Substitutional Isomerism for Trimetallic Clusters Having Icosahedral Ligand Arrangements. The Crystal Structures of (μ-H)Ru₃[μ-CN(CH₃)₂](CO)₉(C₅H₅N) and (μ-H)Ru₃[μ-CN(CH₂C₆H₅)₂](CO)₉(PPh₃)

The crystal structures of two structural isomers (μ-H)ru₃(μ-Cnr₂)(co)₉L (R = Me, L = py; R = Bz, L = PPh₃) have been determined.¹ The complex μ-H)Ru₃(μ-CNMe₂)(CO)₉(py) crystallizes in the centrosymmetric triclinic space group P1̅ (No. 2) with a = 8.0650 (11) Å, b = 9.2237 (13) Å, c = 16.9384 (29) Å, α = 94.274 (19)°, β = 100.837 (13)°, γ = 106.725 (13)°, V = 1174.1 (3) ų, and Z = 2. Diffraction data (Mo Ka, 26 = 5-50°) were collected on a Syntex P2₁ diffractometer, and the structure was refined to R_F= 4.1% for all 3925 data (R_F= 2.4% for those 2649 reflections with |F_o| > 6σ(|F_o|)). The dibridged Ru-Ru bond is 2.802 (1) Å in length, and the nonbridged Ru-Ru distances are 2.855 (1) and 2.844 (1) Å. The py ligand occupies a “semiaxial” site on one of the dibridged ruthenium atoms (Ru(1)-N(1) = 2.250 (5) Å) and causes some asymmetry in the trans μ-CNMe ligand. The species (μ-H)ru₃(μ-Cnbz₂)(co)9(pph₃) crystallizes in the noncentrosymmetric orthorhombic space group Pna2₁(No. 33) with a = 17.879 (5) Å, b = 13.735 (4) Å, c = 17.122 (5) Å, V = 4204 (2) ų, and Z = 4. Diffraction data (Mo Kα, 2θ = 4.5-45°) were collected, and the structure was refined to R_F = 5.9% for all 5311 data (R_F= 4.2% for those 4128 reflections with |F_o| > 6σ(|F_o|). The dibridged Ru-Ru bond is 2.792 (1) A in length, and the bulky PPh₃ ligand occupies an equatorial site on the nonbridged ruthenium atom (Ru(2)-P = 2.384 (2) Å). The Ru-Ru bond cis to the PPh₃ ligand (Ru(2)-Ru(3) = 2.885 (1) Å) is lengthened relative to that trans to the PPh₃ ligand (Ru(1)-Ru(2) = 2.844 (1) Å). These structures bring the number of known isomers of (μ-H)M₃-(μ-Cx)(co)₉L to four. Steric effects upon the thermodynamic stabilities of these isomers are examined.