Cobalt photochemistry, mechanisms

Two rather different points of view have been maintained concerning the general question of mechanisms in inorganic photochemistry. This section defines both of these and attempts to indicate the evidence in favor of each subsequent sections will consider in detail the applicability of various mechanistic proposals to the study of the photochemistry of cobalt(III) and chromium(III). 

Electron Transfer Photochemistry.—It has been suggested that electron transfer can bring about base hydrolysis of cobalt(m) complexes. Recent examples include the aquation of sulphite complexes, where detection of dithionate as one of the products is good evidence for an electron transfer mechanism, and of the dicobalt /M-superoxo complex [(H3N)6Co 02 Co(NH3)5] + in the presence of sulphite. ... 

Kinetics of aquation of [Ru(LLLL)X2], with LLLL = cyclam, 2,3,2-tet, en2, or (NH3)4, and X = Cl or Br, have been followed by cyclic voltammetry. Rate constants, and activation parameters (A// and A5 ) have been evaluated, and compared with kinetic parameters for reactions of analogous compounds of ruthenium(III) and cobalt(III). Similar trends obtain for all three sets of complexes. There is retention of stereochemistry, rates decrease as the extent of chelation in LLLL increases, and trans complexes are less labile than cis analogs. Reactivities are determined by solvation of the initial and transition states, by nephelauxetic effects, and by a-trans effects. A limiting dissociative (D) mechanism is proposed for the ruthenium complexes, with square-pyramidal geometry for the transient intermediate [cf. rhodium(III) photochemistry below. Section 5.8.10]. Differences in isomer lability have also been described for... 

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