D orbitals in transition metals

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 five d orbitals in transition metals can be shown to have the following characters under tetrahedral (7 ) symmetry ... 

The colors of several other gemstones are also caused by the splitting of the d orbitals in transition metal ions imbedded within host crystals. For example, the red in garnet, which has Mg3Al2(Si04)3 as a host crystal, and the yellow-green of peridot, which has Mg2Si04 as a host crystal are both caused by electron transitions between d orbitals in Fe. Similarly, the blue in turquoise, which has [Alg(P04)4(0H)g 4 H20] as a host crystal, is caused by transitions between the d orbitals in Cu. ... 

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... 

The colors are characteristic of the ions themselves and are due to transitions between the partly filled d orbitals of transition metals (d-d transitions) or the partly filled / orbitals of lanthanides (f-f transitions). In the 3d transition-metal ions, the 3d orbitals contain one or more electrons. When these ions are introduced into a solid, the orbital energies are split by interactions with the surrounding anions. The color observed is due to transitions between these split energy levels. The color observed varies considerably as the interactions are dependent upon the... 

We shall briefly discuss the electrical properties of the metal oxides. Thermal conductivity, electrical conductivity, the Seebeck effect, and the Hall effect are some of the electron transport properties of solids that characterize the nature of the charge carriers. On the basis of electrical properties, the solid materials may be classified into metals, semiconductors, and insulators as shown in Figure 2.1. The range of electronic structures of oxides is very wide and hence they can be classified into two categories, nontransition metal oxides and transition metal oxides. In nontransition metal oxides, the cation valence orbitals are of s or p type, whereas the cation valence orbitals are of d type in transition metal oxides. A useful starting point in describing the structures of the metal oxides is the ionic model.5 Ionic crystals are formed between highly electropositive... 

In neutral atoms of the first transition series, the 4s orbital is usually filled with 2 electrons, and the remaining electrons occupy the 3d orbitals. In transition metal ions, all the valence electrons occupy the d orbitals. For polyatomic ions, first determine the oxidation number of the transition metal, and then assign the valence electrons to the d orbitals as you would if the metal were a simple ion. 

Many minerals can be made to luminesce under various excitation sources, usually UV light, but in relatively few cases is the mechanism understood in detail. Best understood is luminescence due to transitions between localized states in the unfilled d-orbitals of transition metal ions and localized states in the unfilled f-orbitals of rare earth ions. Rare earth ions, important in the development of synthetic phosphor and laser materials, are uncommon among naturally occurring minerals. 

In Ln3+ ions, the 4f orbitals are radially much more contracted than the d orbitals of transition metals, to the extent that the filled 5s and 5p orbitals largely shield the 4f electrons from the ligands. The result is that vibronic coupling is much weaker in Ln3+ compounds than in transition-metal compounds, and hence the intensities of electronic transitions are much lower. As many of... 

Transition metals do not follow the octet rule simply because their outer electron structures involve both s and d orbitals. The transition metals form cations, so they are all losing electrons. The typical pattern for the loss of electrons in these elements is for the first electron(s) to come out of the s orbital and then for all remaining electrons to come out of the d orbitals. In addition, most of the transition metals can form more than one type of ion. An example would be copper, which can form ions of either 1+ or 2+ charge. 

The Sn 5 s and 5p radial functions, from a nonrelativistic calculation for the free 5sz5pz atom, are plotted in Fig. 7. Roughly 8% of the 5s charge extends outside the Wigner-Seitz radius, rws, for / —Sn the 5s orbital, with much of its density in a region in which Zen is about equal to the valence, is actually somewhat in the interior of the atom. It is not unlike the d orbitals of transition metals, which, as earlier noted, maintain much of their atomic quality in a metal. Thus it is quite plausible that the valence s character in Sn is much like the free atom 5 s, except for a renormalization within the Wigner-Seitz cell. The much more extended 5p component, on the other hand, is not subject to simple renormalization the p character near the bottom of the band takes on a form more like the dot-dash curve of Fig. 7. It nevertheless appears useful to account for charge terms of a pseudo P component and a renormalized s. 

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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