TY - JOUR
T1 - Comparison of C-Cl and Si-Cl bonds. A valence bond study
AU - Lauvergnat, David
AU - Hiberty, Philippe C.
AU - Danovich, David
AU - Shaik, Sason
PY - 1996/4/4
Y1 - 1996/4/4
N2 - VB calculations with breathing orbitals (BOVB) show that the H3Si-Cl and H3C-Cl bonds are qualitatively different. The differences are rooted in the properties of the H3Si+ and H3C+ species. Thus, the H3C+ cation has an evenly distributed charge and relatively large ionic radius, and therefore the cation maintains a long distance from the counterion Cl-. Consequently, the ionic-covalent mixing remains of secondary influence and shortens slightly the RCCI distance in agreement with the Pauling recipe for polar bonds. On the other hand, in H3Si+ the charge is highly localized on silicon. Consequently, the cation acquires a diminished effective size along the missing coordination site. This allows a close approach of Cl- as well as a very large electrostatic interaction between the Si+ and Cl- centers in the ionic VB structure. Consequently, the ionic potential energy curve R3Si+Cl- approaches the corresponding covalent curve to a near-degeneracy. The ensuing VB mixing renders the Si-Cl bond a true charge-shift bond whose major character is the charge fluctuation inherent in the resonating wave function. The effect of ionicity on the Si-Cl bond length does not follow the Pauling recipe. Indeed, by mixing of the ionic structure the RSiCl minimum shifts to a longer distance in comparison with the covalent minimum. The new minimum is simply an intermediate distance between the covalent and ionic minima in keeping with the charge-shift nature of the bond. The manifestations of the diminished effective size of R3Si+ are its strong coordinating ability with electronegative and electron-rich ligands. Implications on the R3Si+ problem are discussed.
AB - VB calculations with breathing orbitals (BOVB) show that the H3Si-Cl and H3C-Cl bonds are qualitatively different. The differences are rooted in the properties of the H3Si+ and H3C+ species. Thus, the H3C+ cation has an evenly distributed charge and relatively large ionic radius, and therefore the cation maintains a long distance from the counterion Cl-. Consequently, the ionic-covalent mixing remains of secondary influence and shortens slightly the RCCI distance in agreement with the Pauling recipe for polar bonds. On the other hand, in H3Si+ the charge is highly localized on silicon. Consequently, the cation acquires a diminished effective size along the missing coordination site. This allows a close approach of Cl- as well as a very large electrostatic interaction between the Si+ and Cl- centers in the ionic VB structure. Consequently, the ionic potential energy curve R3Si+Cl- approaches the corresponding covalent curve to a near-degeneracy. The ensuing VB mixing renders the Si-Cl bond a true charge-shift bond whose major character is the charge fluctuation inherent in the resonating wave function. The effect of ionicity on the Si-Cl bond length does not follow the Pauling recipe. Indeed, by mixing of the ionic structure the RSiCl minimum shifts to a longer distance in comparison with the covalent minimum. The new minimum is simply an intermediate distance between the covalent and ionic minima in keeping with the charge-shift nature of the bond. The manifestations of the diminished effective size of R3Si+ are its strong coordinating ability with electronegative and electron-rich ligands. Implications on the R3Si+ problem are discussed.
UR - http://www.scopus.com/inward/record.url?scp=0030568423&partnerID=8YFLogxK
U2 - 10.1021/jp960145l
DO - 10.1021/jp960145l
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AN - SCOPUS:0030568423
SN - 0022-3654
VL - 100
SP - 5715
EP - 5720
JO - Journal of Physical Chemistry
JF - Journal of Physical Chemistry
IS - 14
ER -