Ru7, 52%, Z:E 58:42 Ru21, traces Ru21, BHT (10 mol%), 100 °C, 20%, Z:E 95:5 Ru24, 31%, Z:E 95:5 Based on the differences in the stabilities of the investigated complexes and good or very good yields achieved in the studied reactions, we thought that perhaps Ru21 initiates very quickly (as its nonstereoselective counterpart ) reaching maximum conversion within the first minutes of the reaction, therefore its stability is less important. To verify this hypothesis, we investigated its activity profile in a model reaction between allylbenzene (2a) with (Z)-2-butene-1,4-diol (3a) and compared the result with Ru5 (Figure ). Complex Ru5 with standard benzylidene ligand reached the maximum conversion within first ten minutes of the reaction, while the same cross-metathesis carried out in the presence of Ru21 proceeded much slower and a similar result (65% of conversion) was obtained only after one hour. This observation leads us to hypothesise that, during the reaction, not all the catalyst molecules immediately enter the catalytic cycle, but are slowly released with time, allowing high yields to be achieved despite the relatively low stability of the complex.