The kinetics of the thermal insertion reaction of SnBrz and SnCI, with the metal-metal bonded dimer [x-C5H5Fe(CO)Z]2 have been studied by following the change in the visible spectrum. The kinetic data are consistent wlth a two-stage mechanism involving the formation of a carbonyl-bridged intermediate followed by nucleophilic attack by the halides on this intermediate. The formation of the intermediate requires an activation enthalpy of 38.0 _+ 1.0 kcal/mole, and an activation entropy of 45.5 _+ 1.5 cal mole-' deg-'. The activation energy required to break the Fe-Fe bond is estimated to be about 32 kcal/mole.Canadian Journal of Chemistry, 48, 3300 (1970) Since the relatively recent discovery of the prepared in a similar manner except that the preparation existence of covalent metal-metal-bonds in was carried out in an atmosphere of nitrogen in a dry bag. Both anhydrous SnC1, and SnBrz were stored in pounds 'Ontaining d-block there has vacuo over phosphorus pentoxide. Tetrahydrofuran was been considerable interest in the synthesis and purified by refluxing over sodium wire, and was freshly structure of metal-metal bonded carbonyl or distilled before use in the kinetic runs. substituted carbonyl compounds. Although the with hexacarb~ny]bis(tri-n-butyl-phosphine) di-Thecell was tightly stoppered and immediately placed in cobalt (7) have been previously investigated. In a thermostated cell holder, making sure that no air bubbles were trapped inside. The change in the visible this paper we report the kinetic data for the spectrum of the reaction mixture was recorded on a thermal insertion reaction of SnC1, and SnBr, Unicam SP800 spectrophotometer, or the decrease in the with [x-c,H,Fe(CO),],.optical density at 550 mp was measured with a Gilford l-he structure of [ n -~,~,~e (~~) 2 I, in solution 2400 spectrophotometer. The infrared spectrum of the reaction products showed no bridging carbonyl frehas been the Of debate for some time. It is quencies. The slow decomposition of [5r-C,H5Fe(CO)2]z now believed that the solution contains cis-in THF was also followed in a similar manner, bridged and trans-bridged species in comparable In all cases the reaction solutions were handled so that quantities which interconvert rapidly via a non-they were exposed to a minimum of air and light. bridged intermediate (8). l-he insertion of tin(^^) Exposure to air resulted in varying degrees of oxidation of the tin(I1) halide and the complex, and hence led to into the Fe-Fe bond is substanti-irreproducibility of the kinetics. The rates of the reactions ated (1, 2) and X-ray crystallographic studies of were found t o be greatly accelerated during exposure to [ T~-C , H , F~( C O )~] , S~C~, (9) show that this corn-laboratory light. pound is indeed Fe-Sn-Fe bonded.Pseudo first order kinetics were observed in all cases with SnBrz and SnCI, and the plots of log (o.d., -o.d.,),
Experimental and Resultswhere o.d., and o.d., are the optical densities at 550 mp Materials at timestand co, respectively, against time were linear for All chemical...
