VSEPR rules

Which of the following species are the most likely to violate the VSEPR rules ... 

In many cases, the interactions are directed in accordance with the VSEPR rules, confirming the concept of (donor) —> a (acceptor) overlap. The above collection of structural data reveals that in most cases particular supramolecular networks were observed , not designed . 

The most widely used qualitative model for the explanation of the shapes of molecules is the Valence Shell Electron Pair Repulsion (VSEPR) model of Gillespie and Nyholm (25). The orbital correlation diagrams of Walsh (26) are also used for simple systems for which the qualitative form of the MOs may be deduced from symmetry considerations. Attempts have been made to prove that these two approaches are equivalent (27). But this is impossible since Walsh s Rules refer explicitly to (and only have meaning within) the MO model while the VSEPR method does not refer to (is not confined by) any explicitly-stated model of molecular electronic structure. Thus, any proof that the two approaches are equivalent can only prove, at best, that the two are equivalent at the MO level i.e. that Walsh s Rules are contained in the VSEPR model. Of course, the transformation to localised orbitals of an MO determinant provides a convenient picture of VSEPR rules but the VSEPR method itself depends not on the independent-particle model but on the possibility of separating the total electronic structure of a molecule into more or less autonomous electron pairs which interact as separate entities (28). The localised MO description is merely the simplest such separation the general case is our Eq. (6)... 

The GHO basis can therefore provide a localised, directional set of orbitals (hybrids) which do not have the principal qualitative disadvantage of the usual hybrid sets they can be mutually orientated in any directions. What is more the directions taken up by the GHOs can be decided variationally and not by the unitary properties of a hybridisation matrix . This conclusion means that the use of a GHO basis provides both a localised bonding picture and simultaneously a theoretical validation of the VSEPR rules. Thus, it is not necessary, for example, to contrast the hybrid method and the VSEPR method for molecular geometries (30) they are complementary. 

Next, determine the electron-pair geometry around N using VSEPR rules. Because there are four electrons pairs around N, the electron-pair geometry is tetrahedral (or sp hybridization). 

These simplified VSEPR rules may seem a far cry from the more elegant application of symmetry and molecular orbitals to the beryllium hydride molecule and the nitrite ion (Chapter 5). or the BH2 molecule (Problem 6.27). Although the molecular orbital approach can rationalize these structures, the direct application of the VSEPR rules is by far the easier way to approach a new structure. 

Bader and Gillespie [31] have explained exceptions from the VSEPR rules in terms of core distortion using an analysis of the laplacian of the electronic density [32], VSEPR first assumes that the electron density at the core is... 

Table I. This is the basis of our approach and is really quite a simple one to use, although obviously not as simple as the VSEPR rules (18) in main-group stereochemistry.

Figure 2 Examples of various geometries based on VSEPR rules. Also given is the N-X-L notation (Section 2.3). Note that the lone pair of electrons in SeCle is in a stereochemically inactive 4s orbital...

The most detailed structural work on pentacoordinate structures has been carried out and compiled by Holmes, and some of his findings are summarized here. Basic VSEPR rules can account qualitatively for geometries with simple acyclic ligands for example, the series Me PF5 (n = 1 -3). However, as ligands become more complex, other factors predominate. Preference rules have been derived for both the tbp and sp geometries and are listed here in order of probable importance. 

For xenon fluorides and oxides, for example, the same models can be apphed as for interhalogen and halogen oxy species. Furthermore, the very successful valence shell electron pair repulsion (VSEPR) rules for molecule and ion shapes are as effective for noble gas compounds and their relatives as for classical octet compounds. 

It is interesting to note that the coordination environment in the halidotellurate anions is an almost regular octahedron in apparent violation of the VSEPR rules for a 14 electron AXgE system. It has been suggested that the lone electron pair of tellurium resides in the stereochemically inactive 6s orbital and the six Te-X bonds form three pairs of 3c-4e bonds involving the three 6p orbitals (for a more detailed discussion, see Ref 81). 

The square pyramidal (C4v) form of TaH5 is predicted to be 20kcal mol-1 more stable than its trigonal bipyramidal (D3h) isomer.9 This contradicts the VSEPR rules, a consequence of second-order Jahn-Teller effects. 

Use VSEPR rules to detennine the molecular shape and bond angle(s). (n-bonds are formed from the overlap of atomic orbitals [unhybridized] in a side-by-side arrangement.)... 

S4.8 If the N atom lone pair of (H3Si)3N is delocalized onto the three Si atoms, it cannot exert its normal repulsion influence as predicted by VSEPR rules. The N atom of (HsSifsN is trigonal planar, whereas the N atom of (HaOsN is trigonal pyramidal. The structures of (H3Si)3N and (H3C)3N, excluding the hydrogen atoms are ... 

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