Octahedral VSEPR structure

Lewis structures as well, except that the three lone pairs of electrons on each fluorine atom have been omitted. VSEPR theory predicts that a species with six bonding pairs of electrons, such as IF6, should be octahedral. Possible structures of species with seven bonding pairs of electrons, like IFt, were not covered in Section 2.3, but a reasonable and a symmetrical structure would be a pentagonal bipyramid. 

It is noteworthy that the to-bonded structure for ArF6 differs from that predicted by VSEPR theory. ArF6 is predicted to be of octahedral (Oh) symmetry, with three mutually perpendicular F i- Ar -h F triads and an s-type lone pair. In contrast, VSEPR predicts a pentagonal bipyramid (or other seven-vertex polyhedron) with some or all F-Ar-F angles less than 90°. The calculated equilibrium structure is in agreement with the co-bonding model. 

Only the octahedral W(CO)6 final product of the co-addition sequence has a VSEPR-like structure (but the three additional lone pairs on W would also lead to a different prediction for this case if strict VSEPR reasoning were applied). The intermediate 12e W(CO)3, 14e W(CO)4, and 16e W(CO)s molecules are all perfectly stable as isolated species. However, it is clear that, in the presence of excess CO, only the 18e W(CO)6 species will remain as a final product, and the 18e configuration will therefore appear to exhibit special stability. ... 

D—SiC, is tetrahedral. BrF4 is square planar. C2H2 is linear. TeF6 is octahedral. N03 is trigonal planar. If you are uncertain about any of these, Lewis structures and VSEPR are needed. 

Compounds with six fluorine ligands have provided structural difficulties. Xenon hexafluoride exists as the monomer in the vapour state and has been shown largely by electron diffraction experiments to be non-octahedral, and also fluxional.44 Although VSEPR theory might have... 

The structures of these ions normally conform to those predicted by the VSEPR theory, as shown in Fig. 17.2.2. Since the anion XY has two more electrons than the cation XY+, they have very different shapes. The anion IFj- is planar with lone pairs occupying the axial positions of a pentagonal bipyramid. In [Me4N](IF6), IFg is a distorted octahedron (C3V symmetry) with a sterically active lone pair, whereas both BrFg and ClFg are octahedral. The anion IF " has the expected square antiprismatic structure. 

A selection of the most important halides is show in the following table. With sulphur the fluorides are most stable and numerous, but Se and Te show an increasing range of heavier halides. Compounds such as S2C12 and S2F10 have S S bonds S2F2 has another isomer S = SF2. Sulphur halides are molecular and monomeric with structures expected from VSEPR (e.g., SF4 see-saw, SF6 octahedral). 

In the vast majority of cases in which six coordination is observed, the bonding can be viewed as arising from the interaction of all three cr -orbitals with a halide anion, i.e., all three in S. Because the three orbitals are all trans to the primary E-X bonds, such a situation leads naturally to octahedral coordination. Moreover, in cases in which the primary and secondary bonds are the same length, i.e., where A = 0 and a three-center, four-electron bonding model is appropriate, a regular octahedron is the result. Such a structure is clearly at odds with simple VSEPR theory, which is predicated on the lone pair(s) occupying specific stereochemical sites, but stereochemical inactivity of the lone pair tends to be the rule rather than the exception in six-coordinate, seven-electron pair systems Ng and Zuckerman (102) have reviewed this topic for p-block compounds in general. 

The structures normally are those predicted by VSEPR theory. Triatomics are angular, BrF4 has a structure like SF4, while IF is octahedral C12F is bent and asymmetric, Cl—Cl—F+. ... 

Use the VSEPR model to predict whether XeFs has an octahedral structure. Solution... 

The VSEPR model for predicting structure does not work for complex ions. However, we can safely assume that a complex ion with a coordination number of 6 has an octahedral arrangement of ligands, and that complexes with two ligands are linear. On the other hand, complex ions with a coordination number of 4 can be either tetrahedral or square planar there is no reliable way to predict which will occur in a particular case. 

In the tellurium(IV) fluoride structure with chains of corner-sharing TeFs tetragonal pyramids (100), the lone pair (E) is stereochemically much more active and occupies one position in the (/(-octahedral TeFsE polyhedra, in accordance with simple VSEPR considerations (Fig. 2). 

My research has been mainly in superacids, sulfuric acid, fluorosulfuric acid and hydrogen fluoride, which led me naturally into fluorine chemistry. I didn t do a lot more on VSEPR except for trying to promote it. I was excited by the discovery of the noble-gas compounds, and it was an obvious opportunity to show that it was easy to use VSEPR to predict their structures. At one of the early conferences on noble-gas chemistry, Larry Bartell asked me to predict the structure of XeF. I believe he had already determined the structure by electron diffraction but not yet published it. I told him that it could not be octahedral, which the MO theorists believed they had proved, but was probably a distorted monocapped octahedron, which turned out to be correct. Since then, he has been a great fan of VSEPR and has done much to promote it as a useful theory. 

Six is the most common coordination number. The most common structure is octahedral some trigonal prismatic structures are also known. If a metal ion is large enough to allow six ligands to fit around it and the d electrons are ignored, an octahedral shape results from VSEPR arguments. Such compounds exist for all the transition metals with to d configurations. 

E2.7 The Lewis structure of ICV and its geometry based on VSEPR theory is shown below. The central iodine atom is sunounded by six bonding and one lone pair. However, this lone pair in the case of ICU" is a slereochemically inert lone electron pair. This is because of the size of iodine atom—large size of this atom spread around all seven electron pairs resulting in a minimum repulsion between electron pairs. As a consequence all bond angles are expected to be equal to 90° with overall octahedral geometry. 

The amended VSEPR model predicts two forms of five-coordination, and experimental chemistry has clearly identified many examples of both forms. These limiting structures are square-based pyramidal (or, simply, square pyramidal) and trigonal bipyramidal (Figure 4.13). The classical square-based pyramidal shape is formed simply by cleaving off one bond from an octahedral shape, which leaves the metal in the same plane as the four square-based ligands. In reality, almost no complexes exhibit this shape, but rather adopt a distorted... 

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