Trigonal bipyramidal intermediate

Trippett and Stewart have shown that the phosphonium salts (11) derived from the reaction of phenyl di-t-butylphosphinite (12) with alkyl halides are highly resistant to hydrolysis and they suggest that this is due to the reluctance of phosphorus to accommodate two t-butyl groups in a trigonal-bipyramidal intermediate. 

Figure 3. Scheme of the photochemical behavior of M(CO)5 in a mixed NJAr matrix via a trigonal bipyramidal intermediate. Key O, the matrix cage and, 1E excited state of Cr(CO)5. (Reproduced from Ref. 25. Copyright 1978, American... 

The stereochemistry of substitutions at sulfinyl sulfur that proceed with inversion has usually been explained in terms of the reaction proceeding through a trigonal bipyramidal intermediate (or transition state) [46] in which... 

Oae and co-workers (288) were the first to show that nucleophilic displacement at sulfur is accompanied by retention of configuration. They found that chiral 0-labeled alkyl aryl sulfoxides exchange oxygen with dimethylsulfoxide at about 150°C, almost without racemization. To explain the steric course (retention) of this reaction, the formation of a trigonal-bipyramidal intermediate 246 was postulated in which the entering and departing oxygen atoms occupy apical and equatorial positions, respectively. 

With specially designed reactants, the determination of the product structure can be very informative. Mercury(II)-catalyzed aquations of Co(III) complexes are believed to proceed via a 5-coordinated intermediate (Sec. 4.3.2). The shape of this intermediate is of interest. The Hg +-catalyzed aquation of Co(NH3)4 (NDj)X + in which the ND, and X groups are trans to one another gives substantially rrans-Co(NH3)4(ND3)(H20). This is excellent evidence for a square pyramidal intermediate in the reaction. A trigonal-bipyramidal intermediate would be expected to lead to substantial scrambling of the NDj and NHj groups (Fig. 2.1). The definite but very small amount (2.8 0.4%) of cis product recently reported using NHj instead of NDj attests to the sensitivity of current nmr machines. 

Fig. 2.1 Expected products from the Hg(II)-catalyzed aquation of tran5-Co(NH3)4(ND3)X on the basis of a square-pyramid or trigonal-bipyramid intermediate. ...

If my memory serves me correctly, two different mechanisms have been suggested for planar substitution the first one, by a trigonal bipyramidal intermediate, was suggested in 1954 and 1955 by Chatt and Orgel. In 1958, an alternative mechanism for planar substitution was suggested in which 7r-bonding is not very... 

Briefly, as Prof. Basolo points out in his paper, and Prof. Basolo and Pearson pointed out nicely in a review on the trans effect, the situation in the ir-bonding theory is a stabilization of the trigonal bipyramidal intermediate because there are more orbitals in the trigonal plane available for 7r-bonding in the trigonal bipyramid than in the square planar complex. The same is also true in the --system. The trans effect is directional, and in my opinion, it has to be orbital or quantum mechanical. 

Archer et al.206 surveyed all phosphonium salts known in 1981 in a correlation analysis of the positioning of cation and anion. Exclusive face orientation of the anion with respect to the cation centre was found, i.e. the anion was positioned along the direction of approach leading to the apical position in a trigonal bipyramidal intermediate. The positioning of the anion did not correlate with nucleophilic attack at the phosphorus centre but did correlate with a-hydrogen abstraction (ylide formation)206. 

Hydrolysis at higher temperatures yields mainly trimethyl phosphate and acetoin. A tentative reationale for these results features a trigonal bipyramidal intermediate which may be represented with the hydroxyl group apical, viz. 

Fig. 13.3 Reaction coordinate/energy profile for a square planar substitution reaction having (a) a trigonal bipyramidal activated complex and (b) a trigonal bipyramidal intermediate. [From Burdetl, 1. K. Inorg. Chem. 1977, 16, 3013-3025. Used with permission.)...

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