reducing agents before they are used as a catalyst examples of suitable reducing agents include hydrides of alkali metals and alkaline earth metals or aluminum hydrogen or boron hydrogen compounds in particular those of the formula iv met is a mono or bivalent metal preferably zinc lithium sodium or potassium and e is aluminum or boron and r 11 is c 1 u2013 c 8 alkyl and q is 1 2 3 or 4 preferably 4 or 1 and p is the valency of met very particularly preferred reducing agents are libh 4 nabh 4 zn bh 4 2 lialh 4 li bhethyl 3 and li alh sec butyl 3 na alh 2 2 methoxyethoxy 2 sodium hydride and lithium hydride the amount of the reducing agent is preferably selected in such a way that the molar ratio of hydridic hydrogen in the reducing agents to the transition metal complex is 0 5 to 5 larger ratios are possible but uneconomic further particularly preferred transition metal complexes are those which are obtained by reacting compounds of the formula i with zinc compounds zny 2 or znyhal where y is in each case independently hydrogen bh 4 or an organic radical for example and with preference c 1 u2013 c 8 alkyl phenyl and hal is bromine chlorine or iodine preferred zinc.