Rotation trigonal bipyramidal molecules

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). 

In addition to rotation and inversion, there are other mechanisms by which conformational interconversion may occur. One of these, pseudorotation , is most easily seen in a molecule like phosphorous pentafluoride which adopts a trigonal bipyramidal equilibrium geometry with distinct equatorial and axial fluorines. 

S-F bond. The structure of the resulting molecule F3SSF is, in a sense, intermediate between those of S2F2 and SF4, being based on a trigonal bipyramid with the equatorial F atom replaced by an SF group. The fact that the F nmr spectrum at —100° shows four distinct F resonances indicates that the 2 axial F atoms are non-equivalent, implying restricted rotation about the S-S bond. 

CO)4Fe=Si(OCMe3)2(HMPA) revealed a trigonal-bipyramid coordination geometry for the iron atom, and a Fe-Si bond distance of 2.289 (2) A. The Si-O(HMPA) distance of 1.730 (3) A is significantly longer than the other Si-0 distances [1.610 (3) and 1.636 (4) A] in the molecule. Rotation about the Fe-Si bond is unrestricted down to -30°C118. 

Single-crystal X-ray diffraction (at 109 K) data show that PF5 has a trigonal bipyramidal structure, 15.32. In solution, the molecule is fluxional on the NMR spectroscopic time-scale and one doublet is observed in the NMR spectrum, i.e. all F environments are equivalent and couple with the P nucleus. This stereochemical non-rigidity is another example of Berry pseudo-rotation (see Fig. 4.24). 

CONTEXT Phosphorus pentachloride, PF5, is a popular molecule in general chemistry classes for exemplifying the trigonal bipyramidal shape in VSEPR theory The structure is highly symmetric, having two equivalent axial fluorines (so-called because they lie on the principal rotational axis of the molecule) and three equivalent equatorial fluorines. However, studies of the molecular symmetry led to the discovery of an unexpected type of molecular motion. 

A similar series of five-coordinate rhodium(I) molecules has been examined. [RhCl(CO)(i7 -C2H4)(biL)] (biL = diimine) are trigonal bipyramidal with axial Cl and CO in CDCI3. The ethylene (which rotates on the nmr time scale) is readily lost. The related compounds [RhCl(CO)2(biL)] and [RhCl(PF3)2(biL)] also contain trans-axial Cl and CO or Cl and PF3. Intramolecular exchange of the CO sites is rapid, however, suggesting an easier pseudorotation type of motion for these complexes. 

Consider a trigonal bipyramidal [CuCls] ion, as shown in Figure 7. Here, rotation about a C3 axis in clockwise and anticlockwise directions restores the identity of the molecule. However, both these operations yield the molecules with the positions of chlorines that are different from the... 

Coming back to the mentioned PF5 and Fe(CO)5 molecules, the fact is that NMR spectra do not register different distances between the central metal atom and the equatorial and axial ligands. The explanation was given by Berry, and the suggested mechanism was named Berry s mechanism or pseudo-rotation. Figure 4.1.1 is a schematic illustration of Berry s mechanism for the trigonal bipyramid ... 

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