Electronic structure halide valency, solids

The silver halides are semiconductors, that is, their electronic structure is best described by the solid state model in which atomic orbitals overlap extensively to form occupied valence bands together with conduction bands that are unoccupied in the absence of external perturbation (i.e. in the dark at normal temperatures). All the halides absorb light in the UV region of the electromagnetic spectrum, with AgBr, Agl and mixed salts also absorbing into the visible out to ca. 500 nm (Fig. 11.7). 

Cadmium is a member of Group 12 (Zn, Cd, Hg) of the Periodic Table, having a filled d shell of electrons 4valence state of +2. In rare instances the +1 oxidation state may be produced in the form of dimeric Cd2+2 species [59458-73-0], eg, as dark red melts of Cd° dissolved in molten cadmium halides or as diamagnetic yellow solids such as (Cd2)2+ (AlCl [79110-87-5] (2). The Cd + species is unstable in water or other donor solvents, immediately disproportionating to Cd2+ and Cd. In general, cadmium compounds exhibit properties similar to the corresponding zinc compounds. Compounds and properties are listed in Table 1. Cadmium(TT) [22537 48-0] tends to favor tetrahedral coordination in its compounds, particularly in solution as complexes, eg, tetraamminecadmium(II) [18373-05-2], Cd(NH3)2+4. However, solid-state cadmium-containing oxide or halide materials frequently exhibit octahedral coordination at the Cd2+ ion, eg, the rock-salt structure found for CdO. 

The discussion so far has concentrated on the structural chemistry of the electron rich clusters of the [MgXg] type. To prepare for the next paragraph, interest is now focussed on metals with smaller numbers of valence electrons which then prefer to form [MgX j type clusters. A tremendous amount of work has been performed on [MgX,2] type halide clusters of the metals Nb and Th both in the solid state and in solution. Ousters with 14, 15, and 16 electrons in M-M bonding states are well known. Examples of compounds with this cluster type but formed with divalent anions had been very scarce, being represented by only two which contained the [NbgOi2] cluster. [57, 58] Recently, a rather broad chemistry of reduced oxoniobates based on this cluster has emerged. [16]... 

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