Transition metal complexes charge-transfer transitions

R. D. Brown, B. H. James, and M. F. O Dvvycr, Tbeor. Chim. Acta, 17, 264 (1970). Molecular Orbital Calculations on Transition Element Compounds. W. T. A. M. V an der Lugt, hit. J. Quantum Chem., 6, 859 (1972). Molecular-Orbital Calculations on Transition-Metal Complexes, Charge-Transfer Spectra, and the Sequence of Metal and Ligand Orbitals. J. J. Kaufman and R. Predney, hit. J. Quantum Chem., Quantum Chem. Symp., 6, 231 (1972). Extension of INDO Formalism to d Orbitals and Parameters for Second-Row Atoms. 

Basicity, ir and a Complexes with Transition Metals and Charge-Transfer Complexes... 

The electronic spectra of allyl complexes (Table 7.4) exhibit relatively low intensity bands due to the d-d ytansitions as well as intense bands resulting from ligand metal and metal ligand charge transfer transitions. In palladium complexes of the type Pd2Cl2(allyl)2, the first three low intensity bands at 29,500 (fi = 543), 34,000 (e = 618), and 40,500 cm (a = 2006) were assigned to the d-d transitions. The two more intense bands at 44,000 cm ( = 5222) and 46,500 cm(8 = 2286) are due to Cl->Pd and Pd allyl electron transfer transitions, respectively. ... 

In dealing with the complexes of metals containing partly filled d shells, it has become customary to label absorption bands as d-d transitions, where the initial and final electronic states can be considered as mainly localized on the metal, and charge-transfer transitions, where one state is mainly localized on the metal and the other on the ligand(s). The classification of spin-allowed and spin-forbidden bands is retained, as are parity restrictions, although these are often termed symmetry restrictions. There are, in addition, several confusing references to the Laporte rule. ... 

Vogler, A., Kunkeley, H. Photochemistry of Transition Metal Complexes Induced by Outer-Sphere Charge Transfer Excitation. 158, 1-30 (1990). 

The colors that we have described arise from d-d transitions, in which an electron is excited from one d-orbital into another. In a charge-transfer transition an electron is excited from a ligand onto the metal atom or vice versa. Charge-transfer transitions are often very intense and are the most common cause of the familiar colors of d-metal complexes, such as the transition responsible for the deep purple of permanganate ions, Mn04 (Fig. 16.33). 

In heterogeneous photoredox systems also a surface complex may act as the chromophore. This means that in this case not a bimolecular but a unimolecular photoredox reaction takes place, since electron transfer occurs within the lightabsorbing species, i.e. through a ligand-to-metal charge-transfer transition within the surface complex. This has been suggested for instance for the photochemical reductive dissolution of iron(III)(hydr)oxides (Waite and Morel, 1984 Siffert and Sulzberger, 1991). For continuous irradiation the quantum yield is then ... 

J. N. Demas and G. A. Crosby, Quantum efficiencies of transition metal complexes. II. Charge transfer luminescences, /. Am. Chem. 93, 2841-2847 (1971). 

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