Tricarbonylrhenium(I) Complexes with Thiosemicarbazone Derivatives of 2-Acetylpyridine and 2-Pyridine Formamide Showing Two Unusual Coordination Modes of Tridentate Thiosemicarbazone Ligands

[NEt4]2[Re(CO)3Br3] reacts with 2-acetylpyridine phenylthiosemicarbazone (HL1) and 2-pyridine formamide thiosemicarbazone (HL2)
under formation of air-stable, neutral rhenium(I) complexes of the
compositions [Re(CO)3(L1-N,N,S)] and [Re(CO)3Br(HL2-N,N)]. Spectroscopic studies and X-ray crystallography show that the potentially
tridentate thiosemicarbazones adopt unusual coordination modes.
Whereas HL1 deprotonates and binds to the metal in a nonplanar
fashion, HL2 acts as neutral N,N donor ligand. The bond lengths
inside the chelate rings are almost uninfluenced by the overall
bonding situation.