Trigonal bipyramidal intermediate substitutions

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... 

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... 

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.)...

Stereochemical studies of the basic hydrolysis of the cis and trans isomers of Co(en)2(NH3)Y2+, Co(en),(NCS)Y+, Co(en),ClY+, and C o(en)2(NO )Y+ (where Y is halogen or nitrate) have been carried out. With the ammonia and tbiocy nato-substituted complexes, the products predominantly cis complexes, irrespective of the configuration of he reactants. This suggests that these substitutions proceed primarily through a trigonal bipyramidal intermediate such as XI. It is a little... 

Restrictions on Direct Substitution or Formation of Trigonal Bipyramid Intermediates... 

Stereochemical control of the substitution reaction in cis- and frans-[CoCl(en)2(OH)]+ via different transition state geometries. The square pyramidal intermediate offers only one choice to the entering group and 100% retention of geometry, the trigonal bipyramidal intermediate offers three entry points around the triangular central plane, two leading to cis entry and one to trans entry, for a 2 1 ratio. 

An optically active cis complex can yield products that retain the same configuration, convert to trans geometry, or create a racemic mixture. Statistically, the product of substitution of a di-[M(LL)2BX] complex through a trigonal-bipyramidal intermediate should be one fifth trans if both intermediates were equally likely and one third trans if the axial B form is not formed at all. Experimentally, aquation of di-[M(LL)2BX] in acid results in... 

The ligand substitution reactions of square planar complexes of Pt(II) of the type PtA2XL are stereospecific. If the substrate is a cis-isomer, the product is invariably a cis-isomer and vice versa. The reaction occurs via trigonal bipyramidal intermediate, as shown in Figure 10. 

Nucleophilic substitution at RSO2X is similar to attack at RCOX. Many of the reactions are essentially the same, though sulfonyl halides are less reactive than halides of carboxylic acids. The mechanisms are not identical, because a tetrahedral intermediate in this case (148) would have five groups on the central atom. Though this is possible (since sulfur can accommodate up to 12 electrons in its valence shell) it seems more likely that these mechanisms more closely resemble the Sn2 mechanism, with a trigonal bipyramidal transition state (148). There are two major experimental results leading to this conclusion. 

---