On the unexpected isotope effect on the exchange coupling constant in partially deuterated transition-metal trihydride complexes

Abstract

In this paper we attempt to explain the unusual isotope effect displayed by the ¹H-NMR scalar H-H coupling of the hydrides in complexes [CpIrL(H)₃]+ (L=various phosphine and phosphite ligands) when one of the equivalent protons is turned into a deuteron (D.M. Heinekey, J.M. Millar, T.F. Koetzle, N.G. Payne, and K.W. Zilm, J. Am. Chem. Soc. 112, 909 (1990)]. It is found that in the case of the perprotio species the system can be described as a three-coupled well system, but when a deuterium is introduced the system is better described as a double-coupled well. Finally, it is shown that the coupling in the case of a double-well system is greater than in an equivalent triple-well system, in this way giving a plausible explanation to the experimental facts reported.