Flash photolysis of Cr

Relatively little work has been done on the flash photolysis of gas phase metal carbonyls, partly because of the low volatility of many of the compounds. Early work by Callear (41,42) provided some evidence for Ni(CO)3 generated from Ni(CO)4 in the gas phase (41) and Fe atoms produced from Fe(CO)5 (42). This latter process has even been used as the basis of an Fe atom laser (43). More recently Breckenridge and Sinai (44) studied the flash photolysis of Cr(CO)6. Their results, interpreted largely on the basis of data from matrix isolation experiments, were in broad agreement with Kelly and Bonneau s solution work (JJ). In particular, they found no evidence for loss of more than one CO group [Eqs. (4) and (5)]. 

It is worth noting that one of the great advantages of the matrix technique is that trace impurities are rarely a problem because there is little or no diffusion in the matrix. Recently Bonneau and Kelly Q ) have obtained definitive results on this solution system, partly by reference to data obtained in solid matrices (21), which suggest that saturated perfluorocarbons should interact with Cr(C0)5 less than other practicable room temperature solvents. Thus, Bonneau and Kelly have investigated the laser flash photolysis of Cr(C0>5 in perfluoromethylcyclohexane (C7F14) at room-temperature. A transient species 620 nm) formed... 

Figure 6. Summary of the results of flash photolysis of Cr(CO)6 in C7FH in solution. (Reproduced from Ref. 36. Copyright 1980, American Chemical Society.)...

The reactivity of a number of alkane complexes has been examined and this field has been reviewed through 1996 by Hall and Perutz. Flash photolysis of Cr(CO)6 in cyclohexane showed that solvation occurs within the first picosecond after photolysis, a fact that appears to rule out spin crossing as an important component in the dissociation of CO from Cr(CO)6. The stability of CpRe(CO)2(alkane) is particularly striking. Comparison of the rate constants for heptane solvated metal complexes with CO, Table 1, reveals that the rate constant for CpRe(CO)2(heptane) is five orders of magnitude slower than that of CpV(CO)3 (heptane). In fact, the stability of the CpRe(CO)2(alkane) complexes is so high that it has been possible to carry out low-temperature NMR on the cyclopentane complex generated by continuous photolysis of... 

The results of early flash studies were irreproducible and led to various conclusions concerning the nature of the primary photoproduct. For example, flash photolysis of Cr(CO)g in room temperature degassed cyclohexane solutions and in polystyrene film was reported to produce a transient having a maximum absorbance in the visible at 450 nm and which decayed with second-order (equal concentration) kinetics (83). The following scheme was proposed for the overall mechanism ... 

Under supposedly the same conditions (i.e., degassed cyclohexane at room temperature) flash photolysis of Cr(C0)5 was reported by Nasielski et al. (84) to give two successive transients. The first showed maximum visible absorption at 483 nm and decayed in a first-order process with a lifetime of T = 6 ms. The second transient decayed much more slowly with approximately second-order kinetics. Added CO enhanced the rate of decay of the second transient but did not affect that of the first. It was suggested that the transients were the same species supposedly found by Stoltz et al. (85,86) in low temperature photolyses of Cr(CO) C and D3JJ Cr(C0)5. The following mechanism was proposed ... 

Similar results were obtained by Kelly and Morris (87) for the flash photolysis of Cr(C0)g and Mo(CO) in room temperature degassed cyclohexane solutions. With Cr(C0)g the decay of the first transient followed first-order kinetics with a half-life... 

More extensive studies of CrCCO) in highly purified cyclohexane have demonstrated that the earlier contradictory and ir-reproducible results were almost certainly due to reactions of Cr(C0)3 with impurities in the solutions (88). Flash photolysis of Cr(C0)g produced a transient species within 50 ns of excitation and having = 503 5 nm. In degassed cyclo-... 

There have also been a number of IR spectra of metal carbonyls using flash photolysis to investigate unsaturated intermediates. Three such papers are mentioned here. A paper by Church et al [20] looked at flash photolysis of Cr(CO)5 in a solution of cyclohexane, showing proof of a C v Cr(CO)5 photoproduct, another [21] looked at flash photolysis in cyclohexane solution saturated with H2, showing production of Cr(CO)5(H2), and a third looking at flash photolysis of Fe(CO)5 in a solution of benzene [22]. 

Solution studies of transient alkane binding at room temperature were first performed in 1973 by flash photolysis of Cr(CO)6 in cyclohexane, which gave a UV/vis band at 503 cm-1 similar to that seen in CH4 matrices71 72 ... 

The products of flash photolysis of Cr(CO)g have been re-examined using apparatus with a higher time resolution and more extensively purified solvent than in previous studies. A species was shown to be formed within 50 ns of the excitation of Cr(CO)g which has a broad absorption at 503 + 5 nm, a lifetime of > 200 ps, and is believed to be Cr(CO)5. The species is the precursor of two other species, previously thought to be the and forms of Cr(CO)5, but now known to arise from reaction with impurities. The structure of Cr(CO)5 is not presently known.Co-condensation at 20 K of sodium atoms and Cr(CO)e diluted in argon leads to the formation of [Cr(CO)5] ", the i.r. spectrum of which is consistent with a structure. ... 

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... 

A photochemical route can also be used to effect the oxidative addition of a carbon-fluorine bond to rhodium(I). Such a conversion has been achieved by the photolysis of (>/ -cp )Rh(C2H4)(PMe3) in the presence of hexafluorobenzene, where the initial step involves displacement of C2H4 to give a / -complexed CeFe, followed by the photochemical oxidative addition of an aromatic C-F bond (Scheme 7.6). A similar tj -complexed aromatic moiety has been observed in the pulsed laser flash photolysis of Cr(CO)6 in benzene ... 

Flash photolysis of Cr(CO)e produces two transient intermediates, which are thought to be isomeric pentacarbonyls. Modes of dissociation and other subsequent reactions of M(CO)6, generated photolytically from M(CO)e, where M = Cr, Mo, or W, have been investigated. ... 

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