Ligand to ligand charge transfer in (hydrotris(pyrazolyl)borato)(triphenylarsine)copper(I)

Emission and UV-vis absorption spectra of (hydrotris(pyrazolyl)borato)(triphenylarsine)copper(I), (CuTpAsPh₃), (hydrotris(pyrazolyl)borato)(triethylamine)copper(I), (CuTpNEt₃), and (hydrotris(pyrazolyl)borato)(triphenylphosphine)-copper(I), (CuTpPPh₃), are reported. The spectra of the arsine complex contain low-energy bands (with a band maximum at 16 500 cm^-1 in emission and a weak shoulder centered at about 25 000 cm^-1 in absorption) that are not present in the corresponding spectra of the amine or phosphine complexes. The lowest energy electronic transition is assigned to ligand to ligand charge transfer (LLCT) with some contribution from the metal. This assignment is consistent with PM3(tm) molecular orbital calculations that show the HOMO to consist primarily of π orbitals on the Tp ligand (with some metal orbital character) and the LUMO to be primarily antibonding orbitals on the AsPh₃ ligand (also with some metal orbital character). The absorption shoulder shows a strong negative solvatochromism, indicative of a reversal or rotation of electric dipole upon excitation, and consistent with a LLCT. The trends in the energies of the electronic transitions and the role of the metal on the LLCT are discussed.