Lattice molecular compounds

The chemistry of silicon in very low oxidation states is one of the most fascinating research areas, which can be located between molecular compounds of silicon and elemental (perhaps amorphous) silicon [190-194]. Most interesting results have recently been obtained by structural investigations of siliddes in Zintl phases. However, compounds of silicon with negative oxidation states and very low coordination numbers of 1, 2, and 3 are so far only known in the composite of a crystal lattice. 

The lattice energy of a molecular compound corresponds to the energy of sublimation at 0 K. This energy cannot be measured directly, but it is equal to the enthalpy of sublimation at a temperature T plus the thermal energy needed to warm the sample from 0 K to this temperature, minus RT. RT is the amount of energy required to expand one mole of a gas at a temperature T to an infinitely small pressure. These amounts of energy, in principle, can be measured and therefore the lattice energy can be determined experimentally in this case. However, the measurement is not simple and is subject to various uncertainties. 

The + 8 oxide is a molecular compound, unlike many transition metal oxides that have giant lattice arrangements, and is quite soluble in water. It has considerable oxidizing properties and is used as an oxidizing agent in many organic reactions. The +4 oxide is insoluble in water. 

A key property of these polymetallic complexes is geometrical structure, more variable and diverse than the coordination stereochemistries of monometallic complexes or the symmetrical lattices of non-molecular compounds. The following well-defined aggregation modes illustrate the geometrical scope of the field. 

The range of inorganic chemistry includes both molecular compounds, which exist as discrete molecules, and crystals, whose structures are described by infinite lattices of regularly-ordered atoms and which are studied by crystallography and solid-state chemistry. Sec also Solid-State Chemistry. 

Molecular compounds consist of two or more stable species held together by weak forces. In clathrittes-, a gaseous substance, such as SO . HC1. CO.-, or n rare gas is held in the crystal lattice of a solid, such as beta-quinol. by Van der Waals-London dispersion forces. The gas hydrates, e.g.. CU- 6H 0. contain halogen molecules similarly trapped in ice-like... 

Water molecules often occupy positions within the lattice of an ionic crystal. These compounds are called hydrates, and the water molecules are known as water of hydration. The water of hydration is added after a centered dot in a formula. In a name, a number-prefix (listed below for molecular compounds) indicating the number of water molecules is followed by the root -hydrate. 

Yet more important was the publication by Schottky and Wagner (1930) of their classical paper on the statistical thermodynamics of real crystals (41). This clarified the role of intrinsic lattice disorder as the equilibrium state of the stoichiometric crystal above 0° K. and led logically to the deduction that equilibrium between the crystal of an ordered mixed phase—i.e., a binary compound of ionic, covalent, or metallic type—and its components was statistical, not unique and determinate as is that of a molecular compound. As the consequence of a statistical thermodynamic theorem this proposition should be generally valid. The stoichiometrically ideal crystal has no special status, but the extent to which different substances may display a detectable variability of composition must depend on the energetics of each case—in particular, on the energetics of lattice disorder and of valence change. This point is taken up below, for it is fundamental to the problems that have to be considered. 

In the former case, it was assumed to proceed within the crystal lattice of the molecular compound. However, in a series of combinations, a simple mixture and a molecular compound gave the same types of photoproduct. Furthermore, essentially the same photoreaction occurred in solution. Thus, there is still much to be done to elucidate the progress of mixed bimolecular photoreactions in the solid state. 

The binary halides of the elements span a wide range of stoichiometries, stmcture types and properties which defy any but the most grossly oversimplified attempt at a unified classification. Indeed, interest in the halides as a class of compound derives in no small measure from this very diversity and from the fact that, being so numerous, there are many examples of well-developed and well-graded trends between the limiting cases. Thus the fluorides alone include OF2, one of the most volatile molecular compounds known (bp —145°), and Cap2, which is one of the least-volatile ionic compounds (bp 2513°C). Between these extremes of discrete molecules on the one hand, and 3D lattices on the other, is a continuous sequence of oligomers, polymers and extended layer lattices which may be either predominantly covalent [e.g. CIP, (MoPs)4,... 

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