Organometallic photochemistry

Organometallic compounds are those which contain at least one direct metal-to-carbon bond. The main classes of photochemical reaction of importance to organometallic compounds are given below. 

The dominant photochemical reaction of metal carbonyl compounds is loss of carbon monoxide, which is usually followed by substitution of another ligand to replace the expelled carbon monoxide. 

Preparative reactions involving photosubstitution of CO are carried out by irradiation of the compound in a weakly-coordinating solvent such as tetrahydrofuran (THF) and then displacement of the solvent ligand at room temperature with the chosen ligand, L. 

On photolysis, dinuclear single metal-to-metal-bonded carbonyls undergo homolysis of the metal-to-metal bond  

The radical-like products readily abstract halide ligands from halo-carbon solvents  

Photochemical conditions are widely used in synthetic organometallic studies, and there is a steadily increasing number of quantitative studies concerned with quantum yields and elucidation of the photochemical mechanism. Most of the systematic work has been done on metal carbonyls and their derivatives, and the following discussion will be limited to this class of compounds. 

Nasielski and Colas studied the following reaction in benzene and cyclohexane with M = W  

Flash photolysis of Cr(CO)gin cyclohexane produces the transient solvent complex CrCCOjjfCjHjj) which was identified by IR. The Cr—(CgHjj) bond strength has been estimated as -53 kJ mof by photoacoustic calorimetry. This species reacts with CO and HjO with rate constants of 3.6x10° and 4.5x10 M s at 25°C, respectively, both with AH = 22 kJ mol . The reaction of CrfCOlsfOHj) with CgH,2 has 

Other work using picosecond laser spectroscopy has shown that these reactions proceed via a solvent intermediate, M(CO)5(solvent), which forms in a few picoseconds after the laser pulse and then decays to products. Lee and Harris have observed formation of the solvated species Cr(CO)5(C5H,2) with t = 17 ps and the decay of the vibrationally excited Cr(CO)j with t 21 ps (apparently at ambient temperature). These observations are at variance with those of Spears and co-woikers, who claim that the bare Cr(CO)j persists on the 100-ps time scale at 22°C. Hopkins and co-workers have used resonance Raman detection to show that the 100-ps process is due to thermal relaxation of the excited vibrational state, probably of Cr(CO)5(CgH,2). 

The kinetics of ligand substitution on Cr(CO)5(heptane) was studied by Yang et al. and the rate constants vary by 20 for different entering groups. As noted above, the AH for CO and H2O substitution on Cr(CO)5(CgH,2) is smaller than the Cr—(CjH,2 bond strength. These observations seem most consistent with associative activation. On the other hand, van Eldik and co-workers have done several studies in mixed alkane/amine solvents and interpret the observed values of AV in terms of dissociative activation. 

As was also the case for coordination compounds, the excited states of organometal-lic molecules have distinct physical and chemical properties that differ from those of the ground state. The photochemistry of organometallic compounds can often open reaction mechanisms that are thermally inaccessible. 

---

Hancock received his B.A from Harvard and his Ph.D. from the University of Wisconsin in 1968. After a National Institutes of Health Postdoctoral Fellowship at Yale, he worked as assistant and associate professor in the chemistry department of the University of California—Davis from 1968 to 1979, where he taught graduate and undergraduate chemistry and did research in organic and organometallic photochemistry. His work opened a new field of study in organoboron photochemistry. 

G. L. Geoffroy and M. S. Wrighton, Organometallic Photochemistry. Academic Press, New York, 1979. 

Fig. 2. Schematic diagram of an IR kinetic spectrometer and details of the four sets of equipment which have been used successfully for organometallic photochemistry. The spectrometers at the Max Planck Institut fur Strahlenchemie in Miilheim (60) and the University of Nottingham (61) are for use with solutions, while those at Northwestern University (60) and the University of California (Davis) (69) are for gas phase samples.

Geoffroy, G.L. Wrighton, M.S. In Organometallic Photochemistry Academic Press New York, 1979. 

IR spectroscopy is a powerful spectroscopic technique for examining the structure and behavior of intermediates involved in organometallic photochemistry. Examples are given of the combination of IR spectroscopy with matrix isolation, with liquid noble gases as solvents, and with flash generation, for probing novel transients and intermediates. 

These examples show that IR spectroscopy continues to be a powerful technique for the characterisation of unstable intermediates important in organometallic photochemistry. 

DR. ANTHONY POE (University of Toronto) As Dr. Geoffroy mentioned, there is a fair bit of work being done on dinuclear metal-metal bonded carbonyls but rather less on metal clusters [Geoffroy, G. L. Wrighton, M. S., "Organometallic Photochemistry," Academic Press New York, 1979]. We have been interested for some time in the thermal fragmentation of metal clusters, and have recently looked at some photochemical reactions as well. I would like to present some results here today which are very preliminary. 

Spectroscopic Investigations of Organometallic Photochemistry in Supercritical Fluids... 

GL Geoffrey, MS Wrighton. Organometallic Photochemistry. New York Academic, 1979. 

---