Coordination shifts

TABLE 15. 29Si chemical shifts (S) and coordinative shifts (AS) for (O—Si) chelates... 

Focusing on As, we note that in effect, the discrete coordinate shift Ax > (defined below Equation (3.77)) is replaced here by the continuum coordinate shift D(Ap), in which Z)(Ap) 2 is proportional to a reorganization energy density (per unit volume) [39], Equation (3.89) is a generalization of Equation (3.88), in which D is a function of the dielectric constant (i.e., with inclusion of the image effects neglected in Equation (3.88) [12,22]) ... 

The typical olefinic carbon coordination shift to higher field is therefore negative. In the case of the i) -alkyl metal complexes (M-R) it is useful to compare the shifts with those of the corresponding hydrocarbon (H-R). 

The olefin signal in the tris(ethylene)metal complexes experiences a coordination shift of approximately -60 ppm. Interestingly, the magnitude of the complexation shifts increases in the order Pd < Ni < Pt. This result has also been observed for the bis(r/3-allyl)metal complexes discussed in Section IV. 

In addition to those complexes listed in Table IV, a number of electron-rich systems involving lithium (5J) have been studied, and in these cases unusually large coordination shifts are observed. For example the value... 

The effect of the pyrrolyl ligand may be due to a type of hemilability (see Box 8) arising from an to t coordination shift, the former facilitating metallacycle formation and the latter favouring further ethylene insertion and formation of the chromium-bonded 5-hexenyl species. 

Whereas the variation of p value in solvolyses is often ascribed to the shift of the transition state coordinate, this coordinate shift does not appear to be important for the whole range of substituent change, e.g. from bis-(p-methoxyphenyl) to bis-(m-chlorophenyl) in the series [29(X = Y)]. 

Coordination shifts are the difference between the model tetra-coordinate molecule. A5 = 6 —5 Relative XSi(OCHjCHj)3 Relative (CH3)2Si(OCH2CH3)j " Relative CHjSifNfCHjlJ,... 

The concepts developed for compounds of tetrahedral silicon do not explain many experimental peculiarities of Si NMR spectroscopy in the case of penta-coordina-tion. Thus, for example although the interatomic Si... N distance in 1-chloro-methylhomosilatrane is much greater than in 1-chloromethylsilatrane, the shielding of silicon in the former is greater (—85.1 ppm) than in the latter (—77.2 ppm). Even in a narrow series of 1-substituted silatranes the coordination shifts A8 . 

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