Hypervalent compounds

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. 

In recent years, progress has been made in extending these methods to rf-orbitals (e.g. MNDO/d [23, 24]) so that many of the hypervalent compounds of main group elements can now be treated using these techniques. More importantly, the addition... 

The main purpose of this chapter is to review the development of hypervalent chalcogen chemistry after the book of "Chemistry of Hypervalent Compounds edited by Akiba (1999).5 Although several types of compounds may belong to hypervalent ions or molecules by the definition,3a such hypervalent chalcogen compounds (E = S, Se, and Te) will be mainly discussed here that contain CT-type linear bonds.15... 

Hypervalent compounds of 35 13, which belong to 10-E-4 (C4) are also well investigated by the X-ray analysis and NMR. The kinetics, reactivity, and the dynamic properties of the compounds are studied. The results are summarized in Scheme 4.10b,15b... 

K.-y. Akiba (ed), Chemistry of Hypervalent Compounds, Wiley-VCH, New York, 1999. 

PPh2).282 Configurationally stable diastereomeric hypervalent compounds 54-56 and related compounds with... 

MNDO, AMI, and PM3 employ an sp basis without d orbitals [13, 19, 20]. Hence, they cannot be applied to most transition metal compounds, and difficulties are expected for hypervalent compounds of main-group elements where the importance of d orbitals for quantitative accuracy is well documented at the ab initio level [34], To overcome these limitations, the MNDO formalism has been extended to d orbitals. The resulting MNDO/d approach [15-18] retains all the essential features of the MNDO model. 

Limitations of AMI. AMI is still a relatively new method. As such, only a few limitations have become apparent thus far. One of the more important of these are the continued inability to adequately represent hypervalent chemistry. Very recently, AMI parameters for phosphorus and sulfur have become available. Preliminary results indicate these are a considerable improvement over the MNDO method for predicting the AHf of hypervalent compounds. Unfortunately, the phosphorus, parameters include a barrier at 3.0A which complicates the interpretation of reaction mechanisms. For example, in the reaction PF3 + F2 = PF5. there are two spurious barriers which must be crossed before the real barrier to the reaction is encountered. 

Abstract The term Lewis acid catalysts generally refers to metal salts like aluminium chloride, titanium chloride and zinc chloride. Their application in asymmetric catalysis can be achieved by the addition of enantiopure ligands to these salts. However, not only metal centers can function as Lewis acids. Compounds containing carbenium, silyl or phosphonium cations display Lewis acid catalytic activity. In addition, hypervalent compounds based on phosphorus and silicon, inherit Lewis acidity. Furthermore, ionic liquids, organic salts with a melting point below 100 °C, have revealed the ability to catalyze a range of reactions either in substoichiometric amount or, if used as the reaction medium, in stoichiometric or even larger quantities. The ionic liquids can often be efficiently recovered. The catalytic activity of the ionic liquid is explained by the Lewis acidic nature of then-cations. This review covers the survey of known classes of metal-free Lewis acids and their application in catalysis. 

Limiting (6-311+G basis set) Hartree-Fock models consistently overestimate the magnitudes of dipole moments in hypervalent compounds. The largest error is 1 debye (for dimethylsulfoxide), but errors for several other compounds are 0.5 debye or greater. The 6-3IG model provides comparable (and generally slightly better) dipole moments, whereas results from STO-3G and 3-2IG models... 

In terms of mean absolute errors, local density models with the 6-3IG basis set perform better than the corresponding Hartree-Fock model, as well as (and generally better than) any of the density functional models, and better than MP2 models. This parallels previously noted behavior for equilibrium geometries of hypervalent compounds (see Table 5-8). 

In the case of the other hypervalent element compounds, these structural differences are strictly reflected in their terminology. For instance, sulfonium salts such as Me3S+Cl are clearly differentiated from sulfuranes such as Ph2SCl2. The latter is a hypervalent species of decet structure [10-S-4] and pseudotrigonal bipyramid with a linear Cl-S-Cl hypervalent bond however, the former is not a hypervalent compound and has pseudotetrahedral geometry with octet structure [8-S-3]. 

Fig. 4) becomes more pronounced at the phosphorus atom in the hypervalent compound. Calculation of the aromaticity% [127] revealed similar tendencies. While phosphole itself exhibits 18% aromatic character, for phosphole oxide -13% has been reported [176], It is noteworthy that for 1-methoxyephosphole and 1-fluoro-phosphole small negative values (-4.9% and -9.2%) were also obtained, in agreement with the conclusions of Mathey and coworkers [169]. Also, the phosphole sulfide and selenide exhibited smaller antiaromaticity (-11.6% and - 9.7%, respectively) [176] than the oxide, in agreement with the data in Table 2 [61]. It is also worth noting that the decrease of the NICS value and the conjugation upon sulfur addition has been noted in a phosphole-thienyl and a phosphole-pyridyl oligomer by Delare et al. [177],... 

However, the reactivity of the phosphinines is somewhat enhanced in comparison with benzene. Not only the lone pair at P is a reactive center, but phosphorus can also be attacked by electrophiles to form hypervalent compounds. Furthermore, stabilization of the unoccupied levels, which was discussed in conjunction with the electron transmission spectra of phosphinine and 1,3,5-triphosphinine [14], also allows for an easy nucleophilic attack [4, 279],... 

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