Covalent bonding of the transition elements

Theories of chemical bonding —fall into one of two categories those which are too good to be true and those which are too true to be good 

In earlier chapters, allusions were made to die effects of covalent bonding. For example, covalent interactions were invoked to account for the intensification of absorption bands in crystal field spectra when transition metal ions occupy tetrahedral sites ( 3.7.1) patterns of cation ordering for some transition metal ions in silicate crystal structures imply that covalency influences the intracrystalline (or intersite) partitioning of these cations ( 6.8.4) and, the apparent failure of the Goldschmidt Rules to accurately predict the fractionation of transition elements during magmatic crystallization was attributed to covalent bonding characteristics of these cations ( 8.3.2). 

Another manifestation of covalent bonding relates to the sulphide mineralogy of the transition elements. Although earlier chapters have stressed properties of transition metal ions in oxides and silicates, an important feature of these elements is the frequency of their geochemical association with B-sub-group non-metal and metalloid elements such as sulphur, selenium, tellurium, phophorus, arsenic and antimony. The chalcophilic properties of iron, cobalt, nickel and copper in the crust are well known and are important eco- 

Number of 3d electrons Cation Racah B parameter Racah C parameter Ratio C/B 

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The higher oxidation states of the transition elements may be considered to be hydrolysis products of hypothetical more highly charged cations in which the central metal ion is sufficiently electronegative to be able to participate in covalent bonding. For example, the hypothetical Mn7 + ion interacts with water to give an oxoanion, the manganate(VII) ion ... 

As a univalent ion of medium size, CN , in simple compounds like K+CN , behaves as a chlorine ion, especially if the positive ion has a rare-gas or an 18-electron structure, and covalent bonds cannot be formed. If, however, the positive ion is one of the transition elements, covalent bonds are formed by the lone electron-pair of the CN ion. 

From various sources Dowden (27) has accumulated data referring to the density of electron levels in the transition metals and finds an increase from chromium to iron. The density is approximately the same from a-iron to /3-cobalt there is a sharp rise between the solid solution iron-nickel (15 85) and nickel, and a rapid fall between nickel-copper (40 60) and nickel-copper (20 80). From Equation (2), the rates of reaction can be expected to follow these trends of electron densities if positive ion formation controls the rates. On the other hand, both trends will be inversely related if the rates are controlled by negative ion formation. Where the rate is controlled by covalent bond formation, singly occupied atomic orbitals are deemed necessary at the surface to form strong bonds. In the transition metals where atomic orbitals are available, the activity dependence will be similar to that given for positive ion formation. In copper-rich alloys of the transition elements the activity will be greatly reduced, since there are no unpaired atomic d-orbitals, and for covalent bond formation only a fraction of the metallic bonding orbitals are available. 

Almost simultaneous with the publication of Kossel s paper there appeared a rival electronic theory. The American chemist Lewis introduced the idea of the covalent electron-pair bond. Like Kossel, he was impressed by the apparent stability of the noble gas configuration. He was also impressed by the fact that, apart from many compounds of the transition elements, most compounds when rendered as molecules have even numbers of electrons, suggesting that electrons are usually found in pairs. Lewis devised the familiar representations of molecules and polyatomic ions (Lewis structures, or Lewis diagrams) in which electrons are shown as dots (or as noughts and crosses) to show how atoms can attain noble gas configurations by the sharing of electrons in pairs, as opposed to complete transfer as in Kossel s theory. It was soon apparent from the earliest X-ray studies that Kossel s theory was more appropriate... 

In the simplest CF approach, the ligands are represented by negative point charges. No covalency is explicitly considered, so that the bonding is deemed to be ionic this may seem unduly simplistic, but CFT does not purport to be a theory of bonding in compounds of the transition elements. In a quantitative treatment, the array of point charges around the central ion is formulated as an electrostatic potential, the crystal field, whose effect on the nd subshell can be calculated by means of perturbation theory. Here we give only a qualitative treatment. 

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. 

Directional natiire oj bonds involving d orbitals— The atoms of the transition elements may take part in both c and n bonds in the formation of a compound. The number of a bonds formed by the central atom cannot exceed the number of surrounding atoms or groups since between any pair of atoms, only one a bond is permissible, but the number of less than the number of neighbours, since bonds other than the covalent are possible, e.g, bonds due to ion-dipole and dispersion forces. 

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