Reaction of SiH(Me)(CH2CH2CH2PPh2)2, “biPSiH”, with Ru(PPh3)2(CO)3 affords quantitatively the phosphinoalkylsilyl complex RuH(biPSi)(CO)2, as a mixture of the crystallographically characterized syn diastereomer 3-s and its anti correspondent 3-a; subsequent treatment with CCl4 then LiAl2H4 provides access to the deuterioisotopomer Ru2H(biPSi)(CO)2, 3-d1. Slow stereomutation of pure 3-s to an equilibrium mixture 3-s/3-a (4.5:1, 295 K) occurs with ΔG⧧295, 102(5) kJ mol-1; ΔH⧧295, 113(7) kJ mol-1; ΔS⧧295, 37(2) J K-1 mol-1; this is not accompanied by incorporation of external 13CO, shows only a minor isotope effect vs 3-d1, and is concluded to be nondissociative.