Square-based pyramidal transition state

If this reaction were to take place by formation of a square-based pyramid transition state, the product would have a trans configuration. However, if the transition state is a trigonal bipyramid, the incoming ligand, Y, could enter either cis or trans to A. 

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... 

It appears that the square-based pyramid transition state is consistent with the fact that substitution in these second- and third-row metal complexes occurs without isomerization. This is primarily because less LFSE is lost in forming a transition state when this structure is formed. On the other hand, Co3+ and Cr3+, being first-row metals, may have the same number of Dq units lost in forming the transition state as do Ir3+ or W3+, but Dq is much... 

Fig. 3.13 Berry pseudo-rotation interconverts one trigonal bipyramidal structure into another via a square-based pyramidal transition state. The numbering scheme illustrates that axial and equatorial sites in the trigonal bip5ramid are interchanged.

Let us first consider the case of a substitution reaction in a complex of a d6 ion such as Co3+ in a strong field. If the process takes place by an SN1 process, the five-bonded transition state may be presumed to have either a trigonal bipyramid or square-based pyramid structure. The orbital energies will be determined as follows ... 

Thus, the fact there is no isomerization during substitution is consistent with the transition state being a square based pyramid for second and third row metals. As shown in Figure 20.4, substitution would give a product having the same configuration as the starting complex. 

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... 

Finally, a square-based pyramid structure is reached (the transition state for pseudorotation) in which the two ax/a/ fluorines and two of the equatoriar fluorines are equivalent. Motion continues until the two equatorial fluorines occupy axial positions and the two axial fluorines occupy equatorial positions (the remaining equatorial fluorine does not move). 

Hartree-Fock 6-3IG calculations show that pseudorotation in phosphorous pentafluoride is a very low energy process (6 kcal/mol) and that the square-based pyramid structure is indeed a transition state. 

Some of the manifestations of the ligand field stabilization energy (LFSE) have already been described. It can be shown that producing a five-bonded transition state in a dissociative process or a seven-bonded transition state in an associative process would invariably lead to a loss of LFSE except for a few cases such as d° or d5 high-spin ions where the LFSE is zero. However, the loss of LFSE is different in a dissociative process depending on whether the five-bonded transition state is a trigonal bipyramid or a square-based pyramid (sometimes called a tetragonal pyramid). 

Figure 15 The SDDS rearrangement of square-based pyramidal CsHs, which proceeds through a C2v symmetry transition state. The diagram is arranged as for Figure 13 and the triangulation cutoff is 2.1 A. (This figure was produced using Mathematica 2.0 Wolfram Research Inc., 1990.)...

Table 26.5 Changes in CFSE (ACFSE) on converting a high-spin octahedral complex into a square-based pyramidal (for a dissociative process) or pentagonal bipyramidal (for an associative process) transition state, other factors remaining constant (see text).

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