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.

Experimental and computational results for the two-step redox system [Cl5Os(NO)]n- (n = 1-3) are reported and discussed in comparison to the related one-step redox systems [Cl5Ru(NO)]n- and [Cl5Ir(NO)]n- (n = 1, 2). The osmium system exhibits remarkably low oxidation and reduction potentials. The structure of the precursor (PPh4)2[Cl5Os(NO)] is established as an {MNO}6 species with almost linear OsNO arrangement at 178.1 degrees. Density-functional theory (DFT) calculations confirm this result, and a comparison of structures calculated for several oxidation states reveals an increased labilization of the trans-positioned M-Cl bond on reduction in the order M = Ir < Os < Ru. Accordingly, the intact reduced form [Cl5Os(NO)]3- could not be observed in fluid solution even on electrolysis at -70 degrees C in n-butyronitrile solution, as confirmed both by DFT calculations and by comparison with the electron paramagnetic resonance and infrared spectroelectrochemically characterized redox pairs cis-[(bpy)2ClOs(NO)]2+/+ and [(CN)5Os(NO)]2-/3-. The DFT calculations indicate that the oxidation of [Cl5Os(NO)]2- occurs largely on the metal, the highest occupied molecular orbital (HOMO) of the precursor being composed of Os 5d (58%) and Cl(eq) 3p orbitals (41%). As for the related [(CN)5Os(NO)]2-, the reduction is largely NO centered, the lowest unoccupied molecular orbital (LUMO) of [Cl5Os(NO)]2- has 61% pi*(NO) character with significant 5d Os contributions (34%). A rather large degree of metal-NO back-donation is estimated to occur in the {OsNO}7 configuration of [Cl5Os(NO)]3- which leads to an unusual low value of 1513 cm(-1) calculated for nu(NO), signifying contributions from an Os(III)(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. axial ligand relative to the Os(II)Cl4 equatorial plane. The staggered form is calculated to be 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 pi*(NO) LUMO. The oxidized form [Cl5Os(NO)]- contains Os(III) in an {OsNO}5 configuration with a spin density of 0.711 on Os. In all three states of [Cl5Os(NO)]n-, the N bonded form is vastly preferred over the NO-side-on bonded alternative.