The preparation, characterization, and properties of new ruthenium silyl and diphenylsilylene derivatives of Cp*(PMe3)2Ru (Cp* = η5-C5Me5) are described. The silyl complexes Cp*(PMe3)2RuSiR3 (SiR3 = SiEt3 (1), SiPh3 (2), SiPh2Me (3), SiPh2Cl (4)) are obtained directly by reaction of Cp*(PMe3)2RuCH2SiMe3 with the appropriate silane HSiR3 at 80-100 °C. Other conditions lead to loss of 1 equiv of PMe3 to afford the ruthenium(IV) bis(silyl) species Cp*(PMe3)Ru(SiR3)2H (SiR3 = Si(OEt)3 (5), SiPh2Cl (6), SiMe2OEt (7)). The nucleophilic reagents MeMgCl and LiAlH4 displace chloride from 4 to afford 3 and Cp*(PMe3)2RuSiPh2H (8), respectively. The triflate derivative Cp*(PMe3)2RuSiPh2(OTf) (9, OTf = OSO2CF3) is obtained in high yield from reaction of 4 and Me3SiOTf in dichloromethane. Solubility properties and solid-state infrared spectroscopy indicate that 9 possesses a covalent silicon-triflate interaction. Compound 9 exhibits a triplet resonance in the 29Si NMR spectrum at δ 112.39 (2JPSi = 33 Hz). In acetonitrile solution 9 exists mainly as the ionic silylene adduct [Cp*(PMe3)2RuSiPh2(NCMe)]OTf, as determined by 1H NMR spectroscopy, solution infrared spectroscopy, and conductivity measurements. Addition of NaBPh4 to an acetonitrile solution of 9 allows isolation of the silylene adduct [Cp*-(PMe3)2RuSiPh2(NCMe)]BPh4 (10) as a dichloromethane solvate. This procedure also produces significant quantities of [Cp*(PMe3)2Ru(NCMe)]BPh4 (11) as byproduct. Compound 11 was prepared independently by reaction of Cp*(PMe3)2RuCl and AgBPh4 in acetonitrile. Compound 10 exhibits a single broad resonance in its 29Si NMR spectrum at δ 95.75. The thermal stability of 10 in solution and in the solid state is described. 10 reacts with water rapidly to give 11 and the disiloxane Ph2HSiOSiHPh2. It also reacts with LiAlH4 to afford 8 in 37% yield. The acetonitrile of 10 is quite labile and exchanges rapidly with free acetonitrile in solution. A line-shape analysis of variable-temperature 1H NMR spectra of 10 in the presence of 1 equiv of NCMe afforded activation parameters for this exchange process of ΔH? = 14.5 ± 0.3 kcal mol-1 and ΔS? = 14 ± 2 eu. This positive value for the entropy of activation is consistent with a dissociative mechanism and provides evidence for existence of the base-free silylene complex Cp*(PMe3)2Ru=SiPh2+ in solution. Structures of complexes 8, 9, and 10·CH2Cl2 have been determined by single-crystal X-ray diffraction experiments. Crystals of 8 are triclinic, Pl?, with a = 9 362 (1) A?, b = 9.748 (2) A?, c = 16.376 (3) A?, α = 98.18 (2)°, β = 103.71 (2)°, γ = 94.14 (2)°, V = 1428.4 (6) A?3, Z = 2, RF = 2.59%, and RwF = 2.98%. Crystals of 9 are triclinic, Pl?, with a = 10.171 (3) A?, b = 10.802 (3) A?, c = 16.880 (5) A?, α = 86.82 (2)°, β = 88.03 (2)°, γ = 64.00 (1)°, V = 1664.2 (7) A?3, Z = 2, RF = 4.39%, and RwF = 5.61%. Crystals of 10·CH2Cl2 are monoclinic, P21/n, with a = 21.565 (8) A?, b = 10.215 (4) A?, c = 25.59 (1) A?, β = 108.40 (3)°, V = 5349 (3) A?3, Z = 4, RF = 6.32%, and RwF = 6.37%. The Ru-Si distances for these complexes are 2.387 (1) (8), 2.349 (2) (9), and 2.328 (2) A? (10·CH2Cl2). Whereas 8 and 9 exhibit similar staggered conformations with the Cp* and X (Cp*(PMe3)2RuSiPh2X, X = H or OTf) groups in a gauche relationship, 10·CH2Cl2 has a conformation that is skewed toward an eclipsed arrangement of substituents (Cp* centroid-Ru-Si-N dihedral angle = 38.0°). The Si-O distance in 9 is rather long, at 1.853 (5) A?. The Si-N distance in 10·CH2Cl2, 1.932 (8) A?, is consistent with a dative interaction between nitrogen and silicon. Other geometrical parameters for these complexes are compared and discussed.