Reacting 1-phenyl-2-(diphenylphosphino)ethanone (P 1 ∼OH), 1-tert-butyl-2-(diphenylphosphino)ethanone (P 2 ∼OH) and 1-phenyl-2-(diphenylphosphino)propanone (P 3 ∼OH) with ReOCl 3 (PPh 3 ) 2 and Re(NPh)Cl 3 (PPh 3 ) 2 in toluene and ethanol in the presence of NEt 3 yields complexes in which these ligands bind as monoanionic enolato chelating agents. Bromo and iodo analogues are prepared similarly. The reactions are solvent dependent. The disubstituted ReOCl(P∼O) 2 compounds obtained in toluene adopt the 'twisted' cis-(P,P) octahedral structure. With Re(NPh)Cl 3 -(PPh 3 ) 2 , P 1 ∼OH and P 2 ∼OH give the corresponding Re(NPh)Cl(P∼O) 2 compounds and the trans-(P,P)-trans-(Cl,Cl) isomer of monosubstituted Re(NPh)Cl 2 (PPh 3 )(P∼O) as the major products, together with small amounts of 'twisted' trans-(P,P) Re(NPh)Cl(P∼O) 2 . The monosubstituted complex is the only species isolated with P 3 ∼OH. The major structural difference between the two systems is the small stability gap between the cis-and the trans-(P,P) isomers of the imido complexes. In ethanol, unexpected cis-ethoxo-oxo complexes ReO(OEt)(P∼O) 2 are isolated, resulting from stereoselective substitution of the halide in the 'twisted' cis-(P,P) octahedral complexes. The reactions with Re(NPh)Cl 3 (PPh 3 ) 2 are less selective and give rise to mixtures of complexes. The trans-ethoxo-phenylimido compound Re(NPh)(OEt)(P 1 ∼O) 2 is obtained in good yield. A crystallographic study reveals an unexpected 'equatorial' trans-(P,P) structure with the ethoxo ligand trans to the Re᎐ ᎐ NPh bond. 1 H NMR indicates that the ethoxo ligand acts as an electron reservoir in these complexes, since its protons are strongly deshielded in the cis-oxo-ethoxo species and strongly shielded in the trans-phenylimido-ethoxo complex.

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