Five new complexes [RuCl2(SIMes)(Ind)(O-pXC5H4)] bearing different para-substituted triphenylphosphites (X = H, OCH3, CF3, Cl, SF5 and CN) were synthesised and used to study the effect of the electronic properties of the phosphite on olefin metathesis activity. Investigations of the physical properties of the new ligands and complexes were performed using physicochemical and DFT calculations. The catalytic activity of the complexes was benchmarked in challenging ring closing metathesis transformations featuring the formation of tetra-substituted double bonds. Complex [RuCl2(SIMes)(Ind)P(O-pCF3C5H4)3] (3c) exhibited a particularly high catalytic activity, superior to state-of-the-art catalysts, and was further tested on a wide range of substrates.