Linear vs. Tilted / Bent Nitrosyl Paradox: an EXAFS and XANES study to determine the nature of Spin Isomers

FLORENCIA DI SALVO; JOSE MARTIN RAMALLO LOPEZ; FÉLIX GREGORIO REQUEJO; FABIO DOCTOROVICH

Campinas, Brasil

Workshop; 18ª Reunião Anual de Usuários LNLS; 2008

Laboratorio Nacional de Luz Sincrotron

A linear vs. tilted/bent metal-nitrosyl isomerism is observed for pentachloronitrosyliridate(III) ([IrCl5(NO)]?) by EXAFS. While its Na+ and K+ salts comprise octahedral anions with linear Ir?NO moieties, the crystal structure of PPh4[IrCl5(NO)] shows a nitrosyl with an off-axis tilting. A multiple scattering EXAFS fitting was used to determine the alignement of the Ir-N-O atoms. The pentachloronitrosyliridate(III) anions are spatially distributed in a curious arrangement in which one is over the other forming an infinite ?cable?, with a short distance between the N-O moiety of one anion and the trans-chloride of the nearby upper one. In addition XANES data shows that the oxidation state for PPh4[IrCl5(NO)] is notably higher than that for K[Ir(III)Cl5(NO)]. This results as well as DFT calculations suggest an unprecedented spin isomerism for this third row transition metal complex, IrIII?NO+ ↔ IrIV?NO●, in contrast to the Na+ and K+ salts. The spin isomerism could be seen as an electron promotion from a metal-chloride orbital to a metal-NO one due to the small HOMO-LUMO gap in PPh4[IrCl5(NO)]. This is probably induced by electrostatic interactions acting as a result of the closeness and ?cable-like? spatial arrangement of the Ir metal centers, imposed by lattice forces due to π?π stacking interactions among the phenyl rings.