A process for the prepn. of .mu.-3-carbopentazane-N1,N4:N2,N5- bis[bis(triphenylphosphine)rhodium(I)] dinitrate, claimed as a stereospecific hydrogenation catalyst, involves reaction of tris(triphenylphosphine)nitratorhodium(I) with hydrazine in degassed MeOH under an inert atm.