Synthesis and Spectroscopic Properties of Dihydrogen Isocyanide Niobocene [Nb(η5-C5H4SiMe3)2(η2-H2)(CNR)]+ Complexes. Experimental and Theoretical Study of the Blocked Rotation of a Coordinated Dihydrogen

Synthesis of stable hydride isocyanide derivatives Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (H)(CNR) has been achieved through the formation of coordinatively unsaturated 16-electron species Nb(η 5 -C 5 H 4 SiMe 3 ) 2 H by thermolytic loss of H 2 followed by the coordination of an isocyanide ligand. Low-temperature protonation with a slight excess of CF 3 COOH leads to the η 2 -dihydrogen complexes [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (η 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.