Synthesis, Crystal Structure, High-Resolution Optical Spectroscopy, and Extended Hückel Calculations for [Re(CO)₄(thpy)] (thpy^- = 2-(2-Thienyl)pyridinate). Comparison with Related Cyclometalated Complexes

We have synthesized the new cyclometalated complex [Re(CO)4(thpy)] (thpy- = 2-(2-thienyl)pyridinate) and determined the crystal and molecular structure by X-ray diffraction. By application of high-resolution optical spectroscopy to single crystals and glasses at cryogenic temperatures, the first excited state is identified as a nominally ligand-centered 3π−π* state with some 1MLCT character mixed in by spin−orbit coupling. The coefficient α describing the charge-transfer character in the wave function of the first excited state has a value of 0.019. This admixture leads to a reduced luminescence lifetime (258 μs at 10 K) and the occurrence of sidebands in the absorption and luminescence spectra which are due to metal−ligand vibrations. The parameters characterizing the first excited state are compared with the corresponding parameters of related cyclometalated and noncyclometalated complexes of Rh(III), Ir(III), and Re(I). An extended Hückel calculation was performed to identify the relevant one-electron excitations.