D-orbitals in transition metal complexes

Experimentally, spin-allowed d-d bands (we use the quotation marks again) are observed with intensities perhaps 100 times larger than spin-forbidden ones but still a few orders of magnitude (say, two) less intense than fully allowed transitions. This weakness of the d-d bands, alluded to in Chapter 2, is a most important pointer to the character of the d orbitals in transition-metal complexes. It directly implies that the admixture between d and p metal functions is small. Now a ligand function can be expressed as a sum of metal-centred orbitals also (see Box 4-1). The weakness of the d-d bands also implies that that portion of any ligand function which looks like a p orbital when expanded onto the metal is small also. Overall, therefore, the great extent to which d-d bands do satisfy Laporte s rule entirely supports our proposition in Chapter 2 that the d orbitals in Werner-type complexes are relatively well isolated (or decoupled or unmixed) from the valence shell of s and/or p functions. 

This chapter and the next will introduce the use of d orbitals in transition metal complexes. First of all w e shall build up the orbitals of octahedral ML and square-planar ML4 complexes. These molecular levels will be used to develop the orbitals of fragments which is the topic of Chapters 17-20 so considerable time will be spent on this aspect. How the octahedral splitting pattern and geometry is modified by the numbers of electrons and the electronic nature of the ligands is also undertaken. 

Explaining the effect of different ligands on the splitting of the d-orbitals in transition metal complexes and colour observed using the spectrochemical series... 

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. 

Two other, closely related, consequences flow from our central proposition. If the d orbitals are little mixed into the bonding orbitals, then, by the same token, the bond orbitals are little mixed into the d. The d electrons are to be seen as being housed in an essentially discrete - we say uncoupled - subset of d orbitals. We shall see in Chapter 4 how this correlates directly with the weakness of the spectral d-d bands. It also follows that, regardless of coordination number or geometry, the separation of the d electrons implies that the configuration is a significant property of Werner-type complexes. Contrast this emphasis on the d" configuration in transition-metal chemistry to the usual position adopted in, say, carbon chemistry where sp, sp and sp hybrids form more useful bases. Put another way, while the 2s... 

In the early 1970s, Demas and Crosby [73, 74] postulated that in transition metal complexes with unfilled d shells luminescence should only be observed from the lowest excited or thermally populated higher excited states. The condition for this thermalization is a fast radiationless relaxation between the excited states. It was then suggested by Watts et al. [75] that the relaxation between the states may be hindered if the excited states have a different orbital parentage and the energy difference between the states is small ( < 300 cm-1). 

The actual mixing of rcd and (n + l)p orbitals is, of course, of crucial importance in providing a mechanism by which the Laporte forbidden d-d transitions in transition metal complexes may gain in intensity. This may occur in the static situation (e.g., tetrahedral complexes), where the p orbitals and one set of the d orbitals transform as t2 or in the dynamic situation (as in octahedral complexes) where such mixing is only possible when the point symmetry has been reduced by an asymmetric vibration (vibronic coupling). 

The unoccupied d orbitals of transition metals are suitable for monomer coordination. A certain structure of the complexes of these metals can result in an extremely useful link between space-oriented monomer coordination and polymerization. 

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