Trigonal bipyramidal transition state

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. 

Today it is widely accepted that fivefold coordinated silicon plays a key role in the reaction mechanisms of the nucleophilic substitution having a trigonal bipyramidal transition state species which ressemble these transition states can be isolated in some special cases. The structural features fit well to kinetic data and possibly explain the significantly higher reactivity (proved by experimental data) of Si-pentacoordinated compounds compared to their tetracoordinated analoga. 

Having rationalized that the transition state should be a square-based pyramid, it should be mentioned that there are numerous cases in which the transition state appears to be a trigonal bipyramid. We know that because the substitution occurs with a change in configuration. From the foregoing discussion, we would expect this to occur with first-row transition metals because if 11.48 Dq must be sacrificed, this would be more likely if Dq is smaller (which it is the case for first-row metals). If a trigonal bipyramid transition state forms, there would be more than one product possible. This can be... 

FIGURE 20.5 Attack on a trigonal bipyramid transition state during SN1 substitution. 

If back donation occurs to a ligand, the flow of electron density from the metal leaves less electron density to be donated in the opposite direction. It seems that this should have little effect on the donation of a pair of electrons on the ligand in the trans position to form a a bond. Accordingly, the major factor appears to be the stabilization of a five-bonded (trigonal bipyramid) transition state as a result of 7r bond formation. Ligands that readily form 1r bonds include some of those that generate the largest trans effect. 

Several interesting observations have been made on this reaction. First, the rate of isomerization was found to be the same as the rate of dehydration. All attempts to dehydrate the starting complex by conventional techniques were found to lead to isomerization. On the basis of this and other evidence, the mechanism proposed involves the aquation in the complex followed by anation. In this process, water first displaces Cl- in the coordination sphere and then is displaced by the Cl-, possibly by an SN1 mechanism. A trigonal bipyramid transition state could account for the Cl- reentering the coordination sphere to give an cis product. The rate law for this reaction is of the form... 

On the basis of ligand field effects, would it be easier to form a trigonal bipyramid transition state from a square planar or a tetrahedral starting complex ... 

The ammonia exchange proceeds as in the case of water exchange through an associative interchange mechanism, showing a trigonal bipyramidal transition state structure [Be(NH3).5r1,... 

The mechanism of phosphate ester hydrolysis by hydroxide is shown in Figure 1 for a phosphodiester substrate. A SN2 mechanism with a trigonal-bipyramidal transition state is generally accepted for the uncatalyzed cleavage of phosphodiesters and phosphotriesters by nucleophilic attack at phosphorus. In uncatalyzed phosphate monoester hydrolysis, a SN1 mechanism with formation of a (POj) intermediate competes with the SN2 mechanism. For alkyl phosphates, nucleophilic attack at the carbon atom is also relevant. In contrast, all enzymatic cleavage reactions of mono-, di-, and triesters seem to follow an SN2... 

Retention of configuration can only occur, in a reaction that has a trigonal bipyramidal transition state, if the leaving group occupies an apical, and the entering group a radial position (or vice versa), as in 13. Three cases of retention... 

Whereas the rapidly equilibrating mixture of diastereomers 63 B,B was optically stable at room temperature, racemization occured when solutions in toluene were heated for several hours. A kinetic study revealed a racemization barrier of AG 8 = 27.6 2 (115.7. + 8.2) kcal(kJ)/mol, for which high energy Berry processes running through the decisive, achiral trigonal-bipyramidal transition states 66, 67 with diequatorial 2,2 -biphenylylene ligands were made responsible 74). Equivalent... 

These reactions show that there is no loss of configuration as substitution occurs. For these second- and third-row metals, splitting of the d orbitals produced by en and Cl- is considerably larger than it is in the case of first-row metals. As mentioned previously, the formation of a square-based pyramid transition state is accompanied by a smaller loss in LFSE than is the formation of a trigonal bipyramid transition state. Thus, attack by the... 

In the trigonal bipyramid transition state, the attack can be along any side of the trigonal plane (where there is usually more free space), so it can result in either a cis or trans product. This situation is shown as follows where the ligand A is in an axial position in the transition state ... 

This anion was shown to undergo an intramolecularligand via a trigonal-bipyramidal transition state with the hydrogen atom in an axial position. The energy barrier for this process was calculated to be AG = 66.5 kJ mol [2]. The coalescence phenonema observed in the NMR studies of 4-6 might be... 

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