Ionic bonding in d-block elements

Let us first consider the charge and spin distributions for atoms and ions of the first transition series (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn). The neutral ground-state TM electron configurations are of generic form s2d , except at n = 4 (Cr sM5) and n = 9 (Cu s d1 ) where the well-known anomalies associated with the special stability of half-filled and filled d shells are manifested. The simplest picture of ionic bonding therefore involves metal ionization from an s orbital to give the 

The electron affinity (EA) of fluorine can be estimated from the first two entries of Table 2.2 as follows  

Similarly, the energy differences of Table 2.2 can be used to compute the first ionization potential of TM atoms for comparison with experiment, as shown in the plot below  

Since energy differences involving configurational changes are among the most challenging to compute accurately, one may conclude that B3LYP/6-311++G reproduces the chemical trends fairly satisfactorily (mean absolute deviation 0.22 eV). 

Let us first consider the charge and spin distributions in TM monofluorides MF for the first transition series. Calculated geometrical and energetic properties of these species are summarized in Table 2.3. 

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From the polarities of the maximum-valency MH NBOs, one can infer the natural electronegativity Xm(N) of each transition metal M, following the procedure outlined in Section 3.2.5. For cases in which two or more inequivalent M—H bonds are present (e.g., RcH ), we employ the average value of cm2 (or of the bond ionicity z mh) to evaluate xm(N) from Eq. (3.60). Table 4.7 presents the natural electronegativity values for all three series of the d-block elements. 

The primary difference between covalent and ionic bonding is that with covalent bonding, we must invoke quantum mechanics. In molecular orbital (MO) theory, molecules are most stable when the bonding MOs or, at most, bonding plus nonbonding MOs, are each filled with two electrons (of opposite spin) and all the antibonding MOs are empty. This forms the quantum mechanical basis of the octet rule for compounds of the p-block elements and the 18-electron rule for d-block elements. Similarly, in the Heider-London (valence bond) treatment... 

Attempts to classify carbides according to structure or bond type meet the same difficulties as were encountered with hydrides (p. 64) and borides (p. 145) and for the same reasons. The general trends in properties of the three groups of compounds are, however, broadly similar, being most polar (ionic) for the electropositive metals, most covalent (molecular) for the electronegative non-metals and somewhat complex (interstitial) for the elements in the centre of the d block. There are also several elements with poorly characterized, unstable, or non-existent carbides, namely the later transition elements (Groups 11 and 12), the platinum metals, and the post transition-metal elements in Group 13. 

Consideration of metal-nitrogen bond lengths in light of the ionic-bonding model advanced by Raymond (10) leaves little doubt that the bonding in the binary silylamide derivatives of the lanthanide elements is predominantly ionic (11). Indeed, all of the tris-silylamide derivatives of the p-, d-, and f-block elements can be viewed as being mainly ionic. 

Figure 3.2 shows a rearranged periodic table centred on phosphorus. While phosphorus with other p-block elements, generally forms bonds which are covalent, the bonds to atoms in the metallic s, d and f blocks are more varied. They include covalent, metallic, ionic and hybrid or at present ill-defined combinations of these. 

In contrast to the d-block, vhere there are many examples of metal-ligand multiple bonds, f-element-ligand multiple bond chemistry is in its infancy. In part this reflects the entire area of molecular f-element chemistry which is much less developed than that of the transition metals, and, in part, it is the result of difficulties in evaluating the nature of the bonding in heavy metals vhich are highly ionic, do not follow simple electron counting rules (e.g. the 18... 

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