Trigonal-bipyramidal molecules

Antimony Pentachloride. Antimony(V) chloride [7647-18-9], SbQ, is a colorless, hygroscopic, oily Hquid that is frequently yeUow because of the presence of dissolved chlorine it caimot be distilled at atmospheric pressure without decomposition, but the extrapolated normal boiling point is 176°C. In the soHd, Hquid, and gaseous states it consists of trigonal bipyramidal molecules with the apical chlorines being somewhat further away than the... 

Arsenic pentachloride [22441-45-8] (arsenic(V) chloride), AsQ., is produced by irradiation of a solution of AsQ. chlorine at —105°C using ultraviolet light (22). Arsenic pentachloride is a pale yellow soHd (presumably because of an entrained chlorine impurity) which melts with partial decomposition at —50° C. Raman spectra show that it is a trigonal—bipyramidal molecule both in solution and in the soHd state. 

The axial bonds in a trigonal bipyramidal molecule are longer than the equatorial bonds. 

In silicon derivatives, X-ray studies of compound 27 were consistent with a covalently bonded trigonal bipyramidal molecule <2000CC565>. In addition, nucleophilic substitution at silicon for similar compounds was modeled either by NMR or X-ray techniques and both methods correlate in the calculation of % Si-O bond formation <2003JOM66, 2003JOM154>. 

In many respects the chlorine oxyfluorides resemble the chlorine fluorides. For example, they exhibit little or no self-ionization, but are amphoteric. With strong Lewis acids or bases they can form stable adducts. The tendency to form adducts was found (64) not to be so much a function of the relative acidity of the parent chlorine oxyfluoride but rather to depend on the structure of the amphoteric molecule and of that of the anion or the cation formed. The preferred structures are the energetically favored tetrahedron and octahedron. Consequently, a trigonal bipyramidal molecule, such as CIF3O (64), exhibits a pronounced tendency to form either a stable pseudotetrahedral cation or a pseudo-octahedral anion ... 

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). 

A trigonal bipyramidal molecule belongs to point group It differs significantly from the tetrahedral AB4 and octahedral ABh in that all of the ligands, the B atoms, are not equivalent. If we use the numbering scheme... 

Finally, some UPS data and tentative assignments pertaining to the paramagnetic, trigonal bipyramidal molecule WC1S are included in Table XLVI1. 

These considerations are well illustrated by looking at the Group 15 pentahalides. The bonding in the trigonal bipyramidal molecules has been discussed in Chapter 6, and the postulate of nd orbital participation helps to rationalise their occurrence. No nitrogen(V) halides are known, although NF5 may have been detected as a short-lived species at low temperatures. The cation NF4 occurs in solids, prepared by reactions such as ... 

The crystal structure of antimony pentachloride, SbCls, is hexagonal (D (, P63 jmmc, a0 = 7.49, and c0 = 8.01 A) with two molecules in the cell. The molecules are hep giving the simple notation 2P(h). In Figure 4.26, we see that the trigonal bipyramidal molecules have their C3 axes parallel to c0, the packing direction. The axial Sb—Cl distance is 2.34 A and that in the equatorial plane is 2.29 A. The closest Cl—Cl distances (3.33 A) are between axial Cl atoms between adjacent molecules. 

Lastly, we consider a system with non-equivalent positions. An example of such a system is the trigonal bipyramidal molecule AX5 with symmetry. As discussed previously, one possible scheme is the dsp3 hybridization, where dz2 is the only d orbital participating. 

Crystallization may in rare cases result in chemical transformation of the constituents of a chemical compound. For example, PCI5 is a trigonal bipyramidal molecule, but its crystal structure is composed of a packing of tetrahedral PCl and octahedral PClg ions. In contrast, PBrs in the crystalline state comprises PBr and Br ions formation of the PBr ion is precluded by steric repulsion between the bulky peripheral bromide ions. 

The most electronegative substituents of those surrounding the M atom participate in a hypervalent bond and tend to occupy the axial positions in a trigonal-bipyramidal molecule. 

Square pyramidal compounds can be derived formally from the corresponding trigonal bipyramidal molecules by addition of an electron pair. 

According to X-ray structural investigations, the two apical positions in these trigonal bipyramidal molecules are occupied by two sulfur atoms, one from each of the two xanthate groups2. 

The TR mechanism could also explain the positional exchange of ligands in other pentacoordinated, trigonal bipyramidal molecules where the central atom is an element other than phosphorus.133,136... 

When we say that a molecule has symmetry, we mean that certain parts of it can be interchanged with others without altering either the identity or the orientation of the molecule. The interchangeable parts are said to be equivalent to one another by symmetry. Consider, for example, a trigonal bipyramidal molecule such as PF5 (A5-I). The three equatorial P—F bonds, to Fj, F2, and... 

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