One of the most important breakthroughs in this field was the introduction of dithiolate ligands instead of chloride ones; the resulting catalysts exhibited high stereoretentivity, maintaining double bond configuration present in a substrate in a resulted product. Replacement of two hydrogen atoms at positions 3 and 6 of the dithiocatecholate ligand with two chlorides (Ru4 → Ru5 and Ru7, Figure ) reduced the electron density on the sulphur atoms limiting the formation of a catalytically inactive complex (typically formed upon attack of the electron-rich sulphide on the methylidene ruthenium complex) and increasing substrate conversion by about 20 percentage points (for recent reviews on Ru dithiolate catalysts, see refs. ). Despite significant advances that increased the activity and stability of stereoretentive complexes, they remained relatively unstable, especially in the presence of oxygen, and their use required considerable dexterity from the experimentalist and often the use of a glovebox. Further modifications of these systems using sterically hindered dithiolate ligands based on naphthalene and phenanthrene did not constitute a significant step forward in the search for more user-friendly catalysts. An interesting attempt to improve their catalytic properties was provided by the introduction of a phenyl substituent in the benzylidene moiety; unfortunately, the resulting complexes (Ru6 and Ru8, Figure ), despite their excellent activity and high selectivity, turned out to be even less stable than their parental complexes Ru5 and Ru7. On the other hand, replacing the dithiocatechol ligand with squaric acid analogue (Ru9) significantly increased the stability of the resulting catalyst (it decomposed completely in 30 days in comparison to 18 days for Ru5), unfortunately at the expense of reduced activity. An alternative strategy to overcome an issue of low stability of stereoretentive complexes, including the necessity to use a glovebox, involved the in situ generation of the catalyst and subsequent addition of this freshly prepared mixture to the substrate(s) solution.