Hypercoordinate compounds

Silylium ions R3Si+ are known to be more stable in the gas phase than their carbon analogues, which is due to the fact that silicon is more electropositive than carbon. It is easy to observe them by mass spectrometry or ion cyclotron resonance spectroscopy of triorganosilyl compounds. [5] However, their existence in solvent phases is difficult to prove and, therefore, is still a matter of dispute. Early attempts to synthesize silylium ions in solution with methods successful in the preparation of carbocations were not successful. [78] The lack of stability of R3Si+ in solution was attributed to the strong tendency of Si to form hypercoordinated compounds, in which Si has five or even six bonding partners. [79]... 

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). 

It has been noted that the chemical shift of Ge NMR signals is rather insensitive to hypercoordination however, the width of the NMR peak changes dramatically. On the basis of the width of the NMR signal, a number of germanes containing Ge-Y (Y = N, O, S) contacts that are all < 15% shorter than sum of the van der Waals radii are also assigned as hypercoordinate compounds. ... 

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

Hypercarbon compounds contain one or more hypercoordinated carbon atoms bound not only by 2e-2c but also 2e-3c (or >3c) bonds. 

Carbon is known with all coordination numbers from 0 to 8 though compounds in which it is 3- or 4-coordinate are the most numerous. Some typical examples are summarized in the Panel (p. 291). Particular mention should also be made of hypercoordinate non-classical carbo-nium ions such as 5-coordinate CHj", square pyramidal CsHs (cf. the isoelectronic cluster B3H9, p. 154), pentagonal pyramidal C6Me6 " (cf. iso-electronic Bf,Hio, p. 154) and the bicyclic cation 2-norbomyl, C7H] 1... 

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. 

As already was observed for hypercoordinated adducts MX3(ER 3)2, no stibine and bismuthine adducts of low-valent alanes, gallanes or indanes have been prepared, to date. According to the lability of low-valent group 13 compounds toward disproportionation into M(III) and elemental M, stibines and bismuthines are expected to be too weak as Lewis bases, preventing them from the stabilization of such compounds. 

Besides rhodium catalysts, palladium complex also can catalyze the addition of aryltrialkoxysilanes to a,(3-unsaturated carbonyl compounds (ketones, aldehydes) and nitroalkenes (Scheme 60).146 The addition of equimolar amounts of SbCl3 and tetrabutylammonium fluoride (TBAF) was necessary for this reaction to proceed smoothly. The arylpalladium complex, generated by the transmetallation from a putative hypercoordinate silicon compound, was considered to be the catalytically active species. 

The chemistry of hypercoordinated silicon compounds has continued to be an extremely active research field in recent years. Some of this interest is driven by the observation that many hypercoordinated silicon species possess a... 

Some atoms take on more than a full octet s worth of electrons. These atoms are said to be hypervalent or hypercoordinated. The phosphorus of phosphorus pentachloride, PCI5, is an example. These kinds of situations require an atom from Period (row) 3 or higher within the periodic table. The exact reasons for this restriction are still debated. Certainly, the larger atomic size of these atoms allows room to accommodate the bulk of all the binding partners that distribute around the central atom s valence shell. In some cases, even noble gases like xenon (Xe) form compounds. Because noble gases already have a filled valence shell, they automatically violate the octet rule. 

Attempts to stabilize a sila-ylide (hypercoordinated silylene) by intramolecular Lewis-base coordination did not result in sufficiently stable compounds for actual isolation. Formation of the sila-ylide was demonstrated by trapping with 2,3-dimethylbutadiene123. 

Finally, a group of hypercoordinated silicon compounds, the decamethyl silicocenes, in which the formal silicon coordination number is ten, is worthy of mention in connection with this chapter. These analogs of ferrocene have been studied extensively232, and are described in detail in Jutzi s chapter in this volume. 

Another important feature of the extracoordinate silicon compounds (Scheme 7.14) is the increase in natural atomic charge at the central atom compared to the tetracoordinate precursors [69]. The counter-intuitive increase in the positive charge on silicon, which becomes even more substantial in the case of anionic nucleophiles, such as F , is compensated by a more negative character of the surrounding groups (X), and this results in an enhanced ionic nature of the Si-X bond. This polarization then favors intermolecular charge-dipole interaction, which results in an increased Lewis acidity of the hypercoordinate silicon [70]. 

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