Electronic spectra of metal complexes

A low-lying excited state in TiF occurs upon excitation of an electron from — eg(n ). It is now established that the 

Higher electronic excited states arise on excitation of an electron from a bonding or nonbonding MO into t2g(7r ) or eg (a ). Since the bonding and nonbonding MO s are mainly localized around the ligands, and t2g(7T ) and eg(ff ) are mainly located on the metal, this 

In certain complexes, low-lying excited states arise by the transfer of an electron from an orbital based on the metal to an orbital based on the ligands. This is called metal-to-ligand (M — L) charge transfer, and it is exhibited by complexes containing ligands such as NO, CO, and CN , which have relatively stable tt orbitals. 

A low-lying excited state in TiFf occurs upon excitation of an electron from t2g(77 ) —- eg (a ). It is now established that the colors of many transition metal complexes are due to such d-d transitions. The energy separation between eg (a ) and t2g(w ) is 

---

Evans S, Green MLH, Jewitt B, Orchard AF, Pygall CF (1972) Electronic spectra of metal complexes containing 7t-cyclopentadienyl and related ligands part I. - He(I)... 

A consistent model permitting rationalization and prediction of the solvato-chromic behaviour of coordination compounds with MLCT absorption has been described [428]. According to this qualitative model, the changing relationship between the metal-ligand bond dipolarities in the ground and MLCT excited state determines whether the complex is negatively, positively, or not solvatochromic [428]. For comprehensive reviews on solvent effects on electronic spectra of metal complexes, see references [15, 17]. 

The processes which lead to energy absorption in the electronic spectra of metal complexes... 

Pre-lab 5.2 Electronic Spectra of Metal Complexes—Polyelectronic Systems... 

A characteristic feature of many d-block metal complexes is their colours, which arise because they absorb light in the visible region (see Figure 20.4). Studies of electronic spectra of metal complexes provide information about structure and bonding, although interpretation of the spectra is not always straightforward. Absorptions arise from transitions between electronic energy levels ... 

Electronic spectra of metal-dioxygen complexes. A. B. P. Lever and H. B. Gray, Acc. Chem. Res.,... 

Although numerous investigations of the electronic spectra of metal sandwich complexes have been carried out, few reliable assignments of the charge-transfer spectra have as yet been made. Thus, only for a few d5 and d6 species (Fe(Cp)2+, Fe(Cp)2, and Co(Cp)2+) have the Laporte-allowed bands been systematically studied (48), although some speculative identifications have been given for Co(Cp)2 and Ni(Cp)2 (88), and apart from the metallocene series hardly any assignments have been attempted. 

Electronic and vibrational spectroscopy continues to be important in the characterization of iron complexes of all descriptions. Charge-transfer spectra, particularly of solvatochromic ternary diimine-cyanide complexes, can be useful indicators of solvation, while IR and Raman spectra of certain mixed valence complexes have contributed to the investigation of intramolecular electron transfer. Assignments of metal-ligand vibrations in the far IR for the complexes [Fe(8)3] " " were established by means of Fe/ Fe isotopic substitution. " A review of pressure effects on electronic spectra of coordination complexes includes much information about apparatus and methods and about theoretical aspects, though rather little about specific iron complexes. ... 

The direct vibrational spectroscopy complements the indirect photochemical evidence for MNO bending. Raman spectra of metal complexes in excited electronic states have been obtained by using either pulsed or CW lasers to produce a near saturation yield of excited states and to simultaneously provide the probe beam for Raman scattering from the excited molecule (18-22). 

In contrast to the situation with the 3d transition metals in particular, the 4f-4f transitions in the electronic spectra of lanthanide complexes rarely serve any diagnostic purpose. It may be noted, however, that the spectra of the octahedral [LnXe] " ions (X = Cl, Br) have particularly small extinction coefficients, an order of magnitude lower than the corresponding aqua ions, due to the high symmetry of the environment. 

Question 5.3 Explain why f-f transitions in the electronic spectra of lanthanide complexes are weaker than d-d transitions in the corresponding spectra of transition metal complexes. 

Kutzler, F. W., C. R. Natoli, D. K. Misemer, S. Doniach, and K. O. Hodgson (1980). Use of one electron theory for the interpretation of near edge structure of K-shell x-ray absorption spectra of metal complexes. J. Chem. Phys. 73, 3274-88. 

Platinum(IV) is an oxidant therefore, ligand-metal charge-transfer (LMCT) transitions are prominent in the electronic spectra of its complexes and play important roles in the photosubstitution chemistry. This is especially true for the hexahalo complexes, [PtXj]. The aqueous hexachloroplatinate(IV) ion, [PtCl ] , undergoes both photoaquation ... 

---