EXCHANGE COUPLING IN NIOBOCENE TRIHYDRIDES, NB(C(5)H(3)RR')(2)H-3, AND THEIR ADDUCTS WITH COPPER TRIAD CATIONS, [(NB(C(5)H(3)RR')(2)H-3)(2)M](- R=H, R'=SIME(3) - R=R'=SIME(3) - M=CU, AG, AU)() (R=R'=H )

The reactions of Nb(C(5)H(3)RR')(2)Cl-2 with Red-Al followed by hydrol ysis yield Nb(C(5)H(3)RR')(2)H-3 (R = R' = H, 1; R = H, R' = SiMe(3), 2; R = R' = SiMe(3), 3). These compounds react with Lewis acidic coina ge cationic species, namely, [Cu(MeCN)(4)]PF6, AgBF4, and ''Au(THT)PF6 '', prepared in situ from AuCl(THT) and TIPF6 in a 2 to 1 ratio to yie ld the adducts [{Nb(C(5)H(3)RR')(2)H-3}(2)M](+) (M = Cu, R = R' = H, 7 ; R = H, R' = SiMe(3), 8; R = R' = SiMe(3), 9; M = Ag, R = H, R' = SiM e(3), 10; R = R' = SiMe(3), 11; M = Au, R = R' = H, 12; R = H, R' = Si Me(3), 13; R = R' = SiMe(3), 14). Like 1, but unlike the corresponding tantalum derivatives Ta(C(5)H(3)RR')(2)H-3 (R = R' = H, 4; R = H, R' = SiMe(3), 5; R = R' = SiMe(3), 6), 2 and 3 show exchange couplings in their high-field H-1 NMR spectra due to a hydride tunneling phenomeno n. The magnitudes of exchange couplings are larger in the cases of 2 a nd 3 than in the case of 1 as a result of the decrease of electron den sity upon increasing the number of SiMe(3) substituents on the Cp ring . The addition of a Lewis acidic cation results in the observation of an AB(2) pattern for the hydrides at room temperature, which splits at low temperature into an ABC one in agreement with a fluxional behavio r of the cation which binds to two hydrides of each niobium center. Th e activation energy of these fluxional processes are close to 42-45 kJ . mol(-1) in the case of Cu adducts, 37 kJ . mol(-1) in the case of A g adducts, and 40 kJ . mol(-1) in the case of Au adducts. The magnitud e of exchange couplings is reduced upon addition of copper cation to 1 -3, is of the same order of magnitude after addition of a silver catio n, and is greatly increased by addition of a gold cation. A model is p roposed to explain these variations which involves two isomeric states that are close in energy, one involving two bridging and one terminal hydrides on niobium and one involving one bridging hydride and a dihy drogen molecule. A line shape analysis experiment carried out on 14 al lows determination of the parameters of the classical exchange, the co upling constants at various temperatures which reach 550 Hz at 347 K, and the parameters of the quantum mechanical exchange according to our proposed model. The structure of 14 has been studied by X-ray diffrac tion. The structure has been Solved from diffractometer data by Patter son method and refined by blocked full-matrix least squares on the bas is of 3082 observed reflections to R and R(w) values of 0.0346 and 0.0 381, respectively. The structure shows the presence of two bridging hy drides between the niobium and gold atoms; one of them is found close to the terminal hydride.