Protonation of TpM(PR 3 )H 2 (M ) Rh, Ir) complexes with HBF 4 ‚Et 2 O or [H(Et 2 O) 2 ][B(Ar) 4 ] (Ar ) 3,5-(CF 3 ) 2 C 6 H 3 ) affords cationic complexes which exhibit a single hydride resonance at all accessible temperatures in the 1 H NMR spectrum. Formulation as fluxional dihydrogen/hydride complexes is indicated by short T 1 (min) values of ca. 22 ms (Ir) and 7 ms (Rh). The relaxation times are consistent with H-H bond lengths of 0.88-1.11 Å in the iridium complexes and 0.73-0.92 Å in the rhodium complexes depending on the relative rate of the dihydrogen rotational motion. In the case of the iridium complexes, partial substitution of the hydride positions with deuterium or tritium results in large temperature-dependent isotope shifts and resolvable J H-D or J H-T coupling constants. Analysis of the chemical shift and coupling constant data as a function of temperature is consistent with a preference for the heavy hydrogen isotope to occupy the hydride rather than the dihydrogen site. This analysis also provides the limiting chemical shifts of the dihydrogen and hydride ligands as well as the 1 J H-D coupling constant (ca. 25 Hz) in the bound dihydrogen ligand.Since the first report of a stable molecular hydrogen complex by Kubas, 1 the possibility that a fluxional polyhydride complex might also contain a dihydrogen ligand has been actively investigated. 2 In general, transition metal polyhydride complexes are characterized by high coordination numbers (CN 7-9) and high formal oxidation states. 3 Because several structures of nearly equivalent energy are available to seven-, eight-, and nine-coordinate complexes, rapid permutation of the hydride positions is often observed by 1 H NMR spectroscopy. As a result, structural characterization in solution depends upon indirect methods, in which the observed NMR parameters are a population-weighted average of all the hydride environments. For example, Crabtree and co-workers have employed T 1 measurements to detect short H-H contacts in a range of polyhydride complexes, including [Ir(PCy 3 ) 2 H 6 ] + and Fe-(PEtPh 2 ) 3 H 4 . 4 A quantitative treatment of relaxation in polyhydride complexes has been developed which allows useful structural information to be obtained from T 1 (min) data. [4][5][6] We have previously reported the structure and properties of cationic iridium complexes of the form [CpIr(L)H 3 ]BF 4 (L ) various PR 3 ), which have been shown to adopt iridium(V) trihydride structures in the solid state. 7,8 These complexes undergo a rapid hydride rearrangement which leads to a single hydride resonance in the 1 H NMR spectrum above ca. 220 K. However, at very low temperatures, spectra consistent with the solid state structure are obtained.In this paper we investigate the effect of substituting the Cp ligand of [CpIr(L)H 3 ]BF 4 complexes with the hydrotris(1-pyrazolyl)borate (Tp) 9 ligand. The new Tp complexes, [TpIr-(L)(H 2 )H]BF 4 (L ) PMe 3 , PPh 3 ), are formulated as dihydrogen/ hydride complexes, although only a single hydride resonance is observed...

Cationic trihydride complexes of the form [(η-C5R5)Ir(L)H3]BF4 (R = H, Me; L = various phosphines) have been studied. The 1H NMR spectra of these complexes at low temperature display line patterns in the hydride region consistent with AB2X or A2BX spin systems (X = 31P). The values for the HA−HB coupling constant (J AB) derived by computer simulation of the observed spectra are large, ranging from 20−830 Hz. In general, J AB is inversely proportional to the basicity of the ligand L and strongly temperature dependent. These unusual coupling constants have been attributed to quantum mechanical exchange coupling of the hydride ligands. All of the complexes have been partially deuterated and tritiated at the hydride sites and studied by both 1H and 3H NMR spectroscopy. In contrast to J AB, the values of J HT and J TT are independent of temperature. The observed values for J HT have been used to ascertain the contribution of the magnetic H−H coupling to J AB. The contributions of the exchange coupling to J AB have been derived and the corresponding temperature dependency accurately modeled. Significant isotope effects on the values of J AB and the hydride chemical shifts were observed upon tritium and deuterium substitution. The barriers for thermally activated hydride site exchange have also been determined. No appreciable kinetic isotope effects on the thermally activated rearrangement process were observed upon substitution of D and T into the hydride sites. These results are interpreted in terms of a new two-dimensional model for quantum mechanical exchange coupling of the hydrides in these cationic complexes.