Berry pseudorotation mechanism

Fluxional behavior was demonstrated for pentacoordinate alkylhydridosilicates in the gas phase, by the reaction of deuterium-labeled alkylsilicate with carbon dioxide65. The monodeuteriated silicate was first obtained in the flow tube by D- addition to a neutral alkylsilane, followed by H-/D- transfer to CO2 (equation 28). A statistical distribution of HC02 /DCC>2 (3 1 ratio) indicated complete scrambling of the hydrogens and deuterium in the silicate, presumably by a Berry pseudorotation mechanism. 

FIGURE 11 Steps in the conversion of an asymmetric phosphorane (top left) into its enantiomer (top right) by a Berry pseudorotation mechanism. In each step, labeled Kx), one of the three equatorial ligands (the pivot ligand x) remains equatorial and is at the apex of the square pyramid in the transition structure. 

FIGURE 16.17. The three types of five-coordinate d complexes and their relationship to the Berry pseudorotation mechanism (see Figure 16.9). It appears that five-coordinate complexes of Rh, Ir, and Ni can undergo pseudorotation between a TBP and e-SQP structure. Other five-coordinate complexes of Ni, plus those of Pt and Pd, form f-SQP structures and do not undergo pseudorotation. 

All MO calculations, both semiempirical and ab initio, have demonstrated that the barrier to aZ>3h-C4v-D3h interconversion, the Berry pseudorotation process, is small for acyclic phosphoranes (see also Section 2.1.). Estimates of 1.4 (extended Hiickel MO34 ), 4.8 (ab initio35 ) and 3.5 kcal/mol (CNDO/231 ) have been obtained for PFS. The barrier computed from the ab initio calculation increases to 8.5 kcal/mol if the 3c -orbitals are not included. The turnstile rotation mechanism traverses a much higher energy barrier 10.0 (extended Hiickel MO), 18.1 (ab initio) and 9.1 kcal/mol (CNDO/2). On the basis of these calculations, the Berry pseudorotation mechanism must be the preferred explanation of pentatopal isomerisation in acyclic phosphoranes (see Section 2.1.). 

The great flexibility of the trigonal bipyramidal skeleton displayed in the structures of many cyclic oxyphosphoranes suggests that the Berry pseudorotation mechanism may be an oversimplified representation for the permutational isomerisation of such species. Theoretical calculations and statistical considerations combined with these studies lead to the conclusion that this isomerisation may take place by a continuum of different routes (including turnstile rotation pathways) over a broad relatively flat potential surface, which have similar energy barriers. The Berry pseudorotation will, however, represent the energetically most favourable pathway for acyclic derivatives. 

DFT calculations of the Berry pseudorotation mechanism in SF4 was used to give 19F chemical shifts for various points along the pseudorotation trajectory.1014... 

Phosphorus v) Compounds. According to CNDO/2 calculations, the only way in which the axial and equatorial fluorines in PF, can interchange is by the Berry pseudorotation mechanism,and an estimate of the barrier to such an interchange, (1371 cm or 3.92 kcal moP ) has been obtained from a detailed Raman study of the Vj, equatorial bend, of PF.-,. A new normal-co-ordinate analysis for the PF5 molecule has been published. 

General consistency with pseudorotation should not be taken for granted. As shown in Fig. 7, careful examination of the carbonyl resonances observed in (methyl acrylate)Fe(CO)4 show that all the resonances do not broaden to the same degree at the onset of averaging 61, 78). Solvent variations do not alter this effect, and it is therefore unlikely to arise from averaging with an intermediate (see Section II). The Berry pseudorotation mechanism demands that each of the four carbonyl ligands move to a different environment hence, all should broaden to the same extent. This anomalous result can be accommo-... 

The structures of the hydridorhodium complexes that are present in the catalytic system have been deduced by NMR spectroscopy. Brown showed that HRh(CO)2(PPh3)j exists as an 85 15 mixture of diequatoriakapical-equatorial isomers of HRh(CO)2(PPh3)2 (Scheme 17.11) that undergoes rapid equilibration at room temperature. This rapid equilibration of trigonal bipyramidal complexes could occur by either a Berry pseudorotation mechanism or a turnstile mechanism. In situ IR transmission spectroscopy on the catalytic system demonstrated that these two isomers are the resting state of the catalyst and were by far the predominant species present during hydroformylation of 1-octene (60-100 C, 5-20 atm, [Rh] = 1 mM, PPhj/Rh = 5). °... 

Intramolecular Processes. Two studies of axial-equatorial exchange in sulphur tetrafluoride have been published. Provided that the sulphur tetrafluoride is carefully purified it is claimed that the data are in good agreement with the Berry pseudorotation mechanism. The presence of impurities readily leads to intermolecular fluorine exchange. It has been claimed that the rate of exchange (in the absence of hydrogen fluoride) is the same in the liquid phase ( =11.2 kcal mol 0 as in the gas phase (E a =11.7 kcal mol ). 

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