Inert metal complexes photochemistry

If the photolysis takes place in an inert gas matrix, both the homolytic splitting of the metal-metal bond and the breaking of a metal-nitrogen bond will be followed by a fast backreaction to the parent compound. The radicals formed by homolysis of the metal-metal bond can not diffuse from the matrix site and will recombine to the parent compound. Moreover, the photoproduct obtained by breaking of a metal nitrogen bond, will not be stabilized by a coordinating solvent molecule and therefore react back to the parent compound. Because of this the photochemistry of some of these complexes has also been studied in a Cl-k-matrix at 10K and for comparison in a PVC film, which is a less rigid medium than the matrix especially at room temperature. 

Although in a few tetranuclear complexes fragmentation is the primary result of photolysis , and others, e.g., (h -CjHp Fe fCO), are inert with respect to both photoinduced M—M cleavage and carbonyl loss , substitution without fragmentation is the dominant photoreaction of tetranuclear metal carbonyls, especially of those containing second and third-row metals. Both Ir (CO),2 and H Os (CO),2 " lead to tetranuclear olefin-substituted products when irradiated with alkenes, both with some rearrangement of the Ir moiety in the former case, and a hydride abstraction in the latter. Irradiation of H Ru/CO),j in the presence of P(OMe)j or PPhj leads to stepwise formation of the substituted clusters H Ru/CO), (n = 1-4) with low quantum efficiency . Carbonyl substitution also dominates the photochemistry of HjM CCO) (M = Ru, FeRuj, FeOsj) . 

Mechanistic photochemistry of metal )8-diketonate complexes has received increasing attention. Near-ultraviolet irradiation of diketonate complexes of Mn ", Fe , Co ", No" and Cu" eventuates in one-electron reduction by the ligand of the metal center whereas Cr " complexes photo-isomerize. Irradiation of franj-Rh(CF3COCHCOMe)3 appears to lead to two photoactive excited states which may have significant radical character. Photoisomerization to the cis form occurs in inert solvents whereas decomposition takes place in the presence of potential hydrogen atom donors such as alcohols. ... 

Many of the early principles of inorganic photochemistry have developed from studies carried out on the first-row 3d) series of transition metal ions. The majority of these ions are kinetically labile in aqueous solution, the two exceptions being complexes of the trivalent Cr " (d ) and Co " d ) ions, both of which are kinetically inert. This difference in their solution lability has resulted in complexes of these two ions receiving the majority of interest, because their slow substitution rates allows the stereochemistries of the individual photoprocesses to be determined before subsequent thermal reactions occur. 

Among the second- and third-row transition metal halide complexes, those of platinum have received considerable study. Both the octahedral platinum(IV) complex PtCli and the square planar platinum(II) complex PtCl have been investigated. Both complexes are substitution inert under thermal conditions, and kinetic studies of their substitution chemistry have been important in the development of a general understanding of the mechanisms of substitution reactions in transition metal chemistry. The photochemistry of PtCli" was one of the earliest such studies to be made, and the early discoveries of the photosensitive nature of platinum halides led to these salts being used in photographic processes. The subsequent decision to use a silver-based process was based more on economical rather than on technical reasons. 

---