SYNTHESIS AND SPECTROSCOPIC PROPERTIES OF DIHYDROGEN ISOCYANIDE NIOBOCENE [NB(ETA(5)-C5H4SIME3)(2)(ETA(2)-H-2)(CNR)](- EXPERIMENTAL AND THEORETICAL-STUDY OF THE BLOCKED ROTATION OF A COORDINATED DIHYDROGEN() COMPLEXES )

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 thermolyt ic loss of H-2 followed by the coordination of an isocyanide ligand. L ow-temperature protonation with a slight excess of CF3COOH leads to th e 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 wa s observed for the H-D complexes but not for the H-H ones. By combinin g DFT electronic structure calculations with a monodimensional rotatio nal tunneling model, it has been shown that the absence of decoalescen ce 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 p oint energy corresponding to two nonequivalent (H-D and D-H) positions leads to a slight asymmetry which dramatically reduces the exchange c oupling, allowing decoalescence to be observed. Therefore, the H-D cla ssical rotation and the quantum exchange processes will not be practic ally observed for this complex, whereas only the classical process for the H-H species is quenched out on the NMR time scale.