Mechanisms of the thermal and photochemical decomposition of dimethyl(2,4-pentanedionato)gold were examined by UV-visible and1H NMR spectroscopies in solution. The formation of gold mirrors results from both the thermal and photochemical decomposition reactions. The thermal decomposition reaction is extremely solvent dependent and is not observed in non-coordinating, non-polar solvents (i.e. cyclohexane). The kinetics for thermal decomposition are observed to be first-order in gold complex disappearance although the solvent plays a critical role in the decomposition process. Decomposition by reductive elimination of ethane and protonation of the 2,4-pentandionate ligand are major reaction modes. The mechanism for reductive elimination is examined by deuterium labeling with the perdeuteriodimethylgold compound. The formation of ethane-d3and traces of methane by reaction from the 50:50 mixture of (dimethyl-d0)-and (dimethyl-d6)(2,4-pentanedionato)gold indicates that free radicals are formed from the homolysis of gold-methyl bonds. On the basis of the ratio of ethane-d0to ethane-d3, however, the reaction is believed to proceed predominantly via a concerted reductive elimination and, to a lesser extent, a free-radical mechanism simultaneously. On the other hand, the photochemical decomposition produces more 3-methyl-2,4-pentanedione and less ethane. In labeling studies, the ratio of ethane-d0to ethane-d3is also decreased and is indicative of the greater radical nature of the photolytic mechanism. The UV photolysis does not show the same solvent dependence to reaction as the pyrolysis does, although the observed product ratios do vary with solvent. Thus, solvent-cage effects may be important to the decomposition process. © 1989, American Chemical Society. All rights reserved.