The Photochemistry of Metal Complexes

The principal photochemical reactions of metal complexes include dissociation, ligand exchange and redox processes. Unlike organic photoreactions (which take place almost exclusively from the S3 or T3 states), the excited state formed on irradiation depends on the wavelength employed. Hence the quantum yield often depends on the wavelength of the irradiating source. The excited-state processes give rise to a reactive intermediate which may find application in the synthesis of new compounds. 

Photoinduced isomerisation is sometimes possible where the metal-ligand bond undergoes homolysis followed by rapid combination of the metal and the rearranged ligand. 

A solvent molecule such as water will exchange for a ligand according to  

In mixed-ligand complexes, different products are often obtained in photochemical and thermal reactions. 

Thermally, ammine complexes of chromium(III) containing a coordinated ligand X (where X is CL, CNS , etc.) undergo substitution of X by H20 in aqueous solution with retention of stereochemistry  

---

The photochemistry of metal complexes has been extensively studied since the 1960s and has been the subject of a number of monographs and reviews [1,11,14-19]. Systematic investigations have concentrated on or d ... 

While the area of the photochemistry of metal complexes with DNA is still at an early stage of development, substantial progress has been made in the last ten years. It is apparent from the different chapters of this review that a knowledge of the photophysimetal complex s excited state can be used to predict the type of reaction caused to the DNA. 

Reviews have appeared of the photophysics of molybdenum complexes, primary and secondary processes in organometallic chemistry, flash photolysis of Pe(CO)5 and Cr(CO)g, dinuclear manganese carbonyl compounds, the photochemistry of metal complexes isolated in low temperature matrices, cluster complexes, diene complexes, photoproduction of coordinativeiy unsaturated species containing rhodium or iridium, and redox chemiluminescence of organometallic compounds.Synthetic and metal organic photochemistry in industry has also been reviewed. 

One notable area of chemistry that can offer some prospects for enhanced displacement of ligands is photochemistry. The photochemistry of metal complexes has been extensively studied and reviewed (16, 282), and it is not proposed to cover this area in any detail. One of the complications in photochemistry of metal complexes is the marked change in reactivity even within a triad of the d block. For example, the reaction modes of the simple [M(NH3)5N3]2+ ions for the cobalt triad vary clearly. For cobalt(III), the photochemistry is dominated by an intramolecular redox reaction (93) whereas for iridium(III), reac-... 

The interpretation of pressure effects on the photochemistry of metal complexes in solution is in some cases limited by information on the partial molar volume of ES species and the difficulty to separate intrinsic and solvational volume contributions. The latter can be resolved in more detail by performing systematic solvent-dependence studies, through which corrections via the application of appropriate solvent parameters can be made in order to extract the intrinsic volume changes that, for instance, control the nature of the photosubstitution mechanism. Other difficulties are the fragmentary nature of much of the published pressure data and the need for a better understanding of the effect of pressure on the rates of photophysical processes. 

Finally, the work reviewed in this chapter has clearly demonstrated how the photochemistry of metal complexes in solution can be tuned by pressure. The application of hydrostatic pressure as a further physical variable should therefore become a standard technique in the study of the photochemical behavior of metal complexes in solution. 

Conclusions What s Next for the Photochemistry of Metal Complexes ... 

Weitz and co-workers extended gas phase TRIR investigations to the study of coordinatively unsaturated metal carbonyl species. Metal carbonyls are ideally suited for TRIR studies owing to their very strong IR chromophores. Indeed, initial TRIR work in solution, beginning in the early 1980s, focused on the photochemistry of metal carbonyls for just this reason. Since that time, instrumental advances have significantly broadened the scope of TRIR methods and as a result the excited state structure and photoreactivity of organometallic complexes in solution have been well studied from the microsecond to picosecond time scale. ... 

Mo(r75-C5H5)2H2] and [MoH dppe ]. Our studies of the di- and trihydride complexes of ruthenium and iridium, described above and published previously (27,35), and those of other workers (discussed at the beginning of this chapter), indicate that photoinduced elimination of molecular hydrogen is a common reaction pathway for di- and polyhydride complexes. To demonstrate the photoreaction s generality and its utility for generating otherwise unattainable, extremely reactive metal complexes, we have begun to study the photochemistry of polyhydride complexes of the early transition metals. We focused initially... 

