The reactivity of ylide-like phosphasilene 1 [LSi(TMS)═P(TMS), L = PhC(NtBu)2] with group 10 d(10) transition metals is reported. For the first time, a reaction of a phosphasilene with a transition metal that actually involves the silicon-phosphorus double bond was found. In the reaction of 1 with ethylene bis(triphenylphosphine) platinum(0), a complete silicon-phosphorus bond breakage occurs, yielding the unprecedented dinuclear platinum complex 3 [LSi{Pt(PPh3)}2P(TMS)2]. Spectroscopic, structural, and theoretical analysis of complex 3 revealed the cationic silylene (silyliumylidene) character of the silicon unit in complex 3. Similarly, formation of the analogous dinuclear palladium complex 4 [LSi{Pd(PPh3)}2P(TMS)2] from tetrakis(triphenylphosphine) palladium(0) was observed. On the other hand, in the case of bis(cyclooctadiene) nickel(0) as starting material, a distinctively different product, the bis(silylene) nickel complex 5 [{(LSi)2P(TMS)}Ni(COD)], was obtained. Complex 5 was fully characterized including X-ray diffraction analysis. Density functional theory calculations of the reaction mechanisms showed that the migration of the TMS group in the case of platinum and palladium was induced by the oxidative addition of the transition metal into the silicon-silicon bond. The respective platinum intermediate 2 [LSi{Pt(TMS)(PPh3)}P(TMS)] was also experimentally observed. This is contrasted by the reaction of nickel, in which the equilibrium of phosphasilene 1 and the phosphinosilylene 6 [LSiP(TMS)2] was utilized for a better coordination of the silicon(II) moiety in comparison with phosphorus to the transition metal center.

Heavier alkene analogues possess unique electronic properties and reactivity,e ncouraging multidisciplinary research groups to utilize them in the rational designo fn ovel classes of compounds and materials.P hosphasilenes are heavier imine analogues,c ontaining highly reactiveS i =P double bonds. Recent achievements in this field are closely related to the progress in the chemistry of stable low-coordinate silicon compounds. In this Review,w eh avea ttempted to summarize in ac omprehensive way the availabled ata on the structures, syntheses, electronic andc hemical properties of these compounds, with an emphasis on recent achievements.

An in-depth study of the synthesis and structures of N-heterocyclic carbene (NHC)-stabilized silylene transition-metal complexes is reported. An iron hydrosilylene complex, [tBuSi(NHC)(H)Si:→Fe(CO)] (2), was synthesized starting from the corresponding hydrosilylene [tBuSi(NHC)(H)Si:] (1). Complex 2 was fully characterized, including X-ray diffraction analysis, which showed an unusual long Si-Fe bond length. A very long bond length was also observed in the novel hydrosilylene tungsten complex [tBuSi(NHC)(H)Si:→W(CO)] (3). A series of NHC-stabilized silylene iron complexes ([R(NHC)Si:→Fe(CO)], where R = Cl (4), H (5), and Me (6)) were synthesized and fully characterized to investigate the influence of different substituents. The dihydrosilylene iron complex [H(NHC)Si:→Fe(CO)] (5) represents a new example of a donor-acceptor-stabilized parent silylene (HSi:). Density functional theory calculations were utilized to understand the influence of the electronic and steric effects of the silylene unit and its substituents on the Si-Fe bond in these iron complexes, in particular to rationalize the long Si-Fe bond in 2.

An in depth study of the reactivity of an N-heterocyclic carbene (NHC)-stabilized silylene monohydride with alkynes is reported. The reaction of silylene monohydride 1, tBu3 Si(H)Si←NHC, with diphenylacetylene afforded silole 2, tBu3 Si(H)Si(C4 Ph4 ). The density functional theory (DFT) calculations for the reaction mechanism of the [2+2+1] cycloaddition revealed that the NHC played a major part stabilizing zwitterionic transition states and intermediates to assist the cyclization pathway. A significantly different outcome was observed, when silylene monohydride 1 was treated with phenylacetylene, which gave rise to supersilyl substituted 1-alkenyl-1-alkynylsilane 3, tBu3 Si(H)Si(CHCHPh)(CCPh). Mechanistic investigations using an isotope labelling technique and DFT calculations suggest that this reaction occurs through a similar zwitterionic intermediate and subsequent hydrogen abstraction from a second molecule of phenylacetylene.

An acyl-functionalized phosphasilene, LSi(COtBu)=P(SiMe3) (L = PhC(NtBu)2) was synthesized on a new route by the addition of tBuCOCl to the phosphinosilylene LSiP(SiMe3)2 and subsequent Me3SiCl elimination. DFT studies elucidated its molecular structure, the influence of the acyl group on UV/Vis transitions, and revealed the mechanism. The intermediate LSi(COtBu)ClP(SiMe3)2, with a five-coordinate silicon center, was characterized by NMR spectroscopy and X-ray analysis. On the other hand, phosphasilene LSi(SiMe3)=P(SiMe3) reacted with tBuCOCl by a [2+2] cycloaddition of the silicon-phosphorus double bond and the carbon-oxygen double bond in addition to Me3SiCl elimination, thereby affording the novel, fully characterized compound LSi(SiMe3)[P=C(tBu)O] bearing a Si-P-C-O heterocycle with a phosphorus-carbon double bond. DFT studies suggest that two mechanisms occur simultaneously.