Hypercoordinate molecules

Other kinds of molecules besides 1 (which has unusual bond stereochemistry) for which these methods might fail to give good results are hypercoordinate molecules like NF5, molecules with noble gas atoms, particularly those of helium and neon, molecules with highly twisted C=C bonds, extraordinarily crowded molecules like hexaphenylethane, unknown dimers, trimers etc. of small familiar molecules, like C202 and N6, and very highly strained molecules. All these cases are discussed in a book on exotic molecules [4],... 

In spite of all the theoretical evidence, accumulated over many years, it is still commonplace for students to be taught that the existence of hypercoordinate molecules such as SF and PFri relies on the utilization of d orbitals to expand the octet . Indeed, even models based on d sp , dsp2 and dsp3 hybrid orbitals or pn-dn back-bonding are still in use to describe hypercoordinate bonding to second-row elements. Of course, the consensus view that has emerged from most of the... 

The basic strategy adopted for normal octet and hypercoordinate molecules XYn was first to carry out a standard closed-shell RHF calculation and then to localize the orbitals according to the population or overlap criterion introduced by Pipek and Mezey [12]. In all cases, it was straightforward to identify localized molecular orbitals (LMOs) associated with particular X—Y bonds. 

Schmidbaur, H., Gabba, F.P., Schier, A. and Riede, J. (1995) Hypercoordinate Carbon in Protonated Tetraauriomethane Molecules. Organometallics, 14, 4969 971. 

Summarizing the available bonding information, decamethylsilicocene (1) is regarded as an electron-rich silicon(II) compound containing a hypercoordinated silicon atom which is sandwiched between two rather weakly 7i-bonded pentamethylcyclopentadienyl ligands and thus is effectively shielded the lone-pair orbital at silicon is part of the frontier orbitals of the molecule. 

Recently, a fully characterized hypercoordinated protonated quinoline silanol (counterion = chloride) 804 has been described. This can also be regarded as intramolecularly coordinated triorganosiliconium cation, in which a neutral water molecule fills the vacant position in the trigonal-bipyramidal geometry of the silicon atom (Figure 5).815... 

Siehl, H. -U Fuss, M. Silyl effects in hypercoordinated carbocations IN. Organosilicon Chemistry IV, From Molecules to Materials Editors Auner, N. Weis, J. 2000, Wiley-VCH Weinheim, p. 140 - 149. 

For silylium ions, one cannot expect a similar independence of the medium, since Si easily extends its coordination sphere by forming hypercoordinated compounds with five or six ligands. [79] In solution, nucleophilic solvent molecules S (or counterions X ) will be suitable coordination partners (Scheme 1). 

There are many texts that make the point very clearly that the bonding in a molecule such as SFfi has very little to do with the availability of d atomic orbitals, but this is normally done in the context of MO theory, whereas the general ideas of utilizing d orbitals are much more closely allied with the ideas of classical valence bond theory. This, perhaps, is one of the reasons for the continued survival of such models. The purpose of this Chapter is to describe various calculations which have been performed using modern valence bond theory, in its spin-coupled form, resulting in a useful aide memoire which we term the democracy principle. We argue that there are no significant qualitative differences between the hypercoordinate nature of first-row, second-row and noble gas atoms in appropriate chemical environments. 

The bonds involving hypercoordinate atoms tend to be highly polar. There are no significant qualitative differences between the hypercoordinate nature of first-row, second-row and noble gas atoms in appropriate chemical environments, nor between the descriptions of the bonding in hypercoordinate and so-called normal octet molecules, except for some differences in bond polarity. 

The much more highly charged silicon atom can interact far more readily with nucleophiles. Silyl cations may even be complexed simultaneously and symmetrically by two electron pair donors (hypercoordination), in contrast to carbocations. With ammonia, the methyl cation gives the very stable protonated methyl amine, H3C-NH3 a second ammonia molecule is only weakly bound to this complex. If both NH3 groups are forced to be equidistant from carbon, a Sn2 transition state results, 20 kcal mol" higher in energy than the minimum. 

Tetragoldmethane complexes are obtained from tetra-(boryl)methane compounds upon reaction with gold-halide complexes in the presence of an ionic fluoride (equation 35)." Tetragoldmethane species, such as (1), can only be isolated with bulky tertiary phosphines, L, which shield the molecule from further attack by [TAu]+ nucleophiles. With smaller ligands T, penta- and hexaauration occurs, leading to hypercoordinate carbon compounds. " " " ... 

As is the case with alkyl bridges between aluminum atoms, these bridges between beryllium and magnesium atoms are relatively weak, and the metal orbitals are put to better use by addition of Lewis bases (L), which cleave the polymer chains, forming MR2L2 monomeric molecules, in which carbon atoms are no longer hypercoordinated [Eq. (2.8)]. In weakly basic solvents dimers (30) that retain alkyl bridges (and so hypercoordinate carbon atoms) may be formed. 

Rather less symmetrical tetrameric (LiEt)4 molecules have been found (by X-ray diffraction ) in crystalline ethyUithium, again held together by hypercoordinate carbon atoms forming four-center bonds to three neighboring metal atoms located 2.19-2.47 A distant. The Li—Li distances range from 2.42 to 2.63 A and the Li-C-Li angles range from 66° to 67°. 

Again, as in (LiMe)4 (34), the hypercoordinate carbon atom forms three normal two-center bonds within the alkyl group and one multicenter bond to the bridged metal atoms. The molecules of benzene of crystallization are located over the equilateral triangular faces of the Lig antiprism. 

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