Ligand‐site exchange in intramolecular complexes of silicon: substituent effects

Abstract

The intramolecular complexes containing coordination bonds Si←N or Si←O are distinguished for their stereochemical nonrigidity resulting in interconversion between isomers, that is, ligand‐site exchange. The influence of the substituents bound to the silicon atom on the free energies of activation for ligand exchange ΔG≠ of specific interest is poorly understood. In this work, the literature data on substituent influence on the energies ΔG≠ for 13 series of the complexes have been considered, using the correlation analysis. On the basis of the obedience of the energies ΔG≠ to the linear free energy relationship, it has been established for the first time that the ΔG≠ values depend not only on the inductive and resonance effects but also on the polarizability and steric effects of substituents. The reason for the occurrence of the polarizability effect is the appearance of excess charges on Si and N (or O) atoms as a result of intramolecular coordination consisting in the charge transfer from the donor center (N or O atom) to the acceptor one (Si atom). In some series the contribution of the polarizability or steric effect to the overall change in ΔG≠ because of the influence of substituents is a maximum. An understanding of these effects may give a better insight into the mechanism of nucleophilic substitution, involving organoelement compounds. Copyright © 2012 John Wiley & Sons, Ltd.

Bond dissociation energies in organometallic systems: substituent effects

“…In summary, we will point out that the polarizability effect influences significantly a great variety of physical properties of individual compounds and the complexes thereof, the BDEs in organometallic systems among them. There is good reason to believe that this effect influences the chemical properties of organometallic compounds.…”

“…Over a period of years, we investigate intensively the influence of polarizability effect on a great variety of physical properties such as the ionization potentials, electron affinities, core‐electron binding energies, spectroscopic and thermodynamical characteristics of H‐complexes, transition metal complexes, and radical ions . The polarizability effect consists in an electrostatic attraction between the excess charge q originating on the reaction (indicator) center R C and the dipole moment induced by this charge in the substituents X.…”

“…The polarizability effect consists in an electrostatic attraction between the excess charge q originating on the reaction (indicator) center R C and the dipole moment induced by this charge in the substituents X. The charge q is caused by chemical reaction, complexation, or electromagnetic impact . We have every reason to believe that the geometric and electronic reorganization of center R C going on in the breaking of bonds leads to the arising of the charge q and hence of the polarizability effect.…”

“…As pointed out previously, the polarizability effect is due to this charge arising from the changes in electronic structure of the center R C . Such changes result from the chemical reaction, complexation, or electromagnetic impact . Therefore, taking into account the polarizability effect is indispensable to a detailed analysis of substituent influence on the parameters of the IR, NMR, EPR, ultraviolet photoelectron, X‐ray photoelectron, NQR, and Mössbauer spectra of organometallic complexes.…”

Reactivity of organometallic compounds and polarizability effect

“…The first evidence for this effect has come from the study of ionic gas-phase reactions involving simple organic molecules [28e30]. Further investigations demonstrated that the polarizability effect has a profound influence on the parameters of the IR [31], NMR [32,33], EPR [34], NQR [35], UVeVis electronic [36], ultraviolet photoelectron [37,38], X-ray photoelectron [39], and Mossbauer [40] spectra as well as on the bond lengths [41] and toxicity [42] of organometallic compounds. A necessary condition for arising this effect is initiation of the partial charge q on M atom and/or on bonds MÀB and MÀL.…”