Photoexcitation at 355 nm of [Fe(CO),L] and [Ru,(CO)&,] (L = CO, PPh,, P(OMe)3, and P(O-o-tolyl),)can be used to generate catalysts for the isomerization of 1-pentene to cis-and trans-2-pentene. Each complex gives a different initial ratio of trans-to cis-2-pentene ranging from approximately 6 for [RU~(CO),~] and [RU~(CO)~(P(OM~)J~] to approximately 1 for [Fe(CO)4P(O-o-tolyl)3]. Comparisons of the initial ratios of the isomeric products shows that steric effects, not electronic effects, of the P-donor ligands are responsible for the variation in isomer ratio. The more sterically demanding complexes give the smallest ratio of transto cis-2-pentene. Thus, sterically crowded complexes give enrichment of the less thermodynamically stable alkene. Similarly, beginning with cis-2-pentene, [Fe(C0)3(PPh3)2] gives a much smaller ratio of trans-2-pentene to 1-pentene at low conversion than does [Fe(C0)6] under 355-nm irradiation. Initial quantum yields for isomerization generally exceed unity, and the ultimate distribution of alkenes is the thermodynamic ratio, evidencing the photochemical formation of a thermally active catalyst. For [Fe(CO)4L] the only detected primary photoprocess is diesocitive loas of CO with a quantum yield of -0.4, while for [Ru,(CO)&] the primary photoprocess is proposed to be rupture of a metal-metal bond.
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