Silylenes bond lengths

Surprising thermal and chemical stability of the cyclic silylene (29) may be explained by an enhanced aromaticity. This can be seen by comparison with its percursor (30) the difference in length between the single N—C (1.415 A) and double C=C (1.322 A) bonds in (30) decreases in (29) (N—C 1.400 A, =C 1.347 A) to imply an increased delocalization. Also the Si—N bond length increases in (29) only by 0.056 A (from 1.697 A to 1.753 A) instead of the 0.080-0.100 A typical for transition from tetra- to dicoordinate silicon this may indicate developing partial Si= N double bond character in (29) <94JA269l>. 

The five-membered rings in 122 and 124 are planar, while the ring in 123 is necessarily puckered. The N—C distances in the ring are shorter in 122 and 124 than in 123 by 10 pm. The Si—N bond lengths are similar to the normal Si—N single bond distances ( 172-174 pm). However, bonds to a divalent silicon atom are predicted to be longer, by 8 pm, than those to a tetravalent Si. The bond lengths in the silylenes mentioned above are, therefore, consistent with some multiple bond character in the Si—N bonds. 

TABLE 5. Selected intraring bond lengths (pm) and angles (deg) for silylenes... 

Electron donation from nitrogen to silicon is undoubtedly important in stabilizing all of these molecules. Partial double-bonding between N and Si will tend to occupy the vacant p-orbital on silicon, which is the usual site of electrophilic reactivity of silylenes. This stabilization is shown in resonance drawings in Scheme 16. Greater --donation from N to Si would be expected for 60 than for 59 or 61, since in 60 the nitrogens are more basic. The Si—N bond lengths are consistent with this model, as mentioned above. 

The above theoretical analysis for a variety of dimer structures of silylenes requires inevitably a definition of disilenes different from that of alkenes, molecules with carbon carbon double bonds. Geometry around a typical C=C double bond is planar and the double bond length (134 pm) is shorter than the corresponding single bond (154 pm). BDE of ethylene to two methylenes is ca. 170 kcal mol-1 which is 1.9 times larger than for the C C single bond (90 kcal mol-1 for H3C-CH3) the BDE of ethylene really almost doubles the BDE for ethane ... 

Table 2 Experimental and calculated geometries of N-heterocyclic silylenes using different methods (bond lengths in pm, bond angles in degrees)...

The Mo-Si bond lengths in compounds 6-8 are between 2.516(2)A and 2.5683(12)A, which correlates very well with M-Si (M = Mo, W) bond lengths already observed by Berry and coworkers [8] in similar metallocene complexes. On the other hand, compounds 1 and 2 possess significantly shorter Mo-Si bonds. In fact, to the best of our knowledge the SiBrs derivative 1 has the shortest ever observed Mo-Si bond. Its value of 2.459(3)A is in the range of the distance calculated for the silylene (silanediyl) complex (CO)sMo=SiH2 (DMo=si = 2.46A) [9]. 

Schaefer20 pointed out three major disparities between theory and experiment regarding the C=Si bond. The first concerns the C=Si bond length, the second concerns the isomerization barrier for its rearrangement to the corresponding silylene and the third is the question of the relative stabilities of 40 and of the isomeric silylene H iCH3. Let us discuss first the problem of the geometry. 

FIGURE 54. Transition structures and reaction profiles for hydrogen abstraction from methane and silane by triplet silylene and methylene. Bond lengths are in A, bond angles in deg, energy in kcal mol"1. Reprinted with permission from J. Am. Chem. Soc., 106, 4054 (1984). Copyright (1984) American Chemical Society297. 

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