Electropositive elements bonding

Only the carbon atom can gain four electrons this only happens when it is combined with extremely electropositive elements and this state may be regarded as exceptional. Bonding in carbides is almost invariably predominantly covalent. 

Two types of chemical bonds, ionic and covalent, are found in chemical compounds. An ionic bond results from the transfer of valence electrons from the atom of an electropositive element (M) to the atom(s) of an electronegative element (X). It is due to coulombic (electrostatic) attraction between the oppositely charged ions, M (cation) and X (anion). Such ionic bonds are typical of the stable salts formed by combination of the metallic elements (Na, K, Li, Mg, etc.) with the nonmetallic elements (F, Cl, Br, etc.). As an example, the formation of the magnesium chloride molecule from its elemental atoms is shown by the following sequence ... 

The nature of a binary hydride is related to the characteristics of the element bonded to hydrogen (Fig. 14.8). Strongly electropositive metallic elements form ionic compounds with hydrogen in which the latter is present as a hydride ion, H. These ionic compounds are called saline hydrides (or saltlike hydrides). They are formed by all members of the s block, with the exception of beryllium, and are made by heating the metal in hydrogen ... 

With the exception of ionic sulfides formed from highly electropositive elements (i.e., Na, K, Ca, Mg), sulfur bonding in natural environments is covalent. When fully oxidized, however, the covalently bonded sulfur atom exists... 

For more electropositive elements, which have an inferior number of valence electrons in the first place, and which in addition have to supply electrons to a more electronegative partner, the number of available electrons is rather small. They can gain electrons in two ways first, as far as possible, by complexation, i.e. by the acquisition of ligands and second, by combining their own atoms with each other. This can result in the formation of clusters. A cluster is an accumulation of three or more atoms of the same element or of similar elements that are directly linked with each other. If the accumulation of atoms yields a sufficient number of electrons to allow for one electron pair for every connecting line between two adjacent atoms, then each of these lines can be taken to be a 2c2e bond just as in a common valence bond (Lewis) formula. Clusters of this kind have been called electron precise. 

For a general formulation of the Zintl-Klemm concept, consider an intermetallic AmX phase, where A is the more electropositive element, t3 pically an alkali or an alkaline earth metal. Both A and X, viewed as individual atoms, are assumed to follow the octet rule leading to transfer of electrons from A to X, i.e., A AF, X —> X , so that mp = nq. The anionic unit X arising from this electron transfer is considered to be a pseudoatom, which exhibits a structural chemistry closely related to that of the isoelectronic elements [11]. Since bonding also is possible in the cationic units, the numbers of electrons involved in A-A and X-X bonds of various types (caa and exx> respectively) as well as the number of electrons e not involved in localized bonds can be generated from the numbers of valence electrons on A and X, namely and ex, respectively, by the following equations of balance ... 

For electropositive elements (at the left of the Periodic Table), unfilled molecular oibitals may be bonding in character and electron promotion will not necessarily lead to bond weakening (lengthening). 

Nitrides can be sub-divided into ionic, covalent and interstitial types.An alternate general classification of nitrides, based on bonding classification, as ionic, covalent and metallic has also been applied. Ionic or salt-like nitrides are formed by electropositive elements such as Li, Mg, Ca, Sr, Ba, Cu, Zn, Cd and Hg and possess formulae which correspond to those expected on the basis of the combination of the metal ion with ions. A range of covalent nitrides are known and are exhibited by less electropositive elements such as B, S, P, C and Si. Interstitial nitrides are formed by some transition metals and refer to compounds which can be described in terms of the occupancy of interstitial sites in close packed metallic structures by nitrogen atoms. Oxygen can also be accommodated within these structures and a range of oxynitrides are known to... 

Until recently, the synthesis of ionic/covalent nitrides was relatively unexplored except for the pioneering work of Juza on ternary lithium nitrides.11 However, within the last decade, several groups have begun to explore ternary nitride systems, many of which have relied on the inductive effect. The inductive effect is based on the donation of electron density from an electropositive element to an adjacent metal-nitrogen bond, thereby increasing the covalency and stability of that bond and of the nitride material itself. The success of this method is illustrated by the fact that almost all of the known ionic/covalent ternary nitrides contain electropositive elements. Only recently has a small number of transition metal ternary nitrides been synthesized in the absence of the inductive effect at moderate temperatures, by taking advantage of low temperature techniques, such as the ammonolysis of oxide precursors and metathesis reactions.6,12-17... 

Different elective affinity of elements as compared to carbon. Electropositive elements (Si, B, AI, P) have a considerably larger affinity to electronegative elements than carbon. In other words, silicon, boron, aluminum, phosphorus and other elements form weaker bonds with electropositive elements (H, Si, B, Al, As, Sb, Bi, etc.), and stronger bonds with electronegative elements (O, N, Cl, Br, F, etc.) than carbon. 

Different formation of bond types. Multiple c,71-bonds are essential in organic chemistry, because unsaturated compounds are the main monomers for multiple bond polymerisation, whereas in elemenorganic chemistry the role of 0,71-bonds is insignificant due to the fact that the formation of stable compounds with multiple E-E bonds has not been established with reliability. This demonstrates that electropositive elements, unlike carbon, seem to have little capability to form stable compounds with multiple bonds. 

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