D character

We consider first some experimental observations. In general, the initial heats of adsorption on metals tend to follow a common pattern, similar for such common adsorbates as hydrogen, nitrogen, ammonia, carbon monoxide, and ethylene. The usual order of decreasing Q values is Ta > W > Cr > Fe > Ni > Rh > Cu > Au a traditional illustration may be found in Refs. 81, 84, and 165. It appears, first, that transition metals are the most active ones in chemisorption and, second, that the activity correlates with the percent of d character in the metallic bond. What appears to be involved is the ability of a metal to use d orbitals in forming an adsorption bond. An old but still illustrative example is shown in Fig. XVIII-17, for the case of ethylene hydrogenation. 

Fig. XVIII-17. Correlation of catalytic activity toward ethylene dehydrogenation and percent d character of the metallic bond in the metal catalyst. (From Ref. 166.)...

Figure 7-23. The polarizing effect ofd-orfeitals. a) The energy of the. t-MO can be lowered slightly by shifting the centers of the AOs slightly towards the center of the bond, b) The lobes ca n be directed towards each other by mixing soine d-character into the p-orbitals. The effect is exaggerated for clarity.

Areen silicon and germanium are ascribed to the d electron states silicon does not have 3 d electrons, whereas germanium does. Certain transitions (e.g. carbon /3 hn) do depend upon the d character of the electronic configuration in contrast to subsequent isitions. 

From the analysis of the DOS combined with the interpretation of the spectral details in FeAl, CoAl and NiAl (Botton et al., 1996a) the features at the edge threshold (71-73 eV) have been shown to arise from d character being introduced at the Al sites by the strong interaction with the TM d bands and thus by the presence of "covalent" character. This interaction causes the... 

If the term G(M-B) is not constant, the adsorbability scale turns out to be different. In particular, for pyrazine,913 Au(lll) > Ag(lll) (which is opposite to the effect of hydrophilicity) for uracils,914 Au(100) > Au(lll) > Ag(100) > Ag(lll) > Hg and for pyridine,915 Au(311) > Ag(311) > Hg as well as Au(210) > Au(lll). In all these cases the adsorbate interacts with the metal via its re-electrons. The partial d-character of Au gives to this metal the ability to form stronger bonds. The situation thus resembles that described by Silva et a/.,448 i.e., G(M-B) increases more rapidly than G(M-S). However, just the opposite sequence of that hypothesized by the authors is obtained. 

The contribution of Co A is presumably limited to 35 % by the destabilizing effects of absence of a metallic orbital and decreased d character of the bond orbitals, which oppose the stabilizing effect of the quartet atomic state. In the same way nickel involves resonance between the two following structures, in the ratio 30 70 (saturation moment 0-61) ... 

The quinquevalent state with a pure p metallic orbital would have somewhat more d character in the bond orbitals. 

This system is based on the observations (Pauling 1947) that a linear relation between single-bond radius and atomic number holds for bonds of constant hybrid character, and that for an element the single-bond radius is (at least approximately) linearly dependent on the d character of the dsp hybrid bond orbitals. 

Equations (10a) and (106) and the assumed linear relation between R and the amount of d character ( ) lead to the equation... 

Application of equation (10c) to the observed single-bond radii of scandium, titanium and vanadium (1-439,1-324,1-224 A) leads to 20, 27 and 35 % of d character, respectively (table 5). The gradual increase presumably results from the increasing stability of the 3d orbitals relative to 4s and 4p. 

Table 5. Single-bond radii and amount of d character...

If it is assumed that the valency 3 is divided among one single bond, four -bonds, and two J-bonds the corresponding values of R1 are 1-219,1-263,1-278 and 1-217 A. The first and last values correspond to orbitals with 16 % d character, and the others to sp8 orbitals. This suggests that the 0- 25 d orbital provided by the 25 % contribution... 

The observed bond lengths in some intermetallic compounds can be made compatible with those in the constituent metals by consideration of the possibility of distributing the d character unequally among the bond orbitals of an atom. 

This amount of d character corresponds to the average Rt = 1-318 A, in excellent agreement with the value 1-319 A from the / form. I think that this value should replace the older values 1-299 A (Pauling, 1947) and 1-304 A (Pauling, 1949, 1960), which were ob-... 

It is possible that a small amount of oxygen is necessary to stabilize the -tungsten structure (see, e.g. Moss Woodward, 1959). A consideration of the effect of this oxygen might lead to a small revision in the values of the single-bond radii and amounts of d character. 

For other elements the same dependence of electronegativity will be assumed that is, a change by 0.2 z for electric charge z, increase by 0.3 for sp3 bond orbitals (relative to p bonds), and an equal decrease for d character in transargononic bonds. 

The hybrid orbital has cylindrical symmetry, and accordingly the introduction of d character and f character in the axial bond itself does not lead to an interaction dependent on the relative azimuthal orientation of the two groups. 

The repulsion of the thallium and oxygen lone pairs lead to a distortion of the Tl-6s lone pair, which is no longer totally spherical as one would intuitively expect for an s orbital. The thallium contributions involved in the anti-bonding combination are indeed of 97.7% s-, 1.8% p- and 0.5% d-character. Thus, a much less than expected p-character is found for the thallium orbital, which shows that extensive s-p mixing or hybridization is not essential for the lone pair to become stereochemically active. 

A very important criterion for electron structure is the percent d-character, which characterizes the number of unpaired electrons in the rf-orbitals of the individual metal atom. Because of the vacancies existing in these orbitals, metals will interact with electron-donating species forming electron pairs. It is this interaction that determines the special features of adsorption of these species and, as a consequence, the catalytic activity of a given metal. 

---