The saturated imidazolin-2-ylidene compounds RNC3H4NR, with R = mesityl (SIMes), 2,6-diisopropylphenyl, were synthesized by use of diamines that were prepared by the hydrogenation of the corresponding diimines catalyzed by RuHCl(R-binap)(R,R-dach)/KOtBu in toluene. The ligands IMes (MesNC3H2NMes) and SIMes react with RuHCl(PPh3)3 to give RuH(SIMes-H)(PPh3)2 (1) and RuH(IMes-H)(PPh3)2 (2), respectively, where there is cyclometalation of a C−H bond of an ortho Ar−CH3 group. The reaction of RuHCl(PPh3)3 with ItBu (tBuNC3H2NtBu) produces ItBu·HCl and a red solution that, upon reaction with H2, produces the dihydride Ru(H)2(ItBu)(PPh3)2 as a mixture of the two isomers 3a (trans PPh3 ligands) and 3b (cis PPh3). These isomers have an agostic C−H bond from a methyl group. There is an interesting windshield wiper exchange of coordinated tert-butyl groups occurring in isomer 3a, as monitored in solution by VT NMR with a free energy of activation of 11.8 kcal/mol. The reaction of 2 with CO at 20 °C produces (OC-34)-...