Synthesis and spectroscopic properties of dihydrogen isocyanide niobocene [Nb(eta(5)-C5H4SiMe3)(2)(eta(2)-H-2)(CNR)](+) complexes. Experimental and theoretical study of the blocked rotation of a coordinated dihydrogen

Synthesis of stable hydride isocyanide derivatives Nb(eta(5)-C5H4SiMe3)(2)(H)(CNR) has been achieved through the formation of coordinatively unsaturated 16-electron species Nb(eta(5)-C5H4SiMe3)(2)H by thermolytic loss of H-2 followed by the coordination of an isocyanide ligand. Low-temperature protonation with a slight excess of CF3COOH leads to the eta(2)-dihydrogen complexes [Nb(eta(5)-C5H4SiMe3)(2)(eta(2)-H-2)(CNR)](+). NMR spectra of these H-H complexes and their monodeuterated H-D isotopomers present a single high-field resonance at room temperature. By lowering the temperature to 178 K, decoalescence of the signal was observed for the H-D complexes but not for the H-H ones. By combining DFT electronic structure calculations with a monodimensional rotational tunneling model, it has been shown that the absence of decoalescence of the H-H signal is due to the existence of a very large exchange coupling. Conversely, for the H-D isotopomer, the difference in zero point energy corresponding to two nonequivalent (H-D and D-H) positions leads to a slight asymmetry which dramatically reduces the exchange coupling, allowing decoalescence to be observed. Therefore, the H-D classical rotation and the quantum exchange processes will not be practically observed for this complex, whereas only the classical process for the H-H species is quenched out on the NMR time scale.