Hypervalent molecules geometries

A variation on MNDO is MNDO/d. This is an equivalent formulation including d orbitals. This improves predicted geometry of hypervalent molecules. This method is sometimes used for modeling transition metal systems, but its accuracy is highly dependent on the individual system being studied. There is also a MNDOC method that includes electron correlation. 

Three basis sets (minimal s-p, extended s-p and minimal s-p with d functions on the second row atoms) are used to calculate geometries and binding energies of 24 molecules containing second row atoms, d functions are found to be essential in the description of both properties for hypervalent molecules and to be important in the calculations of two-heavy-atom bond lengths even for molecules of normal valence. 

The cu-bonding model provides a more complete and fundamental description of hypervalent molecules that are often interpreted in terms of the VSEPR model.144 In the present section we examine some MX species that are commonly used to illustrate VSEPR principles, comparing and contrasting the VSEPR mnemonic with general Bent s rule, hybridization, and donor-acceptor concepts for rationalizing molecular geometry. Tables 3.32 and 3.33 summarize geometrical and NBO/NRT descriptors for a variety of normal-valent and hypervalent second-row fluorides to be discussed below, and Fig. 3.87 shows optimized structures of the hypervalent MF species (M = P, S, Cl n = 3-6). 

This applies, of course, only to conventional molecules molecules of exotic structure (note the remarks for the geometries of hypervalent molecules and molecules ofunusual structure in section 6.3.1) may defy accurate SE predictions. 

In a continuation of their momentous work on the structure of hypervalent molecules, Holmes et al have reported the synthesis and structure (by X-ray crystallography) of a unique geometry in a pentacoordinate tetraoxyphosphorane... 

The hypervalent molecule PF5 adopts a trigonal-bipyramidal (TBP) geometry (6-12). The fluorine atoms Fi, F2, andF3 define the base of the bipyramid (the xy plane, written aj,) and are situated at the vertices of an equilateral triangle (the angles between bonds are 120°). The atoms... 

So far our discussion of hybridization has extended only to period 2 elements, specifically carbon, nitrogen, and oxygen. The elements of period 3 and beyond introduce a new consideration because in many of their compounds these elements are hypervalent—they have more than an octet of electrons around the central atom, oco (Section 8.7) We saw in Section 9.2 that the VSEPR model works well to predict the geometries of hypervalent molecules such as PCI5, SFs, or BrFj. But can we extend the use of hybrid orbitals to describe the bonding in these molecules In short, the answer to this question is that it is best not to use hybrid orbitals for hypervalent molecules, as we now briefly discuss. 

Thus, we argue that the fact that SeF is distorted C2y coupled with the realization that the fluorines are too far apart for any appreciable nonbonded interaction, means that the stereochemistry of AF (A = S, Se) "hypervalent" molecules is controlled by an interplay of "nonclassical" bonding effects, which allow ready distortions only in the geometry, and "classical" coulomb nonbonded repulsions which can be minimized without adversely affecting core-ligand bonding in the same geometry. In addition, we conclude that SH is not at all a model of SF at... 

Hund snile, 110, 122 Hiickers rule. 217 Hiickel dieory, 211 Hybrid oiUiais, 77 HydraziiK, 170 Hyperconjugation, 173 Hypervalent molecules, 258 d orbital participation, 261 geometries of, 269... 

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