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)(OH(2))(5), [Cr(NO)(acac)(2)(OH(2))], Cr(NO)(CN)(5)(-), and Cr(NO)(NCS)(5)(-), have been performed, in comparison with the related Cr(III) complexes, Cr(OH(2))(6), [Cr(acac)(3)], Cr(CN)(6)(-), and Cr(NCS)(6)(-). 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 pi-back-bonding within the Cr-NO moiety. Multiple-scattering fitting of X-ray absorption fine structure (XAFS) spectra of [Cr(NO)(acac)(2)(OH(2))] supported the assignment of this complex as a trans-isomer (Keller, A.; Jezovska-Trzebiatowska, B. Polyhedron 1985, 4, 1847-1852). The first crystal structure of a Cr nitroso-isothiocyanato complex, (Ph(4)P)(3)[Cr(NO)(NCS)(5)].2.4(CH(3))(2)CO, has been determined.