Binary covalent compound formation

The atomic and ionic properties of an element, particularly IE, ionic radius and electronegativity, underly its chemical behaviour and determine the types of compound it can form. The simplest type of compound an element can form is a binary compound, one in which it is combined with only one other element. The transition elements form binary compounds with a wide variety of non-metals, and the stoichiometries of these compounds will depend upon the thermodynamics of the compound-forming process. Binary oxides, fluorides and chlorides of the transition elements reveal the oxidation states available to them and, to some extent, reflect trends in IE values. However, the lEs of the transition elements are by no means the only contributors to the thermodynamics of compound formation. Other factors such as lattice enthalpy and the extent of covalency in bonding are important. In this chapter some examples of binary transition element compounds will be used to reveal the factors which determine the stoichiometry of compounds. 

It has been shown that not only the elemental semiconductors of group IV and binary compounds which are their analogs crystallize in tetrahedral structures in fact, a whole series of ternary compounds of various types with an average of four valence electrons per atom have the same property. Thus, the formation of covalent bonds based on the sp hybrids is not peculiar to elemental semiconductors and binary semiconducting compounds, but is also found in ternary semiconducting compounds. 

The stabilizing influence of small amounts of B (M/B > 0.25) in the voids of the metal host lattice varies with the mode of filling (partial or complete) of the interstitial, mostly O, sites and whether the compounds develop from the binary-intermetallic host lattice. The structures of B-rich compounds (M/B < 4) are mainly determined by the formation of regular, covalent B polyhedra (O, icosahedron) and the connections between them (B frame structures). Typical metal (M) borides therefore are found within a characteristic ratio of metal to boron 0.125 < M/B < 4. 

Examples of Sb—H bond formation by reactions of Sb compounds with either covalent or ionic binary hydrides are rare. The alkynlstibine, n-Bu,SbC=CH, reacts... 

Oxygen forms binary compounds with nearly all elements. Most may be obtained by direct reaction, although other methods (such as the thermal decomposition of carbonates or hydroxides) are sometimes more convenient (see Topic B6). Oxides may be broadly classified as molecular, polymeric or ionic (see Topics B1 and B2). Covalent oxides are formed with nonmetals, and may contain terminal (E=0) or bridging (E-O-E) oxygen. Especially strong double bonds are formed with C, N and S. Bridging is more common with heavier elements and leads to the formation of many polymeric structures such as Si02 (see Topics FT and F4). 

In coordination chemistry, azido complexes wherein azide ions are coordinated to metal sites are known for long. The azide ion was found in terminal as well as in bridging (l,l-/r, l,l,l-/r, l,3-/r) coordination geometries. It is well established in the literature that the stability of such compounds decreases with an increase of the covalence of the M-N3 bond and with an increasing M/N3 ratio. Binary systems of the type MtNslx usually decompose under formation of dinitrogen and elemental M. 

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