Reaction of H₂FeRu₃(CO)₁₃ with Alkynes. Preparation and Characterization of Isomeric FeRu₃(CO)₁₂(RC≡CR') Clusters. A Definitive Structural Characterization of the µ₄- η²- Bonding Mode for Substituted Alkynes

H₂FeRu₃(CO)₁₃ reacts with a series of alkynes to form isomeric FeRu₃(CO)₁₂(RC≡CR') clusters (R, R’ = Ph; R, R’ = Me; R = Ph, R’ = Me) which are conveniently separated by chromatography on silica gel. The reaction of H₂FeRu₃(CO)₁₃ with PhC≡CPh produces two isomers of FeRu₃(CO)₁₂(PhO≡CPh) in an overall yield of 65%. Reaction with MeO≡CMe and PhCsCMe produces two and three isomers, respectively, of FeRu₃(CO)₁₂(MeG≡CMe) and FeRu₃(CO)₁₂(PhC≡CMe) in 40-41% total yield in each case. The isomeric products have been fully characterized by elemental analysis and by their IR, ¹H NMR, and mass spectra, and the two isomers of FeRu₃(CO)₁₂(PhC≡CPh) by complete single-crystal X-ray diffraction studies. Each isomer crystallizes in the P2₁/c.C_2h⁵ space group with a = 9.526 (3) Å, b = 16.925 (5) Å, c = 17.037 (4) Å, β = 91.10 (2)°, and Z = 4 for isomer 1a and with a = 9.699 (1) Å, b = 16.785 (2) Å, c = 17.054 (2) Å, β = 90.60 (1)°, and Z = 4 for lb. The structures were refined to R₁ = 0.028 and R₂ = 0.032 for 4983 independent reflections having I > 3.0σ(I) for la and to R₁ = 0.026 and R₂ = 0.028 for 6901 independent reflections having I > 3.0σ(I) for 1b. In each isomer, the alkyne ligand has been incorporated into the cluster core to generate closo-FeRu₃C₂ cluster frameworks. The isomers differ only in the arrangement of the metal atoms with isomer 1a having the Fe atom cis to both alkyne carbons (axial isomer) whereas in isomer 1b, the Fe is cis to one carbon and trans to the other (equatorial isomer). Spectroscopic evidence indicates analogous structures for the other alkyne derivatives. The structural isomers interconvert at elevated temperatures, and for each alkyne the axial isomer is the more thermodynamically stable. However, the equatorial isomers form first during the syntheses of the compounds.