The compounds [Tp(3R,4R,5R)Ir (COD)] (con = cycloocta-1,5-diene; Tp(3R,4R,5R) = hydrotris(pyrazolyl)borate (1), hydrotris(3-methylpyrazolyl)borate (2), hydrotris (3-isopropylpyrazolyl)borate (3), hydrotris(3,5-dimethylpyrazolyl)borate, hydrotris(3-(trifluoromethyl)-5-methylpyrazolyl)borate, hydrotris(3-phenyl-5-methylpyrazolyl)borate, hydrotris(3,5-diisopropylpyrazolyl)borate, hydrotris(3,4,5-trimethylpyrazolyl)borate, hydrotris(4-chloro-3,5-dimethylpyrazolyl)borate (9), hydrotris(4-bromo-3,5-dimethylpyrazolyl) were prepared and characterized by IR and NMR spectroscopy. The X-ray crystal structure of 9 . 2MeOH (triclinic, space group P (1) over bar (No. 2); a = 10.044(1) Angstrom, b = 11.186(2) Angstrom, c = 15.499(3) Angstrom; alpha = 77.90(1)degrees, beta = 73.23(1)degrees, gamma = 66.89(1)degrees; Z = 2; R = 0.0276 for 2469 observed reflections) shows that iridium is four-coordinate with an eta(2)-hydrotris(pyrazolyl)borate. The X-ray crystal structure of 1 (triclinic, space group P (1) over bar (No. 2); a = 7.345(1) Angstrom, b = 7.645(1) Angstrom, c = 15.893(5) Angstrom; alpha = 103.17(4)degrees, beta = 90.30(2)degrees, gamma = 93.59(3)degrees; Z = 2; R = 0.0433 for 2606 observed reflections) shows that iridium is five-coordinate with an eta(3)-bonded tris(pyrazolyl)borate. Equilibria between corresponding four-(eta(2)-Tp(3R,4R,5R)) and five-coordinate (eta(3)-Tp(3R,4R,5R)) species of all the complexes are established in solution. The complex containing the ligand HB(Pz(3Me))(3) (PZ = Pyrazolyl group) (2) rearranged first to the corresponding complex with HB(Pz(3Pr))(2)(pz(5Me)) and then into that with HB(pz(3Me))(pz(5Me))(2). However, 3, which contains HB(Pz(3Pr))(3), gave only the complex with coordinated HB(Pz(3Pr))(2)(Pz(5Pr)).