The rational optimizationo fh omogeneous catalysts requires ligand platforms that are easily tailoredt oi mprove catalytic performance. Here, it is demonstrated that pyridylidenea mides (PYAs) provide such ap latform to custom-shapet ransfer hydrogenation catalysts with exceptional activity.S pecifically,aseries of meta-PYAp incer ligands with differently substituted PYAu nits has been synthezised andc oordinated to ruthenium(II) centres to form bench-stable tris-acetonitrile complexes [Ru(R-PYA-pincer)-(MeCN) 3 ](PF 6) 2 (R = OMe, Me, H, Cl, CF 3). Analytic studies including 1 HNMR spectroscopy,c yclic voltammetry,a nd X-ray crystallographyr eveal ad irecti nfluence of the substituents on the electronic properties of the ruthenium center. The complexes are active in the catalytic transfer hydrogenation of ketones,w ith activities directly encoded by the PYAs ubstitutionp attern. Their perfomance improves further upon exchange of an ancillary MeCN ligand with PPh 3 .W hile complexes [Ru(R-PYA-pincer)(PPh 3)(MeCN) 2 ](PF 6) 2 were only isolated for R = H, Me, an in situ protocolw as developed to generate these complexes in situ for R = OMe, Cl, CF 3 by using a1:2 ratio of the complexesa nd PPh 3 .T his in situ protocol together with as hort catalystp re-activation provided highly active catalytic systems. The most active pre-catalyst featured the methoxy-substituted PYAl igand and reached turnover frenquencies of 210 000 h À1 under an exceptionally low catalyst loading of 25 ppm for the benchmark substrate benzophenone, representing one of the mosta ctive transfer hydrogenation systems known to date. Figure 1. Selected ligands that feature ambiguous donor properties.