Electronic transitions/spectra metal complexes

Keywords TDDFT Excitation energies Excited states Transition metal complexes Electronic spectra Metallotetrapyrroles Metallocarbonyls a-diimine complexes Porphyrins Porphyrazines Phthalocyanines Werner complexes Sandwich complexes... 

Stable Mn(HI) compounds, Mn(R2r fc)3, have been known for a long time (42, 46). The structure of Mn(Et2C tc)3 is elucidated (47). The inner geometry of the Mn(CS2)3 core does not conform to the usual D3 point symmetry of transition metal complexes of this type, but shows a strong distortion attributed to the Jahn-Teller effect. The electronic spectrum (48, 49) and the magnetic properties of this type of complexes are well studied (50). 

Furthermore, the method of orientation selection can only be applied to systems with an electron spin-spin cross relaxation time Tx much larger than the electron spin-lattice relaxation time Tle77. In this case, energy exchange between the spin packets of the polycrystalline EPR spectrum by spin-spin interaction cannot take place. If on the other hand Tx < Tle, the spin packets are coupled by cross relaxation, and a powder-like ENDOR signal will be observed77. Since T 1 is normally the dominant relaxation rate in transition metal complexes, the orientation selection technique could widely be applied in polycrystalline and frozen solution samples of such systems (Sect. 6). 

Transitions between different electronic states result in absorption of energy in the ultraviolet, visible and, for many transition metal complexes, the near infrared region of the electromagnetic spectrum. Spectroscopic methods that probe these electronic transitions can, in favourable conditions, provide detailed information on the electronic and magnetic properties of both the metal ion and its ligands. 

Up to this point we have considered two central issues involved in interpreting electronic spectra of transition metal complexes—the number and intensities of spectral lines. There is a third important spectral feature, the widths of observed bands, which we have not yet discussed. Consider again the visible spectrum for... 

The electronic spectrum is yet another property which illustrates the similarities between the metallocenes and (7r-ollyl) metal compounds. In Table VI are listed some data for a series of Coin(absorption bands with the small extinction coefficients are probably two of the spin-allowed d-d transitions. Scott (34) has developed an approximate axial ligand field model for the carborane-transition metal complexes and has discussed the optical spectra in relation to this bonding theory. The actual assessment of bonding in the (7r-ollyl) metal compound as well as the metallocenes would be greatly aided by accurate assignments of the electronic spectra. 

TR methods were originally developed in om laboratories to study excited-state structures and dynamics of transition metal complexes such as Ru + (bpy)s and metaUoproteins. TR measurements rely on a pump-probe approach in which two separate laser pulses are used, one to excite the system and the other to probe the transient Raman spectrum. The time resolution of the experiment is determined by the width of the laser pulses (typically 7 ns for a Q-switched laser or as short as 1 ps for a mode-locked laser). The pulses are variably delayed with respect to one another to achieve time resolution, either by optically dela)dng the probe pulse with respect to the pump pulse or by electronically delaying two independently tunable lasers. Thus, two different approaches are required depending on the time scale of interest. The fastest timescale (from 10 to 10 s) requires optical delay to achieve sufficiently short separation between the pump and probe pulses. In such a scheme, the probe pulse is sent through a fixed path, but the pump pulse is sent through a variable path that can be scanned. Since hght travels about 1 ft per ns, a difference in pathlength of a few feet is sufficient. The second approach typically uses two Q-switched Nd YAG lasers that are electronically delayed with respect to one another, to access... 

Acetonitrile is a convenient solvent in which to study, by pulse radiolysis, the one-electron reduction of transition metal complexes that are not stable in water or hy-droxylic media. For example, the tantalum compound [Ta2Cl6(4-methylpyridine)4] has been shown to be reduced by CH3CN with A = 1.2 x 10 dm mol s [24], The absorption spectrum of the product shows the characteristic features of d-d transitions and it was suggested that the added electron is delocalized over the double-bonded Ta=Ta moiety. Another example is the radiolytic reduction of the vanadium(III) complex [VCl3(y-pic)3], where y-pic is 4-methylpyridine, to the derivative via the V complex [25]. It was shown by pulse radiolysis that the electron adduct of the complex decayed in a first-order process with k= 1.3 x 10 s which is thought to involve loss of Cl". The intermediate V complex had a... 

The prototypical photochemical system for CO2 reduction contains a photosensitizer (or photocatalyst) to capture the photon energy, an electron relay catalyst (that might be the same species as the photosensitizer) to couple the photon energy to the chemical reduction, an oxidizable species to complete the redox cycle and CO2 as the substrate. Figure 1 shows a cartoon of the photochemical CO2 reduction system. An effective photocatalyst must absorb a significant part of the solar spectrum, have a long-lived excited state and promote the activation of small molecules. Both organic dyes and transition metal complexes have been used as photocatalysts for CO2 reduction. In this chapter, CO2 reduction systems mediated by cobalt and nickel macrocycles and rhenium complexes will be discussed. 

Crystal field theory was developed, in part, to explain the colors of transition-metal complexes. It was not completely successful, however. Its failure to predict trends in the optical absorption of a series of related compounds stimulated the development of ligand field and molecular orbital theories and their application in coordination chemistry. The colors of coordination complexes are due to the excitation of the d electrons from filled to empty d orbitals d-d transitions). In octahedral complexes, the electrons are excited from occupied t2g levels to empty Cg levels. The crystal field splitting Ao is measured directly from the optical absorption spectrum of the complex. The wavelength of the strongest absorption is called Amax and it is related to Ao as follows. E = hv, so Ao = hv = Because en-... 

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