Geometries of Hypervalent Molecules

Density of states plot for La2CdGe2 obtained at the extended Huckel level. The dashed line indicates the projection of La valence s, p, and d levels. The Fermi level is marked by the dotted line. 

Idealized Walsh diagram for the degenerate H—A—H bending modes in a Dsh AH3 molecule. The A—H bonding MOs, a and 1e, lie at lower energies and are not shown in the figure. 

Experiments on the reaction of F with XeF2 in the gas phase result in the production of XeF3 amongst other species [40], This compound now has 12 valence electrons. A singlet electronic state would pair two electrons in the 2e set for a D3h geometry. This will be unstable it is a first-order ]ahn-Teller problem where calculations [40] show that both Y and T geometries, 14.44 and 14.45, respectively, represent more stable structures. This is precisely analogous to the 

Assembly of the molecular orbital diagram for trigonal bipyramidal AH5 from the levels of A and of trigonal planar AH3 and those of H2- 

The axial ligands are clearly attached by three-center-four electron bonds in this molecule. The remaining p AO on A d overlaps with the antisymmetric combination of H s orbitals the bonding combination is filled. The symmetric combination of H s AOs is then left filled and nonbonding. As a result the axial linkages are longer than the equatorial ones in PF5, 14.54 [46]. Recall that one of the results of 

The energy level diagram for the distortion of the square plane, 

Numerical calculations suggest that the Qy structure is the lower energy isomer for the (hypothetical) SH4 molecule but the C2V structure is the lower energy isomer for SF4 (as obser ed). The energetic juxtaposition of these two structures leads to a ready pathway for the isomerization of SF4 (14.18). Notice that the initially axial ligands (of the VSEPR trigonal bipyramid) labeled with asterisks become 

Main group five-coordinate molecules are found cither as trigonal bipyramidal molecules (c.g., PFs) or as square pyramidal species (c.g., BrFs). Geometrically they are quite close as. shown in 14.20, a projection down the fourfold axis of the 

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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. 

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. 

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. 

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). 

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... 

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. 

In addition to halogen bonded complexes or ionic salts, it is also possible for sulfur and selenium electron donors to form complexes in which the electron donor atom inserts into the X2 bond, giving a hypervalent donor atom with a T-shaped geometry. It has been recently reported [147] that for dibromine and selenium, this type of complex is favored over halogen bonded complexes. While no purely halogen bonded complex is reported for dibromine, there is one complex (IRABEI) in which one selenium atom of each of several selenanthrene molecules in the asymmetric unit does insert into a Br2 bond, but for one of the molecules, the other selenium atom forms a halogen bond with a Br2 molecule to form a simple adduct (A). 

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