Valency, maximum permitted

The maximum permitted oxidation state of a complex can never exceed the group number, N, of the metal. Ti can have no higher OS than Ti(IV), corresponding to minimum allowed (T configuration of d°. Neutral TiMe, for example, would be and thus implausible because Ti has only four valence electrons, not six. 

We have already assumed that electron pairs, whether in bonds or as nonbonding pairs, repel other electron pairs. This is manifested in the tetrahedral and trigonal geometry of tetravalent and trivalent carbon compounds. These geometries correspond to maximum separation of the electron-pair bonds. Part of this repulsion is electrostatic, but there is another important factor. The Pauli exclusion principle states that only two electrons can occupy the same point in space and that they must have opposite spin quantum numbers. Equivalent orbitals therefore maintain maximum separation, as found in the sp, sjf, and sp hybridization for tetra-, tri-, and divalent compounds of the second-row elements. The combination of Pauli exclusion and electrostatic repulsion leads to the valence shell electron-pair repulsion rule (VSEPR), which states that bonds and unshared electron pairs assume the orientation that permits maximum separation. 

Without describing all of the structures which have been considered, the one which seems most consistent with the data can be represented by superimposing a layer identical with C of Fig. 1 upon itself in a manner permitting maximum elimination of water by the condensation of silicon hydroxyls. Construction of such a double layer from models indicates that a very high proportion, but not all, of the silicon hydroxyls can condense with each other, and that the formation of rings containing more and fewer than 6 tetrahedra can fit into the structure without disruption of normal valence... 

Lewis structure A valence-bond structure that has (a) the maximum number of electron-pair bonds permitted by the rules of valence (e.g., the Lewis-Langmuir octet rule for first-row elements) and (b) places electron... 

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