Photocatalytic Activation of Dihydrogen

This can be illustrated with the photocatalytic hydrogenation of alkenes by Fe(CO)5, which is relatively nonselective and acts as a photocatalyst for 

Recent gas phase studies have demonstrated catalysis for ethene hydrogenation by Fe(CO)3(C2H4) produced by excimer laser pulse photolysis of a catalytically inert mixtures of Fe(CO)5, H2 and ethene. 

Quantum yields for ethene hydrogenation under these ambient temperature conditions exceeded 20. The photolysis procedure generated a reservoir of Fe(CO)3(ethene)2 which is very labile and dissociates to give the active, unsaturated species Fe(CO)3(ethene). Added CO as well as photoliberated CO inhibits catalysis due to the back reaction to form the stable Fe(CO)4(ethene) [65]. 

An early mechanistic investigation proposed that the NTC product may be the result of CO photolabilization from the tetracarbonyl intermediate M(CO)4(t]4-NBD) 

The ability of these group VI metals to coordinate H2 as M(CO)5(ti -H2) has been demonstrated by flash photolysis TRIR techniques in liquid xenon [67] in alkane solutions [68] and in the gas phase [69]. The binding is nonclassical , i.e., coordination as H2 rather than as a dihydride, evidenced by the presence of a vhh stretching band at about 3000 cm . Notably, reaction of D2 with Cr(CO)5(Ti2-H2) in liquid Xe led to formation of Cr(CO)5(Ti2-D2) but not of Cr(CO)5(ri2-HD). In contrast the species Cr(CO)4(ri2-H2)2, which can also be formed in liquid Xe, reacts with D2 to 

---