Pseudorotation-coordinate model

Summary Bulky monodentate ligands affect the structure and reactivity of siliconium ion complexes in several ways they enhance ionization, they promote a methyl halide elimination reaction, and they severely distort complex geometries. The latter effect enabled the assembly of a reaction coordinate model for the Berry pseudorotation, composed of crystal structures with varying NSiN and OSiO bond angles. In a competition between opposing effects of electron withdrawal by CF3 and steric bulk of a cyclohexyl ligand in the same molecule, a nonionic dissociation of the dative N->Si bond was observed. 

From the results obtained for the model species 3, it is concluded that the TBP, with oxygen atoms in the axial positions, is the energetically preferred geometry of the Si coordination polyhedra of the title compounds. As the pseudorotation according to path 1 needs only a small amount of energy, package effects in the crystal (which are individual for each compound) can easily cause distortions of the TBP toward the SP. This assumption is in agreement with the results obtained in experimental studies [1]. 

Fig. 2. A model Berry pseudorotation reaction coordinate made up of crystal structures of pentacoordinate silicon complexes with varying N-Si-N and O-Si-0 angles.

It has been assumed that in the noncatalyzed transesterification of dialkyl H-phos-phonates, these compounds react with their four-coordinated phosphonate form [28,29], AMI semiempirical calculations [30] of the model transesterification of dimethyl H-phosphonate with methanol indicate that in the first stage of this reaction, dimethyl H-phosphonate and the nucleophile form a pentacoordinated intermediate n via a four-centered cyclic transition structure I. The trigonal-bipyramidal intermediate n undergoes pseudorotation [17c,31] and via a new transition structure in of the same type as I, forms the monotransesterificated product. The rate of transesterification depends both on the type of substituents at phosphorus and the nucleophiUcity of the corresponding alcohol. 

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