Bond lengths carbene reactions

The reaction of lead(n) chloride with 2,4,6-triisopropylphenylmagnesium bromide furnished red crystals of compound 84, the first molecule with a short Pb=Pb double bond length of 305.15(3) pm and trans-bent angles of 43.9° and 51.2°.96,96a Compound 84 is stable in the solid state. In solution, it dissociates into the plumbylene molecules 85 that, for example, react with a nucleophilic carbene to furnish the highly labile, zwitterionic adduct 86 (Scheme 2).97... 

Kuhn and co-workers reported the reaction of a stable carbene with POCl3 to yield the [(NHC)POCl2][Cl] salt which, following partial hydrolysis, affords 36 (Scheme 19).46 The solid state single crystal X-ray structure comprises a phosphorus-NHC bond length of 1.843(2) A, which is consistent with the P-C bond being polarised. 

In 2005, Bielawski and co-workers reported the reaction of a stable carbene with an aryl azide to give the triazine 39 (Scheme 20).48 Both the E-and the Z- isomers were identified in the solid state and were found to have nitrogen-NHC bond lengths of 1.339(3) A and 1.330(3) A respectively, hence is even more polarised than compound 37. 

While many stable carbenes tend to be unreactive towards oxygen in the absence of a catalyst, Denk and co-workers found that a stable carbene could be oxidised to the urea 40 in the presence of a catalyst or by reaction with NO (Scheme 21).49 The solid state structure exhibits an oxygen-NHC bond length of 1.237(3) A, and the C2 carbon resonates at 152.7 ppm in the 13C NMR spectrum, 60.2 ppm upheld from the uncoordinated carbene. The bond length is elongated compared to a typical C=0 double bond... 

Reaction of a stable carbene with SCI2 results in the hypervalent sulfur compound 41 (Scheme 22).50 The sulfur-NHC bond length of 1.732(3) A is significantly longer than that of a typical C=S double bond (ca. 1.60 A) indicating a highly polarised compound. 

Pentavalent tin adducts have also been reported and were isolated from the reaction of (1) (R = i-Pr, R = Me) with Ph2SnCl2. An X-ray structure determination of the Ph2SnCl2 adduct (84) showed that the Sn(IV) center adopts a distorted trigonal bipyramidal geometry and the three organic substituents occupy the equatorial positions. The Sn-C(carbene) bond distance of 2.179(3) A in (84) is shorter than the Sn-C(carbene) bond distances found in the Sn(II) adducts (82 and 83) but longer than the Sn-C(phenyl) bond lengths (2.122(5) A and 2.139(3) A). 

Recently, the formation of the first carbene complex of a phosphorus(V) center was reported. The direct reaction of carbene (1) (R = Mes, R = H) with phenyltetrafluo-rophosphorane (equation 12) results in the formation of perphosphoranide (107) in quantitative yield. An X-ray structural determination showed that the phosphoms center in (107) adopts an octahedral geometry with a long P C(carbene) bond length (P C(carbene) 1.91 A cf. P-C(phenyl) 1.84A). The P resonance in (93,94) and (96) are at a substantially higher field than the value in (107), which is expected and reflects the change from a two-coordinate to a six-coordinate P center. 

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