STRUCTURAL STUDIES ON BISMUTH IODIDE HEXAMETHYL-PHOSPHORAMIDE COMPLEXES BY X-RAY CRYSTALLOGRAPHY AND EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE SPECTROSCOPY

The reaction between BiI3 and one equivalent of hmpa [OP(NMe2)3, hexamethylphosphoramide] in toluene solution affords the neutral orange complex [Bi2I6(hmpa)2] which has been characterised by X-ray crystallography. It exists in the solid state as a polymer of dimers in which a linear chain of weakly interacting Bi2I6 units is flanked on either side by hmpa ligands. An analogous reaction between BiI3 and five equivalents of hmpa in toluene also affords this complex together with small amounts of yellow [Bi2I6(hmpa)4]. It is also formed in the reaction between BiI3 and one equivalent of hmpa in thf (tetrahydrofuran) solution (the X-ray quality crystals were obtained from this solvent), although an additional dark orange, ionic compound [BiI2(hmpa)4][I3] is also formed together with small quantities of black crystalline [BiI2(hmpa)4][I5]. The complex [BiI2(hmpa)4][I3] is the major product from the reaction between BiI3 and five equivalents of hmpa in thf, and was also characterised by X-ray crystallography. The bismuth atom in the cation is co-ordinated by two iodines and four hmpa ligands with an octahedral geometry in which the iodines are mutually trans. The factors affecting whether this or the alternative cis geometry is adopted are discussed. The nature of [BiI2(hmpa)4][I5] was established from a partial X-ray diffraction data solution and microanalytical data, and further support for this formulation was obtained from solid-state extended X-ray absorption fine structure (EXAFS) spectroscopic data, an analysis of which was consistent with the presence of the cation [BiI2(hmpa)4]+. The EXAFS data in thf solution are consistent with a three-co-ordinate bismuth environment with dissociation of most or all of the hmpa ligands.