The present invention refers to complexes prepared by reacting rhodium trinitrate with a suitable hydrazine and a suitable tertiary phosphine, more specifically to the compounds di(.mu.-hydrazine-N.sup.1 :N.sup.2)-bis[bis(triphenylphosphine)rhodium (I)]dinitrate and .mu.-3-carbopentazane-N.sup.1,N.sup.4 :N.sup.2,N.sup.5 -bis[bis(triphenylphosphine)rhodium (I)]dinitrate, which are homogeneous hydrogenation catalysts and their application in the hydrogenation of the exocyclic methylene group of acid addition salts of 6-demethyl-6-deoxy-6-methylene-5-hydroxytetracycline (methacycline) to prepare .alpha.-6-deoxy-5-hydroxy-tetracycline (doxycycline).