Substituent effects on the light-induced C–C and C–Br bond activation in (bisphosphine)(η2-tolane)Pt0 complexes. A TD-DFT study

A theoretical study of the steric and electronic effects of different substituents on the electronic ground and excited states of (bisphosphine)(η2-tolane)Pt0 complexes is presented. A natural-bond-order (NBO) analysis has been performed to describe the bonding nature of the “hapto-like” coordination of the Pt atom to the alkyne bond of the tolane group. The results show an important contribution of the π-back donation in all complexes, amounting to half of the energy associated to the σ-bonding interaction. A TD-DFT study of the absorption spectra helps rationalizing the photochemistry of the complexes. Metal–ligand charge transfer (MLCT) transitions from the Pt atom to the alkyne are assigned as the photochemical “active” states responsible for C–C bond cleavage. Electronic excitations to the σ* orbital of the C–Br bond are involved in C–Br bond activation. It is shown that both steric and electronic effects play an important role in determining the presence of these electronic excitations.