Rhodium(I) complexes of the type Tp(3R,4R,5R)Rh(LL) (LL = 2 CO, NBD (norbornadiene), COD (1,5-cyclooctadiene), 3R = 4R = 5R = H; 3R = Me, 4R = 5R = H; 3R = Me, Ph, CF3, 4R = H, 5R = Me; 3R = Me, 4R = Me, Cl, 5R = Me; 3R = Pr-i, 4R = Br, 5R = H) were synthesized and fully characterized. Isomeric forms of three types were identified in solution for these compounds : type A, square planar complexes in which the Rh(NN)(2)B six-membered ring has a boat conformation, the third pyrazolyl ring being free and occupying an equatorial position; type B, compounds with a coordination geometry and chelate ring as above, but having the third pyrazolyl ring in an axial position, i.e., an arrangement which can easily form penta-coordinate complexes; type C, five-coordinate species which, however, on the NMR time scale, are in a fast exchange with those of type B. In the carbonyl complexes (LL = 2 CO), compounds of type C can be distinguished from the forms A and B by IR spectroscopy, The dynamic equilibria between the corresponding isomeric structures, A, B, and C, are solvent-dependent and can be shifted completely toward the forms B and C by placing a methyl substituent in the 5-position of each pyrazole. For compounds possessing COD as a co-ligand, the formation of square-planar complexes of type B, with kappa(2)-bonded pyrazolylborates is preferred, as indicated by Rh-103 NMR spectroscopy, whereas both four- and five-coordinate species are present in compounds having NBD as a co-ligand, their relative contributions being dependent on the substituents bn the pyrazolyl rings. The X-ray crystal structure of the compound Tp(Me)-Rh(NBD) (space group P2(1)/c, a = 18.152(3), b = 16.732(4), c = 20.591(4) Angstrom; beta = 141.793(7); Z = 8; R = 0.0457, R(w) = 0.0412 for 3904 observed reflections) shows that the tris(pyrazolyl)borate ligand is kappa(3)-bonded and the rhodium atom has a distorted trigonal bipyramidal structure (type C), the two olefinic double bonds being in an equatorial and an axial position, respectively.