An isolable heterocyclic silylene (4) with two different π-donating substituents, namely a classical amino group and a more electropositive and stronger carbon-based π-donating phosphonium ylide, was synthesized and fully characterized. The combination of these two different π-donating substituents confers high thermal stability and an unusual nucleophilic character on silylene 4. Therefore, silylene 4 behaves as a strong donor ligand toward transition metals with a donating character comparable to N-heterocyclic carbenes, in contrast to classical N-heterocyclic silylenes, which generally present a weak donating character.

A borylidene-phosphorane, the lightest phosphoranylidene, which is stabilized by an N-heterocyclic carbene ligand, was synthesized and fully characterized. Experimental electron density analysis and DFT calculations indicate an enhanced ylene character (rather than an ylide character) with an exceptionally strong B→P π-back bonding related to the less electronegative boron compared to phosphorus.

As ilanone substituted by bulkya mino and phosphonium bora-ylide substituents has been isolated in crystalline form. Thanks to the exceptionally strong electron-donating phosphonium bora-ylide substituent, the lifetime at room temperature of the silanone is dramatically extended (t 1/2 = 4days) compared to the related (amino)(phosphoniumylide)silanone VI (t 1/2 = 5h), allowing easier manipulation and its use as precursor of new valuable silicon compounds.T he interaction of silanone with aweak Lewis acid such as MgBr 2 increases further its stability (no degradation after 3weeks at room temperature).Incontrast to carbonyl compounds (I), probably one of the most important functional group in organic chemistry,t he heavier silicon analogue,s ilanones (II), are generally highly reactive transient species [1] that were only observed either in argon matrix at low temperature [2] or in the gas phase [3] and were evidenced by trapping reactions. [4] Silanones present as trongly polarized Si=Od ouble bond (Si d+ ÀO dÀ )w ith ac onsiderably large bond energy difference between SiO-p and SiO-s bonds, [5] and therefore,t hey easily oligomerize to give polysiloxanes. [6] Recently,e asy-to-handle silanones stabilized as donor-acceptor or donor complexes,h ave been reported. [7][8][9][10][11] Nevertheless,t he synthesis of stable threecoordinate silanone derivatives remains ad ifficult task, although the heavier analogue germanone III,well kinetically stabilized by bulky aryl substituents,h as been isolated by Matsuo/Tamao et al. [12] At present only three examples are reported:1 )the transition-metal-substituted silanone IV, isolated at RT by Filippou et al., [13] 2) the cyclic dialkylsilanone V,c haracterized in situ at À80 8 8Cb yt he group of Iwamoto, [14] and 3) the relatively persistent (amino)-(ylide)silanones VI,r eported by us. [15] Therefore,t heir synthetic use is still limited because of the scarcity and limited lifetime (0.5-5 h, at RT for VI). Herein we report that the introduction of as trongly donating phosphonium boraylide substituent considerably extends the lifetime of silanone 2 (t 1/2 = 4days,a tR T), which allows easier manipulation to study its properties and reactivity.Recently,w eh ave described that the electropositive boron-based substituent (phosphonium bora-ylide), [16,17] with an exceptionally strong electron donating character,i s able to efficiently stabilize highly reactive species such as Nheterocyclic silylene 1,the related silylium ion and transitionmetal-silylene complexes featuring at rigonal-planar geometry around the silicon center. [18] We thus have considered the use of the same set of substituents for the stabilization of the silanone moiety.T aking into account the better donating ability of phosphonium bora-ylide function than that of classical (carbo)ylides, [18] we can expect abetter stabilization effect with this substituent set than that used for the previously persistent silanone VI.The( amino)(bora-ylide)silylene 1 [15] immediately reacts with N 2 Oi nT HF solution, at À20...

Electropositive boron-based substituent (phosphonium bora-ylide) with an exceptionally strong π- and σ-electron donating character dramatically increases the stability of a new type of N-heterocyclic silylene 2 featuring amino- and bora-ylide-substituents. Moreover, the related silylium ion 4 and transition-metal-silylene complexes, with trigonal-planar geometries around the silicon center, are also well stabilized. Therefore, the N,B-heterocyclic silylene 2 can be used as a strongly electron-donating innocent ligand in coordination chemistry similarly to N-heterocyclic carbenes.

A borylidene–phosphorane, the lightest phosphoranylidene, which is stabilized by an N‐heterocyclic carbene ligand, was synthesized and fully characterized. Experimental electron density analysis and DFT calculations indicate an enhanced ylene character (rather than an ylide character) with an exceptionally strong B→P π‐back bonding related to the less electronegative boron compared to phosphorus.

Silanones 2 substituted by bulky amino‐ and phosphonium ylide substituents have been synthesized and isolated in crystalline form. Thanks to the steric protection and the strong electron‐donating ability of the substituents, silanones 2 are persistent and only slowly dimerizes at room temperature (t1/2=0.5 or 5 h). Structural and theoretical analysis of 2 indicate a short Si=O bond (1.533 Å) and an enhanced polarization toward the O atom compared to Me2Si=O owing to the strong π‐electron donation from the phosphonium ylide substituent.

The donor-stabilized sila-β-lactone 1 reacts with CO via a remarkable reversible [2+2]-cycloaddition reaction to form the spiro-cyclic silicon carbonate derivative 2. Furthermore, photolysis of 2 under pressure of CO affords the first persistent SiO -CO cycloadduct 3, presenting a Si O -like structure, which is stabilized by a Lewis donor-acceptor type ligand. As predicted by theoretical calculations, in marked contrast to the thermodynamically stable SiO dimer, the SiO -CO mixed cycloadduct 3 is labile and readily releases CO .

Electropositive boron‐based substituent (phosphonium bora‐ylide) with an exceptionally strong π‐ and σ‐electron donating character dramatically increases the stability of a new type of N‐heterocyclic silylene 2 featuring amino‐ and bora‐ylide‐substituents. Moreover, the related silylium ion 4 and transition‐metal–silylene complexes, with trigonal‐planar geometries around the silicon center, are also well stabilized. Therefore, the N,B‐heterocyclic silylene 2 can be used as a strongly electron‐donating innocent ligand in coordination chemistry similarly to N‐heterocyclic carbenes.