Spectra from transition metal complexes

Emission Spectra, The emission spectra from transition metal complexes can be an extremely useful aid in studying molecular electronic states. The binuclear complex Cu2(PAA)2en has a sharp fluorescence emission at 427 nm. This is unusual since Cu(II) compounds rarely, if ever, are observed to emit. It is significant that neither Cu(Acac)2 nor Cu(Acac)2Cn emits under the experimental conditions used to study Cu2(PAA)2en. Indeed, there are no reports in the literature of emission from either of these chelates under any experimental conditions. The fluorescence spectra of the protonated ligand, H4(PAA)2en, and of Cu2(PAA)2en are compared in Table IV. The excitation wavelength in... 

The band at 1600 cm-1 due to a double-bond stretch shows that chemisorbed ethylene is olefinic C—H stretching bands above 3000 cm-1 support this view. Interaction of an olefin with a surface with appreciable heat suggests 7r-bonding is involved. Powell and Sheppard (4-1) have noted that the spectrum of olefins in 7r-bonded transition metal complexes appears to involve fundamentals similar to those of the free olefin. Two striking differences occur. First, infrared forbidden bands for the free olefin become allowed for the lower symmetry complex second, the fundamentals of ethylene corresponding to v and v% shift much more than the other fundamentals. In Table III we compare the fundamentals observed for liquid ethylene (42) and a 7r-complex (43) to those observed for chemisorbed ethylene. Two points are clear from Table III. First, bands forbidden in the IR for gaseous ethylene are observed for chemisorbed ethyl-... 

Over the past 15 years there has been a wealth of research on development and application of transition metal complex sensitizers to the development of dye sensitized photoelectrochemical (solar) cells (DSSCs) [113]. Charge injection from the excited state of many sensitizers has been found to be on the subpicosecond timescale, and a key objective has been to identify chromophores that absorb throughout the visible spectrum. For this reason, Os(II) complexes appear attractive and a variety of attempts were made to make use of these complexes in DSSCs in the 1990s [114-116]. Work has continued in this area in recent years and representative examples are given below. 

Cations come in many shapes and sizes. The simplest is the lone proton which may jump from base to base along a small channel. Then there are inorganic ions with no directional preferences for bonding, such as the alkali or alkaline metals, and NH4+ which is tetrahedral but appears spherical when hydrated. At the other end of the spectrum of structural complexity we have organic cations and hydrated transition metal complexes with non-uniform charge densities. 

Figure 5a shows CD spectra of tartaric acid, which has an absorption i the short wavelength region and thus is prone to suffer from dispersion effect as compared with transition metal complexes. Two solution spectra in solvent of different polarity, water and dioxane, are similar to each other, but the CD C a nujol mull is quite different from that in solution. A KBr disc prepared t avoid dispersion effects gave a solid-state tartaric acid spectrum similar to thi in solution (Fig. 5b). Thus the difference between the nujol mull CD and solutid CD is not due to the different molecular conformation or intermolecular intera tion in the two phases. Most likely, it is due to the dispersion effect in the cas of the nujol mull form. Many nujol mull CD spectra of organic compound have been reported recently, but most of them appear to suffer from substanth dispersion effects. It is to be noted that the dispersion terms for molecules C...

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... 

Two papers looking at the photochemistry of iron carbonyls touching upon aspects of their photochemistry that are central to the work below are discussed now. The paper of Hubbard and Lichtenberger [36] from 1981 examined the photoelectron spectrum of Fe(CO)5 in the gas phase. This paper is of relevance as they claimed to have evidence of Jahn-Teller distortions in the Fe(CO)5 cation. Here for the first time it is explicitly mentioned that highly symmetrical transition metal complexes in general have good potential for observable Jahn-Teller activity with regards to their photochemistry after ionization and/or dissociation. They found that ionization into the E state showed Jahn-Teller activity and discussed this in terms of non-Berry pseudo-rotation. 

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-... 

Color arises when a substance absorbs light from a portion of the visible spectrum and transmits or reflects the rest. Both d-d and charge-transfer transitions can occur in the visible region and either or both processes can contribute to the color of a transition metal complex. 

---