Spectroelectrochemistry and DFT Analysis of a New {RuNO}<i><sup>n</sup></i><sup></sup> Redox System with Multifrequency EPR Suggesting Conformational Isomerism in the {RuNO}<sup>7</sup> State SinghPriti FiedlerJan ZálišStanislav DubocCarole NiemeyerMark LissnerFalk SchleidThomas KaimWolfgang 2007 The compound [Ru(NO)(bpym)(terpy)](PF<sub>6</sub>)<sub>3</sub>, bpym = 2,2‘-bipyrimidine and terpy = 2,2‘:6‘,2‘ ‘-terpyridine, with a {RuNO}<sup>6</sup> configuration (angle Ru−N−O 175.2(4)°) was obtained from the structurally characterized precursor [Ru(NO<sub>2</sub>)(bpym)(terpy)](PF<sub>6</sub>), which shows bpym-centered reduction and metal-centered oxidation, as evident from EPR spectroscopy. The relatively labile [Ru(NO)(bpym)(terpy)]<sup>3+</sup>, which forms a structurally characterized acetonitrile substitution product [Ru(CH<sub>3</sub>CN)(bpym)(terpy)](PF<sub>6</sub>)<sub>2</sub> upon treatment with CH<sub>3</sub>OH/CH<sub>3</sub>CN, is electrochemically reduced in three one-electron steps of which the third, leading to neutral [Ru(NO)(bpym)(terpy)], involves electrode adsorption. The first-two reduction processes cause shifts of ν(NO) from 1957 via 1665 to 1388 cm<sup>-1</sup>, implying a predominantly NO-centered electron addition. UV−vis-NIR Spectroscopy shows long-wavelength ligand-to-ligand charge transfer absorptions for [Ru<sup>II</sup>(NO<sup>-I</sup>)(bpym)(terpy)]<sup>+</sup> in the visible region, whereas the paramagnetic intermediate [Ru(NO)(bpym)(terpy)]<sup>2+</sup> exhibits no distinct absorption maximum above 309 nm. EPR spectroscopy of the latter at 9.5, 95, and 190 GHz shows the typical invariant pattern of the {RuNO}<sup>7</sup> configuration; however, the high-frequency measurements at 4 and 10 K reveal a splitting of the <i>g</i><sub>1</sub> and <i>g</i><sub>2</sub> components, which is tentatively attributed to conformers resulting from the bending of RuNO. DFT calculations support the assignments of oxidation states and the general interpretation of the electronic structure.