Rhodium(I) tris(pyrazolyl)borate complexes TpR2Rh(cod) (R = Me, Ph, i-Pr) were found to serve as efficient catalysts for highly stereoregular polymerization of phenylacetylene derivatives (p-YC6H4C⋮CH; Y = H, Me, Cl, CN, CO2Me, COMe, NO2) to give poly(phenylacetylene) species having a head-to-tail, cis-transoidal structure. The catalytic activity was strongly affected by the substituents (R) at the 3- and 5-positions of the pyrazolyl groups, and the more sterically demanding R groups led to higher catalytic activity.

Reactions of the four alkenylruthenium(II) complexes Ru[C(R1)CH(R2)]Cl(CO)(PPh3)2 (R1 = H, R2 = Ph (1b); R1 = H, R2 = t-Bu (1c); R1 = Ph, R2 = Ph (1d); R1 = CHCH(SiMe3), R2 = SiMe2Ph (1e)) with HSiMe2Ph, which constitute the product-forming step of ruthenium-catalyzed hydrosilylation of alkynes, have been examined. Two reaction courses are operative:  one provides the C−Si coupling product PhMe2SiC(R1)CH(R2) and RuHCl(CO)(PPh3)3 (path A), and the other forms the C−H coupling product HC(R1)CH(R2) and Ru(SiMe2Ph)Cl(CO)(PPh3)2 (path B). The ratio of the two courses significantly varies with substituents on the alkenyl ligands, particularly with the α-substituent (R1). Thus, 1b and 1c, without an α-substituent, react mainly by path A. In contrast, 1d and 1e, bearing an α-substituent, exclusively undergo path B. Kinetic studies using 1b and its para-substituted styryl ligand derivatives have revealed that path A proceeds by direct interaction of the five-coordinated complexes with hydrosilane, without dissociation of the PPh3 ligand. On the other hand, path B involves dissociation of PPh3 prior to the reaction of 1d or 1e with hydrosilane. Mechanisms of the C−Si and C−H bond formation are discussed with kinetic data in detail.

Catalytic hydrosilation of 1-(trimethylsilyl)-1-buten-3-yne (1) with three kinds of hydrosilanes (HSiMePh2, HSiMe2Ph, and HSiEt3) in CDCl3 at 30 °C in the presence of a catalytic amount of RuHCl(CO)(PPh3)3 (2) gave five types of reaction products:  (1E,3E)-CH(SiR3)CHCHCHSiMe3 (3), R3SiCH2CHCHCH2SiMe3 (4), R3SiCHCCHCH2SiMe3 (5), (1Z,3E)-CH(SiR3)CHCHCHSiMe3 (6), and R3SiC⋮CCHCHSiMe3 (7). Detailed investigations on the stoichiometric reactions of intermediate ruthenium species provided definitive evidence for the catalytic mechanism comprised of two catalytic cycles, the Chalk−Harrod cycle A and the modified Chalk−Harrod cycle C, and their interconnecting processes B and D. Product 3 is formed by the insertion of 1 into the Ru−H bond of 2 followed by the reaction of the resulting terminal dienyl complex Ru(CHCHCHCHSiMe3)Cl(CO)(PPh3)2 (8) with hydrosilane. The latter process regenerates 2 and the sequence of reactions proceeds catalytically (cycle A). The reaction of 8 with hydrosilane is accompanied by a side reaction giving Ru(SiR3)Cl(CO)(PPh3)2 (9) and CH2CHCHCHSiMe3 (10), and the latter is further converted to 4 by hydrosilation (process B). Silyl complex 9 thus generated in the system is the key intermediate for catalytic cycle C. Thus the insertion of 1 into the Ru−SiR3 bond of 9 via a formal trans-addition process forms an internal dienylruthenium complex Ru[C(CHSiR3)CHCHSiMe3]Cl(CO)(PPh3)2 (11), which reacts with hydrosilane to give 5 and 6 and to regenerate 9. A part of 11 also undergoes β-hydrogen elimination to give a dehydrogenative silation product 8 and hydride complex 2. Complex 2 thus formed resumes catalytic cycle A (process D). The catalytic intermediates 8, 9, and 11 were identified by NMR spectroscopy and/or elemental analysis. Factors controlling the catalytic cycles are discussed on the basis of the experimental observations.

Alkenylruthenium complexes, Ru{C(R1)=CH(R2)}Cl(CO)(PPh3)2 (R1 = H, R2 = Ph; R1 = H, R2 = t-Bu; R1 = Ph, R2 = Ph; R1 = CH=CH(SiMe3), R2 = SiMe2Ph), react with HSiMe2Ph via two reaction courses (path A and path B), leading to C–Si and C–H bond formation, respectively. Relative ratio of the two courses is strongly dependent upon steric bulkiness of substituent(s) on the alkenyl ligands.