Ligands orbitals

Table 7.10 groups 1 ELECTRONIC SPECTROSCOPY Classification of a ligand orbitals in various point... 

Table 7.10 shows how a ligand orbitals are elassified in various point groups with different ligand arrangements. It shows that, in oetahedral MLg, the six a ligand orbitals are split into aig,Sg and t-effeet of these on the e... 

Bg orbitals to interaet, the erystal field orbital being pushed up and the ligand orbital being... 

The difference between the two extremes is essentially that, in the former, the Re retains its valence electrons in its d orbitals whereas in the latter it loses 6 of them to delocalized ligand orbitals. In either case paramagnetism is anticipated since rhenium has an odd number of valence electrons. The magnetic moment of 1.79 BM corresponding to 1 unpaired electron, and esr evidence showing that this electron is situated predominantly on the ligands, indicates that an intermediate oxidation state is involved... 

Salts of IrClg were used in the classic first ESR experiments to demonstrate delocalization of unpaired electrons onto the chloride ligand (Figure 2.1) the unpaired electron spends 30% or more of its time in ligand orbitals in this case [27],... 

Figure 2.38 The effect of varying the relative energies of the metal and ligand orbitals upon the final molecular orbital scheme for a dimeric rhodium carboxylate. (Reprinted from Coord. Chem. Rev., 50, 109, 1983, with kind permission from Elsevier Science S.A., P.O. Box 564,...

Coordination Numbers and Radii. In the transition metal ions, the interaction of the ligand orbitals with the d orbitals of the metal ions generally determines the coordination number and geometry of the oordination sphere about the metal. The... 

Figure 2-3. Contribution to bonding from energy matching with ligand orbitals (O), and from overlap with ligand orbitals ( ).

Both phenomena attest to the covalency of the chemical bonding in these species. Incidentally, they also highlight the different characters and implications of the spectrochemical and nephelauxetic series. Within either lanthanoid- or (higher oxidation state) J-block species, the ligand orbitals overlap with the metal s functions... 

Figure 3. Molecular-orbital diagrams as obtained by the ROHF method. Dashed lines indicate MOs dominated by the metal d-orbitals, the solid lines stand for doubly occupied or virtual ligand orbitals. Orbitals which are close in energy are presented as degenerate the average deviation from degeneracy is approximately 0.01 a.u. In the case of a septet state (S=3), the singly occupied open-shell orbitals come from a separate Fock operator and their orbital energies do not relate to ionization potentials as do the doubly occupied MOs (i.e. Koopmann s approximation). For these reasons, the open-shell orbitals appear well below the doubly occupied metal orbitals. Doubly occupying these gives rise to excited states, see text.

Let us now consider MMCT for the case in which the donating ion is a lanthanide ion with a partly filled 4/ shell M(/")M(d°)CT. The trivalent lanthanide ions with a low fourth ionization potential are Ce, Pr ", Tb ". Their optical absorption spectra show usually allowed 4f-5d transitions in the ultraviolet part of the spectrum [6, 35]. These are considered as MC transitions, although they will undoubtedly have a certain CT character due to the higher admixture of ligand orbitals into the d orbitals. In combination with M(d°) ions these M(/") ions show MMCT transitions. An early example has been given by Paul [36] for Ce(III)-Ti(IV) MMCT in borosilicate glasses. The absorption maximum was at about 30000 cm ... 

The electron transfer series W(R2fi ic)4° W(R2tfic)4 -> W(R2rfic)4 was detected by voltametric measurements (37), the half wave potentials are 0.24 and 0.36 V lower than the corresponding values of the molybdenum compounds. Both transfers obey the Taft relation with a low value of p, which points to a rather small contribution of the ligand orbitals into the redox orbitals. 

The electron transfer Au(R2voltametric measurements 163). The half-wave potentials of the quasi-reversible process depends on the substituent R according to the Taft relation, as was described for Mo, W and Mn 37). The value of p decreases in the series Au > Mn > Mo = W, which indicates that in this sequence the mixing of ligand orbitals into the redox orbital decreases. The dominant ligand character of the unpaired electron MO in Au(R2dtc)2 relative to those in copper and silver compounds is found from Extended Hiickel MO calculations, as will be discussed later on. 

An explanation of the direct 4s contribution to the charge density at the nucleus requires MO calculations. A simple MO diagram for octahedral complexes is shown in Fig. 4.4. The a-interaction of metal /-orbitals and symmetry-adapted ligand orbitals usually yields the major part to the stability of the bonds. According... 

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