The Metal-NO Interaction in the Redox Systems Cl5Os(NO)n-, n = 1-3, and cis-(bpy)2ClOs(NO)2+/+: Calculations, Structural, Electrochemical, and Spectroscopic Results

, , , , , , und . Inorganic Chemistry 45 (12): 4602--4609 (2006)


Exptl. and computational results for the 2-step redox system Cl5Os(NO)n- (n = 1-3) are reported and discussed in comparison to the related 1-step redox systems Cl5Ru(NO)n- and Cl5Ir(NO)n- (n = 1, 2). The Os system exhibits remarkably low oxidn. and redn. potentials. The structure of the precursor (PPh4)2Cl5Os(NO) is established as an MNO6 species with almost linear OsNO arrangement at 178.1°. DFT calcns. confirm this result, and a comparison of structures calcd. for several oxidn. states reveals an increased labilization of the trans-positioned M-Cl bond on redn. in the order M = Ir \textless Os \textless Ru. Accordingly, the intact reduced form Cl5Os(NO)3- could not be obsd. in fluid soln. even on electrolysis at -70° in n-butyronitrile soln., as confirmed both by DFT calcns. and by comparison with the ESR and IR spectroelectrochem. characterized redox pairs cis-(bpy)2ClOs(NO)2+/+ and (CN)5Os(NO)2-/3-. The DFT calcns. indicate that the oxidn. of Cl5Os(NO)2- occurs largely on the metal, the HOMO of the precursor being composed of Os 5d (58\%) and Cleq 3p orbitals (41\%). As for the related (CN)5Os(NO)2-, the redn. is largely NO centered, the LUMO of Cl5Os(NO)2- has 61\% p*(NO) character with significant 5d Os contributions (34\%). A rather large degree of metal-NO back-donation occurs in the OsNO7vvvvvvv configuration of Cl5Os(NO)3- which leads to an unusual low value of 1513 cm-1 calcd. for n(NO), signifying contributions from an OsIII(NO-) formulation. Detailed analyses of the conformational dependence of the g anisotropy suggest that the different reduced species reported previously for Cl5Os(NO)3- in AgCl host lattices may be distinct in terms of eclipsed or staggered conformations of the bent NO.bul. axial ligand relative to the OsIICl4 equatorial plane. The staggered form is more stable by 105 cm-1. The weak absorptions of Cl5Os(NO)2- at 573, 495, and 437 nm are assigned as MLCT/LLCT transitions to the doubly degenerate p*(NO) LUMO. The oxidized form Cl5Os(NO)- contains OsIII in an OsNO5 configuration with a spin d. of 0.711 on Os. In all 3 states of Cl5Os(NO)n-, the N bonded form is vastly preferred over the NO-side-on bonded alternative. on SciFinder(R)

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