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A series of oxo-rhenium complexes of the type ReO(LLL)(LLLH) with tridentate diacidic ligands (LLLH2) containing the donor set $$O\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\frown}$}}{ } N\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\frown}$}}{ } O\left( N \right)$$ has been synthesized and characterized. X-ray analyses were carried out for benzoylacetone-2-hydroxyanilato(2-)-benzoylacetone-2-hydroxyanilato-oxo-rhenium(V) ReO(BAHA)2 1, [2,6-diphenylacylpyridinato(2-)]-2,6-diphenylacylpyridinato-oxo-rhenium(V) ReO(DPAP)2 2, [2-pyrrolaldehyde-2′-hydroxyanilato-(2-)]-2-pyrrolaldehyde-2′-hydroxyanilato-oxo-rhenium(V) ReO(PAHA)2 3, {4-[1-(N′-benzoylhydrazino)-1-phenyl-methyliden]-3-methyl-1-phenyl-pyrazol-5-onato-(2-)} – {4-[1-(N′-benzoylhydrazino)-1-phenyl-methyliden]-3-methyl-1-phenyl-pyrazol-5-onato}-oxo-rhenium(V) ReO(BHMP)2 5, and for the ligand 4-[1-(N′-benzoylhydrazino)-1-phenyl-methyliden]-3-methyl-1-phenyl-pyrazol-5-one (BHMP). In all the oxo-rhenium complexes the rhenium is in a distorted octahedral coordination environment with one ligand acting as tridentate in the equatorial plane and the other ligand acting as bidentate in the axial position. The Re — Nbond lengths were found to be in the range of 2.01 to 2.18 Å depending on the nature of the bond caused by mesomeric and steric effects of the ligands. In order to check the bonding relationships, molecular orbital calculations with Gaussian 94 were carried out. Additionally force field calculations were performed using the Extensible Systematic Force Field (ESFF). This was done to examine the ability of the ESFF to represent the bonding relationships in order to provide reasonable input data for MO calculations. [ABSTRACT FROM AUTHOR]