Hypervalency

3 Hypervalency. - Dobado et a/.173 used AIM to focus on chemical bonding in hypervalent molecules such as Y3X and Y3XZ (Y = H or CH3 X = N, P or As Z = or S). The nature of the P- bond in particular has been extensively reviewed and explained in terms of a combination of two different descriptions, R3P+-0 and R3P=0. The former structure obeys the octet rule but requires 

21 Hypervalency. - Dobado et used the electron pair density in 

MoUna and Dobado revisited the nature of the 3c-4e bond via AIM and ELF. They performed B3LYP/6-311++G calculations on a series of formally hyper-valent compounds showing linear three-centre geometries. Their results supported the 3c-4e model for the linear structures but revealed only a small contribution from this model for the T-shaped structures. In addition there was no evidence to support the 3c-4e bond scheme for the bipyramidal compounds. 

The standard method of explaining how such molecules can be stable is to invoke the interactions of filled p or hybrid orbitals on the ligands with an empty d orbital on the central element. Like any interaction of filled with unfilled orbitals, interactions with empty d orbitals are bound to be stabilising, but d orbitals are too high in energy relative to the p orbitals for their interaction to have any significant effect. 

4 Polar Interactions, and van der Waals and other Weak Interactions 

Furthermore, polar attractions from one polar molecule to another, or from one strongly hydrogen-bonding molecule to another, lead such molecules to aggregate, and to exclude nonpolar molecules. This is the basis for the well-known hydrophobic effect, in which nonpolar molecules stick together to avoid being in water. 

There is a somewhat similar phenomenon in which the presence of a dipole within a molecule induces a temporary dipole, either elsewhere in the molecule or in another molecule. The induced dipole is then attracted to the inducing charge or dipole, and another small attractive force comes into play that is not included in the molecular orbital picture at the most simple level of calculation, but is included when larger basis sets are used. Weak dipolar attractions like these, both the static and the induced, are not strong, and so nonpolar molecules are not well solvated by polar molecules the polar solvent molecules would rather solvate each other and the nonpolar molecules are left to their own devices. As it happens they do not repel each other as much as one might expect. 

We saw correlation earlier on p. 5, when we learned that two electrons can be placed into one orbital, provided, of course, that their spins are opposed. The correlated movement of the electrons within that orbital keeps them, on average, far apart but there is an energetic penalty from putting a second electron into an orbital already singly occupied. The electron correlation reduces the severity of the penalty, and is often needed in calculations to get the right answers. In the same way, van der Waals interactions often have to be invoked when calculations based on molecular orbitals and dipolar effects do not explain all the attractions or repulsions found in practice. 

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The first quantum mechanical improvement to MNDO was made by Thiel and Voityuk [19] when they introduced the formalism for adding d-orbitals to the basis set in MNDO/d. This formalism has since been used to add d-orbitals to PM3 to give PM3-tm and to PM3 and AMI to give PM3(d) and AMl(d), respectively (aU three are available commercially but have not been published at the time of writing). Voityuk and Rosch have published parameters for molybdenum for AMl(d) [20] and AMI has been extended to use d-orbitals for Si, P, S and Q. in AMI [21]. Although PM3, for instance, was parameterized with special emphasis on hypervalent compounds but with only an s,p-basis set, methods such as MNDO/d or AMI, that use d-orbitals for the elements Si-Cl are generally more reliable. 

Because carbon is a first-row element unable to extend its valence shell, hypervalence cannot exist in carbon compounds, only hypercoordination. 

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. 

We have encountered oscillating and random behavior in the convergence of open-shell transition metal compounds, but have never tried to determine if the random values were bounded. A Lorenz attractor behavior has been observed in a hypervalent system. Which type of nonlinear behavior is observed depends on several factors the SCF equations themselves, the constants in those equations, and the initial guess. 

Oxidation of 4H-pyran-4-thiones with thallium(III) trifluoroacetate was used in the one pot synthesis of l,6-dioxa-6n-thiapentalenes, a hypervalent heterocyclic system [57] (equation 27)... 

A common interpretation of the interaction of chalcogens with nucleophiles considers donation of electron density from a lone pair on the donor atom into the o- (E-X) orbital (Figure 15.1). As the degree of covalency increases, a hypervalent three-centre four-electron bond is formed. Real systems fall somewhere between secondary interactions and hypervalent (three centre - four electron) bonds. The two extremes can be distinguished by the correlation of X-E and E D distances.In the hypervalent case both bond distances decrease simultaneously, whereas in the secondary bond the distances are anticorrelated. This concept has been applied in a study of selenoquinones 15.17 (R = Ph, Me) with short Se 0 contacts,for... 

It includes a significant number of molecules with unusual electronic states (for example, ions, open shell systems and hypervalent systems). 

CF,C02)2lPh, H2O, CH3CN, 85-99% yield. In the presence of ethylene glycol the dithiane can be converted to a dioxolane (91% yield) or in the presence of methanol to the dimethyl acetal. The reaction conditions are not compatible with primary amides. Thioesters are not affected. A phenylthio ester is stable to these conditions, but amides are not. The hypervalent iodine derivative l-(t-butylperoxy)-l,2-benziodoxol-3(l/f)-one similarly cleaves thioketals."... 

The hypervalent silicon hydride anion, SiHj (cf. SiFs below), has been synthesized as a reactive species in a low-pressure flow reactor ... 

