Experimental
and computational results for the two-step redox system [Cl
5Os(NO)]
n- (
n = 1-3) are reported
anddiscussed in comparison to the related one-step redox systems [Cl
5Ru(NO)]
n- and [Cl
5Ir(NO)]
n- (
n = 1, 2). Theosmium system exhibits remarkably low oxidation
and reduction potentials. The structure of the precursor (PPh
4)
2[Cl
5Os(NO)] is established as an {MNO}
6 species with almost linear OsNO arrangement at 178.1
. Density-functionaltheory (DFT) calculations confirm this result,
and a comparison of structures calculated for several oxidation statesreveals an increased labili
zation of the trans-positioned M-Cl bond on reduction in the order M = Ir < Os < Ru.Accordingly, the intact reduced form [Cl
5Os(NO)]
3- could not be observed in fluid solution even on electrolysis at-70
C in
n-butyronitrile solution, as confirmed both by DFT calculations
and by comparison with the electronparamagnetic resonance
and infrared spectroelectrochemically characteri
zed redox pairs
cis-[(bpy)
2ClOs(NO)]
2+/+and [(CN)
5Os(NO)]
2-/3-. The DFT calculations indicate that the oxidation of [Cl
5Os(NO)]
2- occurs largely on themetal, the highest occupied molecular orbital (HOMO) of the precursor being composed of Os 5d (58%)
and Cl
eq3p orbitals (41%). As for the related [(CN)
5Os(NO)]
2-, the reduction is largely NO centered, the lowest unoccupiedmolecular orbital (LUMO) of [Cl
5Os(NO)]
2- has 61%
*(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 [Cl
5Os(NO)]
3-which leads to an unusual low value of 1513 cm
-1 calculated for
(NO), signifying contributions from an Os
III(NO
-)formulation. Detailed analyses of the conformational dependence of the
g anisotropy suggest that the differentreduced species reported previously for [Cl
5Os(NO)]
3- in AgCl host lattices may be distinct in terms of eclipsed orstaggered conformations of the bent NO
axial lig
and relative to the Os
IICl
4 equatorial plane. The staggered formis calculated to be more stable by 105 cm
-1. The weak absorptions of [Cl
5Os(NO)]
2- at 573, 495,
and 437 nm areassigned as MLCT/LLCT transitions to the doubly degenerate
*(NO) LUMO. The oxidi
zed form [Cl
5Os(NO)]
-contains Os
III in an {OsNO}
5 configuration with a spin density of 0.711 on Os. In all three states of [Cl
5Os(NO)]
n-,the N bonded form is vastly preferred over the NO-side-on bonded alternative.