J. L. Burmeister, Coord. Chem. Rev, vol.3, p.225, 1968.

P. Coppens, I. Novozhilova, and A. Y. Kovalevsky, Chem. Rev, p.861, 2002.

J. H. Enemark and R. D. Feltham, Coord. Chem. Rev, p.339, 1974.

M. J. Rose and P. K. Mascharak, Coord. Chem. Rev, 2008.

H. Giglmeier, T. Kerscher, P. Klüfers, D. Schaniel, and T. Woike, Dalton Trans, vol.42, p.9113, 2009.

D. Schaniel, M. Imlau, T. Weisemoeller, T. Woike, K. W. Krämer et al., Adv. Mater, p.723, 2007.

M. Goulkov, D. Schaniel, and T. Woike, J. Opt. Soc. Am. B, p.927, 2010.

D. Schaniel and T. Woike, Phys. Chem. Chem. Phys, p.4391, 2009.

T. Ishikawa and K. Tanaka, Z. Kristallogr, p.334, 2008.

N. Casaretto, S. Pillet, E. Bendeif, D. Schaniel, A. K. Gallien et al., IUCrJ, issue.2, p.35, 2015.

N. Casaretto, S. Pillet, E. Bendeif, D. Schaniel, A. K. Gallien et al., Acta Cryst, p.788, 2015.

N. Casaretto, B. Fournier, S. Pillet, E. Bendeif, D. Schaniel et al., CrystEngComm, vol.18, p.7260, 2016.

A. K. Gallien, D. Schaniel, T. Woike, and P. Klüfers, Dalton Trans, p.13278, 2014.

S. Grimme, J. Antony, S. Ehrlich, and H. Krieg, J. Chem. Phys, vol.132, pp.154104-154119, 2010.

F. Neese, The ORCA program system, Wiley Interdiscip. Rev.: Comput. Mol. Sci, vol.2, pp.73-78, 2012.

. Rigaku-oxford-diffraction, CrysAlis CCD and CrysAlis RED (Versions, Rigaku Oxford Diffraction, 2017.

G. M. Sheldrick, Acta Cryst. C, vol.71, p.3, 2015.

D. V. Fomitchev and P. Coppens, Inorg. Chem, p.7021, 1996.

B. Cormary, I. Malfant, M. Buron-le-cointe, L. Toupet, B. Delley et al., Acta Cryst, vol.65, p.612, 2009.

G. A. Kostin, A. O. Borodin, A. A. Mikhailov, N. V. Kuratieva, B. A. Kolesov et al., Eur. J. Inorg. Chem, p.4905, 2015.

D. Schaniel, T. Woike, B. Delley, D. Biner, K. W. Krämer et al., Phys. Chem. Chem. Phys, vol.9, p.5149, 2007.

D. Schaniel, T. Woike, N. Behrnd, J. Hauser, K. W. Krämer et al., The agreement statistics is close to the values obtained for the ground state refinement, the N1B-O1B bond distance is shorter than for the ground state, and the ADPs for N1B and O1B are consistent with a lower value for the O1B atom attached to Ru, and a larger value for the terminal N1B atom. -in the hypothesis (2), corresponding to MS1a of DFT, N2B-O2B is in a isonitrosyl configuration. The refinement agreement statistics are almost identical to hypothesis (1), but the refinement yields a very small population of the MS with a bent configuration of Ru-O2B-N2B, indicating rather a compensation for some residual density around the molecule than a PLI. This corresponds to more or less a ground state configuration, which we can therefore exclude as valuable hypothesis for PLI-2. -in the hypothesis (3), corresponding to MS11 of DFT, both NO groups are considered as isonitrosyl configurations in the PLI-2 state. The refinement agreement statistics are higher than for hypothesis (1). The resulting ADPs for N2 and O2 are not consistent. As a matter of fact, the O2 atom being attached to the Ru atom leads to higher ADPs compared to the terminal N2 atom. The reverse situation would be expected (higher ADPs for the terminal atom), 11399. the hypothesis (1), corresponding to MS1 of DFT, N1B-O1B is in an isonitrosyl configuration (Ru-O1B-N1B), vol.48, 2009.

, As a conclusion, we can firmly assert that the PLI-2 state corresponds to N1-O1 in an isonitrosyl configuration, and N2-O2 in a nitrosyl configuration: it consists of a single linkage isomerism of N1-O1 solely. Accordingly, the results of hypothesis (1) are discussed in the manuscript