Pyramidal intermediate

JOC1537). The mechanisms of these transformations may involve homolytic or heterolytic C —S bond fission. A sulfur-walk mechanism has been proposed to account for isomerization or automerization of Dewar thiophenes and their 5-oxides e.g. 31 in Scheme 17) (76JA4325). Calculations show that a symmetrical pyramidal intermediate with the sulfur atom centered over the plane of the four carbon atoms is unlikely <79JOU140l). Reactions which may be mechanistically similar to that shown in Scheme 18 are the thermal isomerization of thiirane (32 Scheme 19) (70CB949) and the rearrangement of (6) to a benzothio-phene (80JOC4366). 

Concept of " pseudorotation introduced by R. S. Berry to interpret the stereochemical non-rigidity of trigonal bipyramidal PF5 (and SF4, ClFi) the 5 F atoms are equivalent (1953) due to interconversion via a square pyramidal intermediate. 

Figure 19.5 The interconversion of trigonal bipyra-midal configurations via a square-pyramidal intermediate. Notice that the Li ligands, which in the left-hand tbp are axial, become equatorial in the right-hand tbp and simultaneously 2 of the L2 ligands change from equatorial to axial.

Pi the Berry step is seen as a double bending of an equatorial and an apical angle. The two apical ligands become equatorial and two equatorial ones go to apical positions. One of the equatorial ligands, the so-called pivot, is on the fourfold axis of the tetragonal pyramidal intermediate state. The connectivity i.e. the number of isomers reached from a given one in one step, is three. 

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. 

B. Formation of an intermediate species with prior or subsequent breaking of one metal oxygen bond (probably to form a trigonal pyramid intermediate) followed by loss of a proton and reformation of the stable chelate ring. 

Arsenic pentafluoride (b.p. -52.8 °C) is the only stable well-characterized arsenic(V) halide. Electron diffraction49 shows it to be a trigonal bipyramidal molecule, with equatorial d As—F) 1.656 A and axial d(As—F) 1.711 A 19F NMR shows that the five fluorines are equivalent. The apparent inconsistency displayed by the two techniques is due to the differing time scales of the measured effects. On the slower NMR time scale the axial-equatorial bonds rotate via a square pyramidal intermediate resulting in the observed equivalence (pseudorotation). 

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. 

Addition of triphenylphosphine or t-butylisocyanide to 101 affords the analogous five-coordinate complexes 102b and 102c. These species represent rare examples of stable five-coordinate ds-dialkyl complexes of Ni(II), although stable trigonal-bipyramidal trans-dialkyls of nickel are well known (176-178), and they provide a structural model for a putative square-pyramidal intermediate in reductive elimination reactions of ds-dialkyl... 

Reaction (19) causes larger isotope effect determined for product than effect for substrate due to additional fractionation of analysed carbon atom in the side reaction. For mechanistic analysis 13C KIE based on the substrate analysis was used. DFT calculations of isotope effects for each step of the reaction led to conclusion that the rate-determining step involves breaking of the P-C bond in the tetragonal pyramidal intermediate. 

Similar conclusions were drawn from detailed 1 H-NMR-analyses of the dynamic behaviour of the isopropylphenyl derivatives 39h, i where even the (very improbable) possibility of a tetragonal-pyramidal intermediate with freely rotating isopropylphenyl substituents was taken into consideration 81). 

It may then be asked how the small amount of cis-trans interconversion sometimes observed in aquation can come about. It is quite possible that a fraction of the pyramidal intermediates, X, rearrange to a trigonal bipyramidal intermediate (XI) before reacting with a water molecule. As indicated in Figure 23-2, two of the three most likely positions of attack will result in a cis product, whereas the third will result in a trans product. 

One final point about the mechanism of these reactions should be made. In the previous discussion of Mechanisms 2 and 3, it was assumed that the intermediate was a square pyramid and that no rearrangement to other geometries (such as trigonal-bipyramidal) occurred. Other labeling studies, involving reactions of labeled CH3Mn(CO)5 with phosphines, have supported a square-pyramidal intermediate. ... 

Fig. 4. Reaction profile with energy minima for trigonal-bipyramidal and square-pyramidal intermediates.

Fig, 5. Possible general reaction profile for gold(III) ligand replacements, dominated by a trigonal-bipyramidal transition state but allowing for square-pyramidal intermediates. 

Figure 10. New pathway for the scrambling of metal vertices in 86-electron octahedral metal clusters. The proposed edge-shared bitetrahedral and capped square pyramidal intermediates (or transition states) both maximize connectivity and preserve the 86-electron count...

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