Frederik W. M. Vanhelmont, Geoffrey F. Strouse, Hans U. Güdel, Claudia Stückl and Helmut W. Schmalle
Synthesis, Crystal Structure, High-Resolution Optical Spectroscopy, and Extended Hückel Calculations on Cyclometalated [Re(CO)₄(ppy)] (ppy = 2-Phenylpyridine)
J. Phys. Chem. A 101, 2946-2952 (1997)                       

Abstract: The cyclometalated complex [Re(CO)₄(ppy)], where ppy^– = 2-phenylpyridine, was synthesized. Its crystal and molecular structure was determined by X-ray diffraction. The lowest energy electronic excitations were studied by high-resolution optical spectroscopy at cryogenic temperature. The first excited state in [Re(CO)₄(ppy)] is nominally a triplet ligand-centered state, with 1.8% metal-to-ligand charge transfer character mixed in through spin-orbit coupling. This induces a shortened lifetime (89 ms at 10 K) and the occurrence of metal-ligand vibrations in both absorption and luminescence spectra. The transition moment of the first electronic excitation lies in the molecular plane with a tilt toward the pyridine part of the ppy^– ligand. This is derived from the polarized absorption measurements and confirmed by an extended Hückel calculation. Splittings in the IR absorption bands are resulting from the presence of two crystallographically equivalent complexes in the unit cell which are tilted with respect to each other.