Bonding considerations silene

Despite the differing levels of calculations, the same general conclusions were reached. The silicon-carbon double bonds in 1-silaallene (1.69 A) and 2-silaallene (1.70 A) are shorter than in isolated silenes at the same level of theory. This trend is also observed in the analogous carbon series. 1-Silaallene is thermodynamically more stable than 2-silaallene by 21 kcal/mol (22). Intuitively, this is what would have been expected, realizing the low ability of silicon to participate in multiple bonds. As may be expected from simpler systems (i.e., H2Si=CH2)(i97), silylene isomers (for example, structures 8 and 9) are considerably more stable (approximately IS kcal/mol) than their silaallene counterparts. 

A second category of silene reactions involves interactions with tt-bonded reagents which may include homonuclear species such as 1,3-dienes, alkynes, alkenes, and azo compounds as well as heteronuclear reagents such as carbonyl compounds, imines, and nitriles. Four modes of reaction have been observed nominal [2 + 2] cycloaddition (thermally forbidden on the basis of orbital symmetry considerations), [2 + 4] cycloadditions accompanied in some cases by the products of apparent ene reactions (both thermally allowed), and some cases of (allowed) 1,3-dipolar cycloadditions. 

The calculated Si=C bond lengths in trans-bent silynes are distinctively shorter than Si=C bond lengths in silenes (i.e. 1.632 A and 1.714 A in HSi=CH and in H2Si=CH2, respectively, at MP2/6-31G )2970. A theoretical analysis in the framework of natural bond theory predicts for 668b and substituted derivatives bond orders for the SiC bond which are intermediate between double and triple bonds with considerable ionic contributions29711 0. 

Considerable insight into the broader question of how the kinetic and thermodynamic stability of the Si=C bond is affected by substituents at silicon and carbon was obtained in the late 1970s, when Brook and coworkers reported the isolation and characterization of the first stable silene derivatives (80), prepared by photolysis of the corresponding acylpolysilanes (79) as shown in equation 6017. The unique stability of these compounds is clearly due to steric stabilization effects to some extent, but it was suggested that the siloxy substituent at carbon also reduces the reactivity of these silenes toward dimerization as a result of resonance effects on the degree of positive charge character at silicon. 

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