ORIGIN OF THE HYDRIDIC H-1-NMR CHEMICAL-SHIFT IN LOW-VALENT TRANSITION-METAL HYDRIDES

We present a theoretical study of the H-1 NMR chemical. shifts in low- valent transition metal hydrides based on density functional theory an d gauge-including atomic orbitals (DFT-GIAO). Calculations have been c arried out on the representative hydrides HM(CO)(5) (M = Mn, Tc, Re), H2Fe(CO)(4), HCo(CO)(4), [HCr(CO)(5)](-), and [HCr2(CO)(10)](-). In ge neral, the calculated chemical shifts are in good agreement with the a vailable experimental data. The paramagnetic and diamagnetic contribut ions to the H-1 chemical shielding have been analyzed in detail. Our c alculations show that the paramagnetic current localized in the adjace nt metal fragment, ML(n), is responsible for the negative ''hydridic'' shia observed in transition metal hydrides H-ML(n).