D orbitals in bonding

The physical chemist of today has a wide variety of methods at his disposal for the experimental investigation of electronic structure and all of them have been used in attempts at obtaining evidence of the participation of outer d-orbitals in bonding. One such group of methods is constituted by the various techniques of radiofrequency spectroscopy, which have the advantage that they yield information about the molecule in its ground state. In this they have a distinct superiority over, say, electronic absorption spectra where it is necessary to consider both ground and excited states. Moreover much of the data derived from radiofrequency spectroscopic methods concerns essentially just one part of the molecule so that attention can be concentrated on those atoms of interest in whatever study happens to be under way. 

Several workers have objected to the inclusion of d orbitals in bonding in normetals. The principal objection is to the large promotion energy required to effect... 

As a general rule, then, elements in the third and subsequent periods may use up to two extra d orbitals in bonding. That means that up to six covalent bonds can be formed and up to twelve electrons may surround these atoms in their compounds. In these cases, therefore, the octet rule does not necessarily apply. 

Some of the most important and commonly encountered compounds which involve the d orbitals in bonding are the transition metal complexes. The term complex in this context means that the molecule is composed of two or more kinds of species, each of which can have an independent existence. 

Binding of Cl- to an aryl iodide may well involve sj d hybridization at iodine to accommodate the ninth electron, but the involvement of a d-orbital in bonding at sulfur is more controversial. Recently it was discovered that even first row elements can form Cl- complexes the evidence indicates that th complexes utilize three-electron bonds, not d-orbitals. ° Best explored are templates for radical relay chlorination using nitrogen atoms. 

The studies described in sec. 2.3 on of die orbitals in conjugated systems went along with another line of enquiry, on the question of the tendency of second row elements to display their higher covalencies, five in the case of phosphorus, six in sulphur, towards the more electronegative first row elements. Pauling had proposed the use of d-orbitals in bonds in his classic 1931 paper [240],... 

The participation of inner d orbitals in bonding may be observed with transition metal atoms of the third or higher rows of the periodic table. If these transition metal atoms or ions are coordinated by heteroatoms of ligands which are themselves connected by conjugated multiple bonds, one obtains chelate rings of different ring sizes which may form cyclic (pd)n systems. Some examples of such compounds with different coordinating heteroatoms are collected in Fig. 2. 

One way that has been used to produce a localized bonding picture for molecules of this type is to involve the higher energy valence shell d orbitals in bonding. In the 0/, point group these transform as + 2 - i tl the result of their inclusion is shown in Figure 14.2. Now, of course, there arc six bonding orbitals and the rules... 

It thus remains to discuss the inclusion of groups 11 and 12. The group 11 elements Cu, Ag, Au, and Rg clearly have a too pronounced involvement of their (n —1) d-orbitals in bonding, even in their lower oxidation states, to be safely considered main-group elements. The group 12 elements Zn, Cd, Hg, and Cn are usually considered to be main-group or post-transition elements. Yet recently quantum-chemical predictions [5] of oxidation-state Hg(-l-IV) in the form of the... 

The chemical shift (5) is a sensitive indicator of the chemical environment around the phosphorus atom. It relates closely to the molecular structure, and under some circumstances even facilitates stereochemical identification. The unique chemistry of phosphorus-containing materials is partially attributable to the variable oxidation state of P, the participation of d orbitals in bonding, and the ability of the phosphorus atom to vary its coordination number between 1 and 6. 

All of the halogens form stable compounds in which the element is in the -1 oxidation state. In fluorine compounds, this is the only oxidation state. Chlorine, bromine, and iodine also have compounds in which the halogen is in one of the positive oxidation states +1, +3, +5, or +7. The higher positive oxidation states (> +1) are due to the involvement of d orbitals in bonding. 

---