X-ray Absorption Spectroscopic Studies of Chromium Nitroso Complexes. Crystal and Molecular Structure of (Ph₄P)₃[Cr(NO)(NCS)₅]·2.4(CH₃)₂CO

X-ray absorption spectroscopy (XAS) provides a direct means of solving the controversy on Cr oxidation states in nitroso complexes. The first XAS studies of four known Cr−NO complexes, [Cr(NO)(OH2)5]2+, [Cr(NO)(acac)2(OH2)], [Cr(NO)(CN)5]3-, and [Cr(NO)(NCS)5]3-, have been performed, in comparison with the related Cr(III) complexes, [Cr(OH2)6]3+, [Cr(acac)3], [Cr(CN)6]3-, and [Cr(NCS)6]3-. The X-ray absorption near-edge structure (XANES) spectra of the Cr−NO complexes are distinguished from those of the corresponding Cr(III) complexes by increased intensities of pre-edge absorbancies due to the 1s → 3d transition, as well as with slight shifts (by 0.2−1.0 eV) of the edge positions to lower energies, with no major changes in the edge shape. These features, together with the available structural data on Cr−NO complexes, show that the effective Cr oxidation states in such complexes are close to Cr(III), due to the π-back-bonding within the Cr−NO moiety. Multiple-scattering fitting of X-ray absorption fine structure (XAFS) spectra of [Cr(NO)(acac)2(OH2)] supported the assignment of this complex as a trans-isomer (Keller, A.; Ježovska-Trzebiatowska, B. Polyhedron 1985, 4, 1847−1852). The first crystal structure of a Cr nitroso−isothiocyanato complex, (Ph4P)3[Cr(NO)(NCS)5]·2.4(CH3)2CO, has been determined.