Weak Hypervalent Interactions

Weak hypervalent G- Se-X 3c-4e type interactions17,43,66 are closely related to T-shaped 3c 4c interactions and often control the fine structures of the compounds. 

The Se-X bonds (X=C1 and Br) in 48 are examined by stabilizing through the coordination of X, in which the negative charge brought by X- is neutralized by the ammonium group.69 

If an interaction of the X- -E-E 3c-4e type occurs in both sides of an E-E bond, the X E-E X interaction is expected to be symmetric or close to symmetric under suitable conditions. The interaction is no more treated as 

Extended hypervalent bonds containing chalcogens have been substantially discussed in the last decade, however. 

How do E4 4c-6e stabilize the linear forms Energies of linear and Z-shaped forms are calculated for naphthalene and benzene systems. The results are 

---

The weak SISHA interactions (Si - - H = 2.0-2.5 A) play an important role, as their breaking is responsible for the most energetically demanding step. They allow smooth variations leading to the formation of new cr bonds without the necessity of decoordination of a ligand, which can be of considerable importance in catalytic reactions. Related intramolecular hypervalent interactions (IHI) between halosilyl ligands and hydrides are also relevant in this context [85]. 

However, we have shown how the 18-electron rule is commonly satisfied in the absence of any significant p-orbital participation, on the basis of hypervalent 3c/4e cu-bonding interactions wholly within the framework of normal-valent sd" hybridization. Results of NBO and Mulliken analyses of high-level wavefunctions for transition-metal complexes commonly exhibit only paltry occupation of the outer p orbitals (comparable in this respect to the weak contributions of d-type polarization functions in main-group bonding). 

Hypervalent Cl Sn—Cl bridges are also typical for pentacoordinate tin atoms. For example, the crystal structure of 20 and 21 comprise dimeric pairs of molecules bridged by weak intermolecular Cl Sn interactions. The Cl —Sn distances in 20 (2.475 A ), 21 (average 3.869 A ) and in MeSn(Ar)Cl2 (Ar = 3-methyl-4-nitropyridine-N-oxide) (3.934 A ) are longer than in most comparable structures The Sn NMR resonance (—285.7 ppm) for 21 suggests that it possesses a five-coordinate structure in solution . 

Hydrogen bonds contain linear 4e,3-center E H M bridge bonds with hypervalent hydrogen, while sigma complexes contain bent 2e,3-center E-H-M bridge bonds with electron-deficient hydrogen. In the former, the metal acts as a weak base, while in the latter it acts as a weak acid. In principle, intermediate situations are possible where both interactions compete but information is still sparse in this area. 

---