Tetracoordinated Atoms

MOs form two group orbitals, each with and 4,) and a pair of degenerate p-like 

Gimarc has specified a set of rules for the application of qualitative MO theory [73], particularly for the determination of molecular structures and conformations, following the pioneering work of Walsh [74]. These are repeated here  

Electrons in molecules are completely delocalized and move in molecular orbitals which extend over the entire molecular framework. 

For properties which can be explained by qualitative MO theory, only the valence electrons need be considered. 

Satisfactory MOs can be formed from linear combinations of atomic orbitals (LCAO). This is the well-known LCAO-MO approximation already discussed in Chapter 2. 

The atoms which form the molecules of a particular series or class contribute the same kinds of valence AOs from which MOs can be constructed. Therefore the MOs for each series or type of molecular framework must be qualitatively similar, and individual molecules differ primarily in the number of valence electrons occupying the common MO system. 

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Figure 3.19. Group orbitals for a tetracoordinated atom as a substituent interacting through a bonds. A methylene group is illustrated.

The title anion derives from the Keggin structure by replacing the central tetracoordinated atom by an arsenic(III) atom. Because of the lone pair on this heteroatom, the Tj Keggin structure can no longer be completed, and the species can be only [As W9033] . Two As Wg034 groups can be... 

As a criterion for a systematic discussion, we shall adopt as first classification the dichotomy infinite/finite systems, i. e. macromolecule vs. molecule as a second criterion, we shall discuss systems in terms of hybridization, using the well-known types sp, sp2, sp3 although these are only first approximations for dicoordinated, tricoordinated and tetracoordinated atoms, because the exact hybridization is determined by the valence angles. 

In many reactions at carbonyl groups, a key step is addition of a nucleophile, generating a tetracoordinate carbon atom. The overall course of the reaction is then determined ly the fate of this tetrahedral intermediate. 

Section 7.16 Atoms other than carbon can be chirality centers. Examples include those based on tetracoordinate silicon and tricoordinate sulfur as the chirality center. In principle, tricoordinate nitrogen can be a chirality center in compounds of the type N(x, y, z), where x, y, and z are different, but inversion of the nitrogen pyramid is so fast that racernization occurs vit -tually instantly at room temperature. 

Tricoordinate groups, such as sulfinyl (—SO—) and sulfonio (—S + R2), and a tetracoordinate group like sulfonyl (—S02—), possess partial positive charge on the central sulfur atom and hence are electron-withdrawing. The magnitude of the electron-... 

A cobalt(II)-chitosan chelate has been prepared by soaking a chitosan film in C0CI2 aqueous solution. The chitosan chelated Co(II) through both oxygen and nitrogen atoms in the chitosan chain. The tetracoordinated, high-spin Co(II)-chitosan chelate could be used as a catalyst, and the polymerization of vinyl acetate was carried out in the presence of Na2S03 and water at pH 7 and normal temperature. The polyvinyl acetate possessed a random structure [114,115]. 

For a review of mechanisms of nucleophilic substitutions at di-, tri-, and tetracoordinated sulfur atoms, see Ciuffarin, E. Fava, A. Prog. Phys. Org. Chem., 1968, 6, 81. 

From X-ray crystal structures of the products, the reactions of these stereoisomeric disilenes with episulfides, and with sulfur, were shown to proceed with retention of configuration at silicon. These findings suggest that the reaction proceeds in a concerted fashion, through intermediates or transition states involving tetracoordination for the sulfur atom being transferred (Scheme 13). Similar intermediates are believed to occur in other sulfur-transfer reactions.86... 

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