Pentagonal bipyramidal transition state

In the high resolution crystal structure of the GTP form of Ras protein, a tightly bound water molecule is visible located in an optimal position for nucleophilic attack on the y-phosphate (Wittinghofer et al., 1993). The water molecule is fixed in a defined position by H-bridges with GIn61 and Hir35. As described in 5.4.4 for the a-subunits of the heterotrimeric G-proteins, GTP hydrolysis takes place by an in-line attack of the nucleophilic water molecule on the y-phosphate, for which a pentagonal, bipyramidal transition state is postulated. 

Example 17-1. Some octahedral substitution reactions are believed to occur by an I, mechanism, where the transition state is seven-coordinate. Assuming a pentagonal bipyramidal transition state, calculate the LFAEs for d -d . Which electron configurations will lead to substitutionally inert metal ions, assuming an I, mechanism ... 

Inai et al. [483] suggested that an activated complex with a pentagonal bipyramid-type structure was formed in the transition state in the Volmer— Heyrovksy-type mechanism for chlorine evolution at Ru02. A theoretical activation energy for this reaction was calculated by using the difference in the crystal field stabilization energy between the initial and transition... 

Table 25.5 Changes in CFSE (ACFSE) on converting a high-spin octahedral complex into a square-based p5ramidal (for a dissociative process) or pentagonal bipyramidal (for an associative process) transition state, other factors remaining constant (see text).

Fig. 9. Eight-coordinate transition state in an interchange associative mechanism of substitution reactions on a pentagonal-bipyramidal complex (S=solvent molecule, A =anionic monodentate ligand).

There have been attempts to reflne the theory. Breitschwerdt allowed the effective charge on the metal ion to change widi the number of ligands, so that Dq varied between the ground and transition states. The Dq also was decreased by 40% for ligands in the plane of the pentagonal bipyramid compared to the axial ligands. It was found that a square pyramid was the most stable transition state for all the 2+ ions and that all the CFAEs are positive. 

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