)(4)(MeDAB)] (MeDAB = N,N'-dimethyl-1,4-diazabutadiene), [W(CO)(4)(en)] (en = ethylenediamine), [W(CO)(5)(py)] (py = pyridine) and [W(CO)(5)(CNpy)] (CNpy = 4-cyanopyridine) complexes. Contrary to the textbook interpretation, calculations on the model complex [W(CO)(4)(MeDAB)] and [W(CO)(5)(CNpy)] show that the lowest W MeDAB and W-->CNpy MLCT excited states are immediately followed in energy by several W-->CO MLCT states, instead of ligand-field (LF) states. The lowest-lying excited states of [W(CO)(4)(en)] system were characterized as W(COeq)(2)COax CT excitations, which involve a remarkable electron density redistribution between axial and equatorial CO ligands. [W(CO)(5)(py)] possesses closely-lying W-->CO and W-->py MLCT excited states. The calculated energies of these states are sensitive to the computational methodology used and can be easily influenced by a substitution effect. The calculated shifts of [W(CO)(4)(en)] stretching CO frequencies due to excitation are in agreement with picosecond time-resolved infrared spectroscopy experiments and confirm the occurrence of low-lying M-->CO MLCT transitions. No LF electronic transitions were found for either of the complexes studied in the region up to 4 eV.