Partially Filling d-Orbitals

Thus far, we have focused upon the thermodynamic consequences of the fi-electron configuration. Many everyday observations in transition-metal chemistry have more to do with the relative rates of reactions rather than the position of a thermodynamic equilibrium. So now we consider some of the kinetic manifestations of partially filled d orbitals. 

Transition elements Elements in Groups 3 through 12 in the periodic table. These elements have partially filled d orbitals, but the number of valence electrons varies. Consequently, they have widely different chemical properties. 

The physical reasoning for why these densities were frequently employed in the earlier days of density functional theory was that in this way the degeneracy of the partially filled d-orbitals could be retained. A technical reason why these densities still have to be employed in some recent investigations is that calculations with integral orbital occupations simply do not converge in the self consistent field procedure (see, e. g., Blanchet, Duarte, and Salahub, 1997). Such densities correspond to a representation of a particular state 2S+1L with Mg = S and a spherical averaging over ML. 

As with other first-row transition metals, copper complexes are not expected to be satisfactory singlet oxygen photogenerators, because of the rapid deactivation of excited states in the presence of partially filled d-orbitals. The exceptional case of the copper(II) benzochlorin iminium salt ((18), M = Cu) has already been referred to (Section 9.22.5.6) this showed bioactivity, although the nickel(II) complex ((18), M = Nin) was inactive.195... 

C—Transition elements have partially filled d orbitals. This configuration is for the metal zirconium. 

We first examine the relationships between electron structure and the emission and absorption spectroscopy of metal complexes. Transition metal complexes are characterized by partially filled d orbitals. To a large measure the ordering and occupancy of these orbitals determines emissive properties. Figure 4.2 shows an orbital and state diagram for a representative octahedral MX6 d6 metal complex where M is the metal and X is a ligand that coordinates or binds at one site. The octahedral crystal field of the ligands splits the initially degenerate five atomic d-orbitals by an amount... 

The d block includes all the transition elements. In general, atoms of d block elements have filled ns orbitals, as well as filled or partially filled d orbitals. Generally, the ns orbitals fill before the (n - l)d orbitals. However, there are exceptions (such as chromium and copper) because these two sublevels are very close in energy, especially at higher values of n. Because the five d orbitals can hold a maximum of ten electrons, the d block spans ten groups. 

The Lewis structure is a help-/ V I ful tool for understanding the configuration and chemistry of molecules of main-group elements. In general, Lewis structures cannot be drawn for transition-metal compounds because of the complex way that partially filled d orbitals influence the configuration and chemistry of these compounds. 

When the A ion is kept constant and the B ion is varied, systematics in lattice parameters are not obvious. In Fig. 9, the lattice parameters and da ratios (35, 36) of some Sr2B04 compounds are plotted against the ionic radius of the B ion. The a parameter varies linearly with the radius of the B ion provided that it has partially filled d orbitals. Thus, ions such as Sn4+, Hf4+, and Zr4+ do not fall on this straight line. Poix (3) has, however, found a linear relationship using the /3B parameters. What is important is that there is no linear relationship between the c parameters or the da ratios and the size of the B ion in these compounds. Furthermore, compounds containing B ions with partially filled d orbitals exhibit larger da ratios than those with filled or empty d orbitals. When the B ions have partially filled d orbitals, the da ratio... 

A number of transition metal ion-exchange zeolites are active for acetylene trimerization (159, 160), and the criterion for activity appears to be an even, partially filled d-orbital, i.e., d8 (Ni2 +, Co+), d( (Fe2+), d4 (Cr2 + ). This has led to the suggestion that the mechanism must involve a complex in which there is simultaneous coordination of two acetylene molecules to the transition metal ion. The active oxidation state for CuNaY butadiene cyclodimerization catalysts has been unambiguously defined as monovalent copper (172-180). The d10 electronic configuration of Cu+ is consistent with the fact that isoelectronic complexes of Ni° and Pd° are active homogeneous catalysts for this reaction. The almost quantitative cyclodimerization selec-... 

A characteristic of transition metals is the presence of electrons in partially-filled d-orbitals. This allows them to overcome spin-forbidden transitions in reactivity (e.g., making them able to react with molecular oxygen) it also gives them distinctive spectroscopic properties. Ferrous iron (Fe11) contains 6 electrons in the d-orbitals and each oxidation state higher contains one less electron. Therefore Fem is d5, FeIV is d4, Fev is d3 and FeVI is d2. The latter two oxidation states are not mere curiosities. Fev has been proposed as a shortlived intermediate in many of the proteins that utilise FeIV chemistry (c.f. later) and both Fev and FeVI oxidation states are present in tetraoxopolyanions, e.g. the ferrate ion FeO - [9]. 

Transition metals have partially filled d orbitals, and therefore their compounds are obvious candidates for Jahn-Teller systems. Let us consider an example from among the much studied cupric compounds [64], Suppose that the Cu2+ ion with its electronic configuration is surrounded by six ligands in an octahedral arrangement. We have already seen (Table 6-10 and Figure 6-36) that the d orbitals split into a triply (t2g) and a doubly (eg) degenerate level in an octahedral environment. For Cu2+ the only possible electronic configuration is t e. ... 

Transition metals open up new opportunities for synthesis, because their means of bonding and their reaction mechanisms differ from those of the elements of the s and p blocks. The empty and partially filled d-orbitals that characterize most of these metals enable them to bond reversibly to many functional groups. Thus, transition metals activate many difficult or previously unobserved reactions which are not readily achieved by using conventional reagents. The organometallic chemistry of transition metals has grown explosively in the last decade. 

Transition metal ions with partially filled d orbitals will show the expected d-d spectra which will depend on the degree of perturbation of these orbitals by the coordinating ligand. It has been well established that coordination through sulfur causes less perturbation than coordination through the nitrogen of the thiocyanate ion, or —SON occurs lower in the spectrochemical series than — NCS (see, e.g.. Ref. 664), and a similar distinction may be made between —SeCN and —NCSe (663). 

Transition-metal cations, on the other hand, in which bonding occurs via d-electron orbitals, can generally exist in more than one stable oxidation state the partially filled d-orbitals also give the oxide a component of directional covalent bonding. For example, molybdenum can exist as Mo , Mo, Mo" ", Mo and... 

Finally, we mention studies of the rutile SnO2(110) surface [145,164-166]. Despite the absence of partially filled d orbitals on Sn cations, the surface characteristics of Sn02 are qualitatively similar to those of Ti02- The same is true for the non-stoichiometric (1 x 1) and (1 X 2) surfaces, which present a distribution of defect states in the gap [166]. The authors, however, argue that some quantitative differences with respect to Ti02 take place, which are due to the much larger polarisability of the Tin atoms, compared to the Titaniums. 

It is clear that much work remains to be done to extend our understanding to polax surfaces of transition metal oxides in which the cations have partially filled d orbitals. An especially challenging issue is related to mixed valence metal oxides, such as Fe304, in which the cations exist under two oxidation states. In addition, considering the rapid development of ultra-thin film synthesis and characterization, a simultaneous effort should be performed on the theoretical side to settle the conditions of stability of polar films. More generally, on the experimental side, it seems that one of the present bottlenecks is in a quantitative determination of the surface stoichiometry, an information of prominent interest to interpret the presence or absence of reconstruction. 

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