The bis(ethylene) Rh species Tp(Me2)Rh(C2H4)(2) (1*) (Tp(Me2) = tris(3,5-dimethyl-1-pyrazol-1-yl)hydroborate) has been obtained from [RhCl(C2H4)(2)](2) and KTp(Me2). Complex 1* easily decomposes in solution to give mainly the butadiene species Tp(Me2)-Rh(eta(4)-C4H6). In the solid state its thermal decomposition follows a different course and the allyl Tp(Me2)RhH(syn-C3H4Me) is cleanly obtained as a mixture of exo and endo isomers. The complexes Tp'Rh(C2H4)(2) (Tp' = Tp, Tp(Me2)) afford the monosubstituted species Tp'Rh(C2H4)(PR3) upon reaction with PR3 but react differently with L = CO or CNR: the Tp compound gives dinuclear [TpRh](2)(mu-L)(3) complexes, while, in the case of 1*, Tp(Me2)Rh(C2H4)(L) species are obtained. The ethylene ligand of complexes Tp(Me2)Rh(C2H4)(PR3) is labile, and several peroxo compounds of composition Tp(Me2)Rh(O-2)(PR3) have been isolated by their reaction with O-2 All the mononuclear Rh(I) complexes are formulated as 18e(-) trigonal bipyramidal species on the basis of IR and NMR spectroscopic studies. A. series of dihydride complexes of Rh(III) of formulation Tp'RhH2(PR3) have been prepared by the hydrogenation of the corresponding ethylene derivatives. Complexes [TpRh](2)(mu-CNCy)(3), Tp(Me2)Rh(C2H4)(PEt3), and Tp(Me2)-Rh(O-2)(PEt3) have been further characterized by X-ray diffraction studies.