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

Emission and UV-vis absorption spectra of (hydrotris(pyrazolyl)borato)(trip henylarsine)copper(I) (CuTpAsPh(3)), (hydrotris(pyrazolyl)borato)(triethyla mine)copper(I) (CuTpNEt(3)), and (hydrotris(pyrazolyl)borato)(triphenylphos phine)-copper(I), (CuTpPPh(3)), are reported. The spectra of the arsine com plex contain low-energy bands (with a band maximum at 16 500 cm(-1) in emis sion and a weak shoulder centered at about 25 000 cm(-1) in absorption) tha t are not present in the corresponding spectra of the amine or phosphine co mplexes. The lowest energy electronic transition is assigned to ligand to l igand charge transfer (LLCT) with some contribution from the metal. This as signment is consistent with PM3(tm) molecular orbital calculations that sho w the HOMO to consist primarily of pi 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 absorptio n shoulder shows a strong negative solvatochromism; indicative of a reversa l or rotation of electric dipole upon excitation, and consistent with a LLC T. The trends in the energies of the electronic transitions and the role of the metal on the LLCT are discussed.