The insertion of the [M(CO)3(NN)] fragments (M = Mo, W; NN = 2,2′-bipyridine; 1,10-phenanthroline; ethylenediamine) into the HgX (X = Cl, Br, I, N3, SCN) bonds of the [η5-C5H5Fe(CO)2(HgX)] complexes. Heterotrimetallic compounds with FeHgMX bondings