Jr-Bonds, strength

Silene 2, 1-fluorosilene 602 and 1,1-difluorosilene 603 have been examined by FT ion cyclotron resonance spectroscopy in order to estimate the jr-bond strength. Bracketing studies with various bases (B) and fluoride acceptors (A) gave values for the proton and fluoride affinity of the silenes284. The TT-bond energy was calculated by thermochemical cycles and found to increase with fluorine substitution. The data are given in Table 16. 

TABLE 10. Theoretical jr-bond strengths (kcal mol ) of H2E-E H2 molecules in their equilibrium geometry"... 

Gibbs, G. V., L. W. Finger, and M. B. Boisen, Jr. (1987). Molecular mimicry of the bond length-bond strength variations in oxide crystals. Phys. Chem. Minerals 14, 327-31. 

IR spectra of the isolated adducts, the frequencies of the asymmetric stretching vibration combination Vasym(CO) + Vasym(CC) have been shifted to higher frequencies than those of their mother chelates. The enhanced jr-interactions appeared within the adduct complexes because of hyperconjugation, which result in an increased C—O bond strength. The same reasons can be applied to the shift in lowering the stretching frequencies, v(Ni—O), compared to those assigned to the mother chelates. 

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