The photochemistry of rhenium complexes occupies a prominent position in the photochemistry of transition metal complexes. Along with early preparative studies on photosubstitution of carbonyl species like Re(CO)sX, the preparation of the remarkably stable yellow complex /ac-Re(CO)3(phen)Cl foreshadowed the discovery of the a large class of related luminescent materials by Wrighton and co-workers in the 1970s [ 1 ]. As pointed out by Vogler and Kunkley, the current photochemistry of rhenium complexes is rich, spanning eight oxidation states from formal Re(0) (for example, Re2(CO)io) to formal Re(VII) (for example MeReOs) [2],... 

Photochemical methods offer a convenient tool to study intra- and interprotein ET because of their time resolution and selectivity. Various mechanistic and design approaches based on photochemistry of metal complexes have been undertaken. Most of the studies on protein electron transfer processes have been done for hae-moproteins using among others ruthenium complex as a photosensitizer, modified haemoproteins in which haem iron is substituted by another metal (mainly Zn), and CO-bonded haem proteins [6,7],... 

Photochemical oxidation of square-planar bis(l,2-dithiolene) complexes of Ni, Pd, and Pt is by no means limited to IPCT excitation. Photooxidation also occurs in halocarbon solvents. In 1982, two separate reports addressed the photochemistry of metal bis(l,2-dithiolene) complexes. Vogler and Kunkely (80) investigated complexes of the type M(S2C2R-2)2, where M = Ni, Pd, Pt,... 

J3, 4). Low-lying electronic excited states of metal-metal bonded complexes often involve significant changes in the electron density associated with the metal-metal bond, compared to the ground electronic state. Accordingly, study of the photochemistry of metal-metal bonded complexes not only provides potential new reaction chemistry but also provides insight into, and confirmation of, the electronic structure. This symposium volume affords us an opportunity to record the state of the field of metal-metal bond photochemistry. The aim of this article is to summarize recent research results from this laboratory and to place them in perspective in relation to results from other laboratories. 

The photochemistry of metal-metal bonded complexes has been the subject of several reviews see Luminescence Behavior Photochemistry of Organotransition Metal Compounds and Photochemistry of Transition Metal Complexes Theory). Strong a bonding to a antibonding transitions are often observed in metal - metal bonded systems. These lead to photogenerated fragments that undergo reactions and may ultimately recombine (equation 72). 

The photochemistry of these complexes has been studied extensively (50-63a), and the nature of the primary photoprocess has been a controversial subject. Both carbon monoxide dissociation [Eq. (64)] (50, 51a,b, 52-56, 59, 60, 61-63a) and metal-alkyl bond homolysis [Eq. (65)] (51, 51c, 57, 58, 60a) have been proposed on the basis of different experimental results. However, more recent work has shown that CO... 

Wrighton and co-workers (52-52c) have also investigated the photochemistry of these complexes (M = Mo or W) in hydrocarbon matrices at 77 K and found no evidence for metal-methyl bond homolysis. Irradiation of CpM(CO)3CH3 under these conditions led to CO loss and reversible formation of CpM(CO)2CH3 (63) which was spectroscopically characterized [Eq. (69)]. The coordinatively unsaturated complex 63 was stable... 

The photochemical properties of metal-hydride complexes depend upon the number of hydride ligands on the metal . The photochemistry of monohydride complexes... 

We have attempted to review the photochemistry of metal carbonyls, metallocenes, and olefin complexes under two aspects ... 

An impressive example of the possibilities of metal complex photochemistry is the substantiation of the critical step in corrin synthesis reported by Eschenmoser 145> Ring closure is brought about by antara-facial cycloisomerization of a secocorrinoidic palladium complex by photochemical 1.16-hydrogen shift. 

PRESSURE-TUNING PHOTOCHEMISTRY OF METAL COMPLEXES IN SOLUTION the precursor complex. 

PRESSURE-TUNING PHOTOCHEMISTRY OF METAL COMPLEXES IN SOLUTION TABLE 12 Summary of AV Data for the Riug-Closure Step in Eq. (47) ... 

---