Tris(triisopropylsilyl)silane (iPr 3 Si) 3 SiH has been synthesized and studied by X-ray and neutron diffraction. It possesses an unusual structure in which the four silicon atoms are nearly coplanar, ∠Si-Si-Si ) 118.41(5)°. The Si-H distance is found to have a normal value of 1.506(2) Å. Thermal and room-temperature photochemical decomposition of (iPr 3 -Si) 3 SiH leads to the elimination of iPr 3 SiH and the generation of bis(triisopropylsilyl)silylene, [(iPr 3 Si) 2 Si:]. Reactions of (iPr 3 Si) 2 Si: include precedented insertions into H-Si bonds and addition to the π-bonds of olefins, alkynes, and dienes. Despite theoretical predictions of a triplet ground state for [(iPr 3 Si) 2 Si:], stereospecific addition to cis-and trans-2-butene was observed.

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The reactions of (Cl)Ir(PR& (R = Me, Et) with potassium oxapentadienide, potassium I-methyl-5-oxapentadienide, and potassium 2,4-dimethyl-Boxapentadienide have been investigated. Treatment of (Cl)Ir(PMe& with potassium oxapentadienide initially produces ((1,2,5-+S-oxapentadienyl)Ir(PMe3)3 (l), which rapidly rearranges to ((1,2,3-q)-5-oxapentadienyl)-Ir(PMe3)s (2a). When refluxed in tetrahydrofuran, 2a undergoes metal-centered activation of the aldehydic C-H bond (C4-H) to produce the iridacyclopentenone complex fac-CH2-CH=CH-C(0)-Ir(PMe3)3(H) (3). Treatment of (Cl)Ir(PEt& with potassium oxa-I 1 pentadienide yields the iridapyran complex mer-CH=CH-CH=CH-O-Ii(PEt3)3(H) (4) via activation of a C-H bond on the C-terminus (Cl) of an O-bound 7'-oxapentadienyl ligand.Upon stirring in tetrahydrofuran solution, 4 gradually converts to the iridacyclopentenone complex fac-CH2-CH=CH-C (0)-Ir (PEt3)3( H) (5). Treatment of (Cl) Ir (PMe3)3 with potassium 4-methyl-5-oxapentadienide produces ((1,2,5-q)-4-methyl-5-oxapentadienyl)Ir(PMe3)3 (6). However, upon refluxing in tetrahydrofuran, this species undergoes C-H bond activation at C2 of the 4-methyl-5-oxapentadienyl ligand, generating the iridaoxacyclopentene derivative mer-compounds and anionic heteropentadienide reagents as our building blocks.Our initial studies have focused on electron-rich heteropentadienyl-iridium (1)-phosphine complexes because these species have a propensity to undergo C-H bond activation, generating novel metallacyclic products.4 In this paper, we describe the reactions of (Cl)Ir(PR& (R = Me and Et) precursors with potassium oxapentadienide, potassium 4-methyl-5-oxapentadienide, and potassium 2 , 4 dimethyl-5-oxapentadienide, which yield (oxapentadienyl)Ir(PR& complexes as kinetic products. However, these initially-formed species undergo intramolecular, iridium-centered C-H bond activation to produce fiveand six-membered iridacycles as the thermodynamic products. The site of C-H bond activation is strongly influenced by the location of the methyl substituents on the oxapentadienyl backbone; hence, these reactions can be directed toward a particular metallacyclic product by choosing the appropriate oxapentadienide reagent.Bleeke et al. Results and DiscussionA. Oxapemtadienide Reagents. While a variety of synthetic approaches to (oxapentadieny1)metal complexes have been explored over the years,5 anionic oxapentadienide reagents have not previously been employed to introduce oxapentadienyl ligands onto transition metal centers. These reagents promise to provide a new general route to (oxapentadieny1)metal complexes via nucleophilic displacement of anionic ligands from metal precursors.Potassium oxapentadienide, first reported by Heiazwolf and Kloosterzie16 in 1967, can be readily synthesized by deprotonating crotonaldehyde with potassium amide in liquid ammonia. Analogous treatment of 3-penten-2-one and mesityl oxide with potassium amide in liquid ammonia generates potassium 4-methyl-5-oxapentadienide and potassium 2,4-dimethyl-5-oxapentadienide, respective...