The simplest parent phosphinidene, :PH (1), has been observed only in the gas phase or low temperature matrices and has escaped rigorous characterization because of its high reactivity. Its liberation and transfer to an unsaturated organic molecule in solution has now been accomplished by taking advantage of the facile homolytic bond cleavage of the fragile Si═P bond of the first zwitterionic phosphasilene LSi=PH (8) (L = CH[(C═CH2)CMe(NAr)2]; Ar = 2,6-(i)Pr2C6H3). The latter bears two highly localized lone pairs on the phosphorus atom due to the LSi═PH ↔ LSi(+)-PH(-) resonance structures. Strikingly, the dissociation of 8 in hydrocarbon solutions occurs even at room temperature, affording the N-heterocyclic silylene LSi: (9) and 1, which leads to oligomeric [PH]n clusters in the absence of a trapping agent. However, in the presence of an N-heterocyclic carbene as an unsaturated organic substrate, the fragile phosphasilene 8 acts as a :PH transfer reagent, resulting in the formation of silylene 9 and phosphaalkene 11 bearing a terminal PH moiety.

This account is a review on the synthesis and transition-metal coordination chemistry of N-heterocyclic silylenes (NHSi's) over the last 20 years till the present time (2012). Recently, fascinating and novel synthetic methods have been developed to access transition-metal-NHSi complexes as an emerging class of compounds with a wealth of intriguing reactivity patterns. The striking influence of coordinating NHSi's to transition-metal complex fragments affording different reactivities to the "free" NHSi is a connecting theme ("leitmotif") throughout the review, and highlights the potential of these compounds which lie at the interface of contemporary main-group and classical organometallic chemistry towards new molecular catalysts for small-molecule activation.

The first electron-rich N-heterocyclic silylene (NHSi)-iron(0) complexes are reported. The synthesis of the starting complex is accomplished by reaction of the electron-rich Fe(0) precursor [(dmpe)2Fe(PMe3)] 1 (dmpe =1,2-bis(dimethylphosphino)ethane) with the N-heterocyclic chlorosilylene LSiCl (L = PhC(N(t)Bu)2) 2 to give, via Me3P elimination, the corresponding iron complex [(dmpe)2Fe(←:Si(Cl)L)] 3. Reaction of in situ generated 3 with MeLi afforded [(dmpe)2Fe(←:Si(Me)L)] 4 under salt metathesis reaction, while its reaction with Li[BHEt3] yielded [(dmpe)2Fe(←:Si(H)L)] 5, a rare example of an isolable Si(II) hydride complex and the first such example for iron. All complexes were fully characterized by spectroscopic means and by single-crystal X-ray diffraction analyses. DFT calculations further characterizing the bonding situation between the Si(II) and Fe(0) centers were also carried out, whereby multiple bonding character is detected in all cases (Wiberg Bond Index >1). For the first time, the catalytic activity of a Si(II) hydride complex was investigated. Complex 5 was used as a precatalyst for the hydrosilylation of a variety of ketones in the presence of (EtO)3SiH as a hydridosilane source. In most cases excellent conversions to the corresponding alcohols were obtained after workup. The reaction pathway presumably involves a ketone-assisted 1,2-hydride transfer from the Si(II) to Fe(0) center, as a key elementary step, resulting in a betaine-like silyliumylidene intermediate. The appearance of the latter intermediate is supported by DFT calculations, and a mechanistic proposal for the catalytic process is presented.

A review of all existing N-heterocyclic silylene (NHSi) complexes involved in catalysis. Remarkably few examples exist, but already show promise as an emerging new generation of catalysts with the potential to tune catalyst activities and selectivities.

The first N-heterocyclic carbene adducts of arylchlorosilylenes are reported and compared with the homologous germanium compounds. The arylsilicon(II) chlorides SiArCl(Im-Me(4)) [Ar=C(6)H(3)-2,6-Mes(2) (Mes=C(6)H(2)-2,4,6-Me(3)), C(6)H(3)-2,6-Trip(2) (Trip=C(6)H(2)-2,4,6-iPr(3))] were obtained selectively on dehydrochlorination of the arylchlorosilanes SiArHCl(2) with 1,3,4,5-tetramethylimidazol-2-ylidene (Im-Me(4)). The analogous arylgermanium(II) chlorides GeArCl(Im-Me(4)) were prepared by metathetical exchange of GeCl(2)(Im-Me(4)) with LiC(6)H(3)-2,6-Mes(2) or addition of Im-Me(4) to GeCl(C(6)H(3)-2,6-Trip(2)). All compounds were fully characterized. Density functional calculations on ECl(C(6)H(3)-2,6-Trip(2))(Im-Me(4)), where E=Si, Ge, at different levels of theory show very good agreement between calculated and experimental bonding parameters, and NBO analyses reveal similar electronic structures of the two aryltetrel(II) chlorides. The low gas-phase Gibbs free energy of bond dissociation of SiCl(C(6)H(3)-2,6-Trip(2))(Im-Me(4)) (Delta(calcd) degrees=28.1 kJ mol(-1)) suggests that the carbene adducts SiArCl(Im-Me(4)) may be valuable transfer reagents of the arylsilicon(II) chlorides SiArCl.