Four-ligand compounds, valence-shell

In a similar vein, we observe nickel(O), possessing ten electrons in its valence shell, to require four carbonyl ligands to satisfy the eighteen electron rule and form [Ni(CO)4l, whilst chromium(O), with six electrons in its valence shell forms [Cr(CO)6]. These latter compounds are tetrahedral and octahedral respectively. 

Isolable Oxysulfuranes. Sulfuranes, as described by Musher (1969), are compounds of nlfurlIV) in which four ligands are attached 10 sulfur and have in common with tare-gas compounds such as XeF, an electronic structure involving a formal expansion of the valence shell of the central atom from 8 to 10 electrons, Martin and Perozzi (1976) pointed out that the incorporation of oxygen ligands makes possible a wide range of new structural types that illustrate structure-reactivity relationships in a particularly illuminating way. 

While compoimds with three coordination are known, these species are strong Lewis acids and usually achieve four coordination by formation of three-center two-electron bonds, for example, AI2CI6 (1) (see Bridging Ligand). Aluminum is not restricted to an octet of electrons in its valence shell and compounds with coordination numbers of five and six may be prepared, for example, Cl3Al-2NMe3 (2), [AlFg] " (3). 

Akiba and coworkers have established the existence of hexacoordinate hypervalent compounds, that is, 12-C-6 species with 12 formally assignable electrons in the valence shell of carbon with six ligands (substituents). In 12-C-6 compounds, carbon is sp hybridized, bears two positive charges, and attracts four nucleophiles to form two 3c-Ae bonds. Geometry optimization at the B3LYP/cc-pVDZ level has been performed, with dications having the... 

The many covalent compounds which are formed between elements of the S and p-blocks have stoichiometries that are determined by the sharing of the valence electrons of the central atom such that there are eight electrons shared between the central atom and its ligand atoms. The expected valency is that consistent with the four valence orbitals, one s + three p, and is therefore n + m for values of + nt < 4, and 8 (n + m) for values of n + m > 4, where n is the number of s electrons and m is the number of p electrons in the valence shell of the central atom. The normal outcome of combination between two elements is the maximization of the number of covalent bonds with respect to the above formula. If more or fewer bonds are formed, such cases are exceptions to the rule. 

The ability of an atom in a molecular entity to expand its valence shell beyond the limits of the Lewis octet rule. Hyper-valent compounds are common for the second and subsequent row elements in groups 15 -18 of the periodic table. Hyperva-lent bonding implies a transfer of the electrons from the central (hypervalent) atom to the nonbonding molecular orbitals which it forms with (usually more electronegative) ligands. A typical example of a hypervalent bond is a linear three-center, four-electron bond, e.g., that of the Fap-P-Fap fragment of PF5. 

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