Hypervalent molecules incorporate elements with more than a normal complement of eight valence electrons (an octet). 

Phosphorous ylides such as triphenylphosphine-metJhylidene may either be represented as hypervalent species incorporating a phosphorous-carbon double bond, or in terms of a zwitterion, that is, a molecule with separated positive and negative charges. 

Examine the charge on the methylidene group, as well as the magnitude and direction of the molecule s dipole moment. Are they consistent with representation of the ylide as a hypervalent molecule or as a zwitterion ... 

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. 

G1 theory does badly with ionic molecules, with triplet-state molecules such as O2 and S2 and with hypervalent molecules. Gaussian-2 (G2) theory eliminates some of these difficulties by making the following three changes ... 

Hypervalent molecules, like sulfoxides and sulfones, are too unstable. 

If only one set of polarization functions is used, an alternative notation in terms of is also widely used. The 6-31G=i basis is identical to 6-31G(d), and b-SlG ts is identical to 6-31G(d,p). A special note should be made for the 3-21G basis. The 3-21G basis is basiciy too small to support polarization functions (it becomes unbalanced). However, the 3-21G basis by itself performs poorly for hypervalent molecules, such as sulfoxides and sulfones. This can be substantially improved by adding a set of d-functions. The 3-2IG basis has only d-functions on second row elements (it is sometimes denoted 3-21G(=f=) to indicate this), and should not be considered a polarized basis. Rather, the addition of a set of d-functions should be considered an ad hoc repair of a known flaw. 

Preparation of heterocyclic betaines using hypervalent Te-, I-, and Xe-reagents 99JHC1573. 

Chemical transformations of heterocycles induced by hypervalent iodine reagents 97T1179. 

Synthesis of heterocyclic compounds using hypervalent iodine reagents 98AHC(69)1. 

Recently, the ring enlargement of 4-hydroxy-2-cyclobutenones 5 was promoted by PhI(OAc)2, a popular and accessible hypervalent iodine reagent (99JOC8995). Thus, when 5a-c (R = Me, Bu, Ph) were treated with a slight excess of PhlfOAcja in dichloromethane at room temperature, the 5-acetoxy-3,4-diethoxyfuranones 13... 

On the other hand, the fluorine-induced addition of the diastereomeric silyl-subsliluted sulfides 36 A and 36B to benzaldehyde proceeds without loss of stereochemical information and with retention of configuration32. Since, however, the anionic reagent 35A/35B is known to be configurationally labile, the observed retention of configuration in the fluorine-induced desi-lylative hydroxy alkylation lends experimental evidence to the notion that these reactions proceed via hypervalent silicon species rather than anionic reagents. 

An intermediate allyltin trichloride, possibly stabilized by an oxygen-tin hypervalent interaction, which then reacts with the aldehyde via a chair-like six-membered ring transition state with the substituent a to tin in an axial position, may be involved94. On heating with aldehydes, the 4-benzyloxypentenylstannane shows only modest diastereofacial selectivity22. 

Optically active 1-alkoxyallylstannanes are more readily available by asymmetric reduction of acylstannanes using either ( + )-(/J)-BINAL-Il105 106 or LiAlH4-Darvon alcohol [(2S,3/ )-4-dimethylamino-3-mcthy]-1,2-diphenyl-2-butanol] 06 followed by O-alkylation. The stereoselectivity of the BINAL-H reductions differs from that usually observed, and has been attributed to a tin-oxygen hypervalent interaction107, l08. 

So far, there is no conclusive evidence that a free allyl carbanion is generated from allylsilanes under fluoride ion catalysis. A hypervalent silyl anion, with the silicon still bonded to the allylic moiety, accounts equally well for the results obtained. Based on a variety of experimental results, it is in fact more likely that a nonbasic hypervalent silyl anion is involved rather than the basic free allyl carbanion first postulated14-23. When allylsilanes are treated with fluoride in the presence of enones. 1,4-addition takes place along with some 1,2-addition13. 

The composition of I, and possibly its structure, may be deduced by identifying Q. Certain examples from peroxide chemistry will illustrate the scope of the method. The reactions of ferrous(nitriloacetate) and ferrous(ethylenediamine-N,N -diacetate) with hydrogen peroxide are complicated processes.1 A particular scavenger T did indeed divert the reaction at high concentrations of T. The required levels of T were, however, much higher than those that would have been needed to trap the hydroxyl radical, HO. It is thereby ruled out. With this and with spectroscopic evidence, a reactive hypervalent iron complex was suggested as the intermediate. 

The foregoing discussion indicates that while there are difficulties in the way of a bonding role for 3d orbitals, for certain situations at least it is possible to conceive of ways in which these difficulties may be overcome. However, it is necessary to say that even for hypervalent molecules such as SF6 which seem to require the use of d orbitals, there are molecular orbital treatments not involving the use of d orbitals. In fact, as shown by Bent in an elegant exposition12, the MO model of SF6 involving the use of d orbitals is only one of several possibilities. The octahedral stereochemistry of SF6, traditionally explained in... 

Two polymer-supported reagents have been developed for the oxidation of sulphoxides to sulphones these involve peracid groups150, and bound hypervalent metals activated by t-butyl hydroperoxide151,152. 

Octet expansion (expansion of the valence shell to more than eight electrons) can occur in elements of Period 3 and later periods. These elements can exhibit variable covalence and be hypervalent. Formal charge helps to identify the dominant resonance structure. 

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