Hypercoordinate bonding

Hypercoordinate bonding to main group elements the spin-coupled point of view... 

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... 

Interestingly, solid-state structure investigations on methyl thiosalicylate dialkylaluminum compounds uncovered close intermolecular S - C(ti) contacts (with an average S - C distance of 3.382 A significantly below the sum of the corresponding van der Waals radii [76, 77]) between the Al-S thiolate units and the ester component (28, Fig. 9) that can effectively compete with the putative sulfur-aluminum hypercoordinate bond (27, Fig. 9) [141]. The latter results provide the first evidence for the competition of intermolecular n Ti interactions, involving the thiolate sulfur atom and the electrophilic ester carbon atom, with the hypercoordinate bond in metal complexes it opens up an interesting area for further studies. 

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

Cooper et al. [30] were successful in rationalizing the striking variation in the S-S equihbrium bond lengths of FSSF (189.0 pm), ClSSCl (195.0 pm) and HSSH (205.5 pm) using the spin coupled (modern valence bond) theory. In the disulfur dihalides, but not for HSSH, incipient hypercoordinate character is observed at sulfur, with two partial t-like interactions in approximately perpendicular planes, and some antibonding character in the S-X (X=F or Cl) bonds. In other words, it is the form of t-like orbitals that is most rele-... 

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. 

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],... 

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]. 

The summary of all these characteristic points to a pentacoordinated organogermanium compound where no steric enforcement is involved to enhance the hypercoordination. The rotation about the S—CH2 bond, which should be feasible, would move the CO2H group far apart from germanium and should be noted. This in turn appears to suggest that the Ge—O interaction is strong enough to lead to hypercoordination. 

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