Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC-main group element adducts, and might be useful for the broad research community.

The synthesis of stable heavier analogues of nitriles as monomeric tetrylene−phosphinidenes Mes TerEP(IDipp) (E = Ge, Sn; Mes Ter = 2,6-Mes 2 C 6 H 3 , IDipp = C([N-(2,6-iPr 2 C 6 H 4 )CH] 2 ) was achieved by taking advantage of NHC (N-heterocyclic carbene, here IDipp) coordination to the low-valent phosphorus center. Multiple bonding character of the E−P bonds was examined experimentally and computationally. Both germanium and tin compounds undergo [2+2] cycloaddition with diphenylketene, whereas reaction of the tin derivative with tris(pentafluorophenyl)borane provided unique "push−pull" phosphastannene ( Mes Ter)(Ar)Sn = P(IDipp) (Ar = C 6 F 4 [B(F)(C 6 F 5 ) 2 ]). Going further, we demonstrated the potential of tetrylene−phosphinidene complexes in catalytic hydroboration of carbonyl compounds.

P-Trityl substituted Li/Cl phosphinidenoid tungsten(0) complex (OC)5W{Ph3CP(Li/12-crown-4)Cl} (3) was prepared via chlorine/lithium exchange in complex (OC)5W{Ph3CPCl2} (2) using (t)BuLi in the presence of 12-crown-4 in tetrahydrofuran (THF) at low temperature; complex 3 possesses significantly increased thermal stability in contrast to previously reported analogue derivatives. Terminal phosphinidene-like reactivity of 3 was used in reactions with benzaldehyde and isopropyl alcohol as oxaphosphirane complex (OC)5W{Ph3CPC(Ph)O} (5) and phosphinite complex (OC)5W{Ph3CP(H)O(i)Pr} (6) were obtained selectively. Reaction of 3 with phosgene allowed to obtain the first kinetically stabilized chloroformylphosphane complex (OC)5W{Ph3CP(Cl)C(O)Cl} (4). Density functional theory (DFT) calculations revealed remarkable differences in the degree of P-Li bond dissociation 3a-d: using a continuum model 3 displays a covalent character of P-Li bond (COSMO (THF)) (a), which becomes elongated if 12-crown-4 is coordinated to lithium (b) and is cleaved if a dimethylether unit is additionally coordinated to lithium (c). A similar result was obtained for the case of 3(thf)4 in which also a solvent-separated ion pair structure is present (d). All products were unambiguously characterized by various spectroscopic means and, in the case of 2 and 4-6, by single-crystal X-ray diffraction analysis. In all structures very long P-C bonds were determined being in the range from 1.896 to 1.955 Å.

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.

The first donor-acceptor complex of as ilaaldehyde, with the general formula (NHC)(Ar)Si(H)OGaCl 3 (NHC = N-heterocyclic carbene), was synthesized using the reactiono fs ilyliumylidene-NHC complex [(NHC) 2 (Ar)Si]Cl with water in the presence of GaCl 3 .C onversion of this complex to the corresponding silacarboxylate dimer [(NHC)(Ar)SiO 2 GaCl 2 ] 2 ,f ree silaacetal ArSi(H)(OR) 2 ,s ilaacyl chloride( NHC)(Ar)Si(Cl)OGaCl 3 ,a nd phosphasilene-NHC adduct( NHC)(Ar)Si(H)PTMS unveili ts true potentiala sa synthoni nsilacarbonylchemistry.Supporting information and the ORCID identification number(s) for the author(s) of this articlecan be found under: https://doi.