Advances in the chemistry of bis-cyclopentadienyl hydride derivatives of niobium and tantalum

The bis-cyclopentadienyl hydride chemistry of the elements niobium and tantalum has been dominated by the family of trihydride complexes, namely Cp2MH3 (M = Nb, Ta, Cp=C5H5, C5Me5, C5H4SiMe3, C5H3(SiMe3)(2) and related substituted cyclopentadienyl rings). The chemical and spectroscopic properties of these complexes are strongly influenced by both the nature of the Cp ring and the metal center. In some cases, anomalous NMR spectroscopic behavior has been observed, and values of J(HH) that are largely dependent on the temperature have been found. This behavior can be modulated by means of the interaction of the trihydride complexes with Group 11 metal fragments, and in some cases HH interactions in the molecules can be observed. Several X-ray crystal structure determinations of this kind of complex, as well as different theoretical calculations, have been performed in order to gain a deeper insight into these systems. The reactivity of the trihydride metallocene complexes has been extensively considered. Thus, the easy elimination of H-2 from the Cp2MH3 species, giving rise to the unsaturated species Cp2MH3 has been used to study the activation processes of several kinds of H-X bend, in hydrosilanes, hydrogermananes, etc., which allows the synthesis of complexes such as Cp2MH2X. This type of complex has been extensively studied, especially when X = SiR3, because in several cases these complexes exhibit a special hypervalence behavior. A large family of complexes of stoichiometry Cp2MHL was prepared from the reaction of the Cp2MH3, complexes with different classes of pi-acid ligands, L. The Cp2MHL complexes can undergo insertion reactions into the M-H bond with several classes of unsaturated molecules, and such processes are of interest in the field of organic synthesis. Furthermore, the protonation processes of the Cp2MHL complexes give rise to a new family of eta(2)-H-2-containing cationic species, Cp2M(eta(2)-H-2)L+, which are stable at low temperatures. The NMR data of such compounds indicate that it is possible to block the rotation of the H-2 molecule at low temperatures. Of particular interest are the Cp2MH(olefin) complexes, and these have been widely studied because they can be considered as excellent models for the study of the beta-elimination reaction and the reverse olefin insertion process. Finally, from the trihydride derivatives Cp2MH3 and related species, several heterometallic complexes have been prepared and characterized. (C) 1999 Elsevier Science S.A. All rights reserved.