Square pyramid mechanism

This square pyramidal mechanism or the nonsticky collision says that when a substance like iodine collides with the complex, a square pyramid is formed initially with another solvent which aids the dissociation of the bromide hence the dissociative mechanism. 

XIV) and L [Eq. (50)] may be best rationalized on the basis of the intermediacy of square-pyramidal (XVII), which arises from (XIV) via a concerted approach of Et and CO(l), accompanied by the opening of the Cl(l)—Ir—L bond angle. It is entirely possible that this mechanism of cooperative migration may be as general, if not more so, than that of alkyl migration. 

Fig. 7. Structures of five-coordinate Cu2+ from first principles molecular dynamics. A Berry twist mechanism for the interconversion of the two structures is shown (from left to right) the reorientation of the main axis of a square pyramidal configuration by pseudo-rotations via a trigonal bipyramidal configuration. The grey atoms in the plane of the trigonal bipyramid are all candidates for becoming apical atoms in a square pyramid.

For example, conversion of the hexacoordinate (octahedral) derivatives [MnHCl(PP3)] to the corresponding deprotonated pentacoordinate (trigonal-bipyramidal or square pyramidal) derivatives [MnCl(PP3)]+ (M = Fe, Ru, Os), Scheme 6, follows electrode mechanisms that are more or less complicated depending upon the nature of the central metal.16... 

Pseudorotation is well established in 5-coordinated species involving the main group elements and is best described by the Berry mechanism which interconverts two trigonal bipyramids via a square pyramid. Its operation here is difficult to reconcile with the highly stereospecific nature of substitution in Pt(II). Nevertheless, the mechanism has had substantial support. It may very well be that (a) is favored by polar solvents and that (b) is prevalent in nonpolar media. The associated reaction profiles are shown in Fig. 7.11. 

Both five-coordinate and four-coordinate pathways have been proposed for these reactions. The associative (five-coordinate) mechanism involves the formation of a trigonal bipyramidal or square pyramidal intermediate, which can revert back to tetracoordination by alkene insertion into the Pt—H bond.151 The dissociative (four-coordinate) mechanism involves initial substitution of a ligand other than hydride by alkene, followed by insertion to form the alkyl product. The ligand which is substituted is usually the anionic ligand, and if this group is trans to hydride an isomerization will need to occur prior to insertion of the coordinated alkene into the Pt—H bond. 

Figure 74 (a) The Berry-twist mechanism connecting a trigonal bipyramidal and square pyramidal geometry (b) e modes of vibration (c) correlation of one-electron energy levels... 

---