Such an obtained catalyst in a solid form can be stored under an argon atmosphere for almost an undefined period of time; however, we noted that in air ca. 5% of solid Ru3 is decomposed over 24 h (see SI for details). To evaluate Ru3 stability in a solution, a quantitative NMR thermal stability study has been performed in THF-d 8 (C = 1.0 mM) at 60 °C under argon. As a result of this measurement, it was found that in a solution Ru3 exhibits slightly more pronounced thermal stability as compared to Ru2 (Scheme 2B, see SI for details). Crystals for XRD measurement were obtained by placing a DCM solution of the Ru3 complex in an NMR tube, then layering n-hexane and allowing the solvents to slowly diffuse. Two molecules of the catalyst were found in the asymmetric cell unit. The measurement showed similar values of the most important geometric parameters as compared with those from the reported XRD of a thiocatecholate-complex. The distance between the NHC carbon and Ru in Ru3 was 2.077 Å and 2.107 Å (two molecules in the asymmetric cell unit). The distances between sulphur atoms and ruthenium were slightly shorter and amounted to 2.3190 Å and 2.3199 Å for the Ru-S Tests in reactive distillation conditions. Having a new complex in hand, we attempted to test it in the high-concentration Z-selective mRCM reaction at high temperature. To do so, we used the same model diene 1 and conditions as previously (PAO-6, 200 mM, vacuum, 110 °C, 8 h; see Scheme 1A). Surprisingly, Ru3 performed in the title reactive distillation outstandingly well, giving the musk product 2 in 84% isolated yield and-this time-with very good Z-selectivity (93:7