D Orbital bonding

Transition metal atoms form polyatomic ions that result in colored species. These polyatomic ions exhibit bonding in d orbitals. Specific wavelengths of light are absorbed by these d orbital bonds, and the remaining reflected waves are colors in the visible spectrum. 

The significance of such studies is fourfold (1) realization of the synthesis of hitherto-unknown, difficult-to-prepare novel heterocycles which are of particular theoretical and/or biological-medicinal importance (2) contribution to the understanding of the factors which determine the structure, limits of bonding (including d orbital bonding... 

FIGURE 14.7 Stick models of the staggered arrangement of five-equivalent d-orbital bond directions. The application to the bonding in ferrocene seems obvious (From Powell, R.E., J. Chem. Educ., 45, 45, 1968. With permission.)... 

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. 

In Group III, boron, having no available d orbitals, is unable to fill its outer quantum level above eight and hence has a maximum covalency of 4. Other Group 111 elements, however, are able to form more than four covalent bonds, the number depending partly on the nature of the attached atoms or groups. 

The detailed theory of bonding in transition metal complexes is beyond the scope of this book, but further references will be made to the effects of the energy splitting in the d orbitals in Chapter 13. 

This greater reactivity of the silanes may be due to several factors, for example, the easier approach of an oxygen molecule (which may attach initially to the silane by use of the vacant silicon d orbitals) and the formation of strong Si—O bonds (stronger than C—O). 

Ammonia is a colourless gas at room temperature and atmospheric pressure with a characteristic pungent smell. It is easily liquefied either by cooling (b.p. 240 K) or under a pressure of 8-9 atmospheres at ordinary temperature. Some of its physical and many of its chemical properties are best understood in terms of its structure. Like the other group head elements, nitrogen has no d orbitals available for bond formation and it is limited to a maximum of four single bonds. Ammonia has a basic tetrahedral arrangement with a lone pair occupying one position ... 

The splitting of the d orbital energy levels when ligands are bonded to a central transition atom or ion has already been mentioned (p. 60). Consider the two ions [Co(NH3)g] and [Co(NH3)g] we have just discussed. The splitting of the d orbital energy levels for these two ions is shown in Figure 13.2. 

In sorn e situation s, using this option m ay he im portan t. For exam -pic, if p orbitals on electronegative atoms irileracL with d orbitals, (as for a silicon atom bonded to an amine group), you may want to include d orbitals. 

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