Niobium hydrido complexes

Yasuda and coworkers51 reported a route to niobocene-allyl compounds by hydrometa-lation of conjugated dienes with niobium hydrido-olefin complexes, NbH(C5H5)2(olefin),... 

The coordinated ethylene is readily expelled at 25 °C by the attack of a conjugated diene and the hydride is transferred to the sterically less crowded diene terminus. Thus the niobium hydrido-olefin complexes serve as convenient reagents for the preparation of 1,2- or 1,3-dialkyl-substituted allylniobium compounds starting from butadiene, (E,E) and (/i,Z)-2,4-hcxadicnc, (E)- and (Z)-l,3-pentadiene (equation 1), 3-methyl-l,3-pentadiene and isoprene (equation 2). All the allyl niobium compounds synthesized were isolated as air-sensitive pale-yellow crystals by crystallization from hexane. 

The chemistry of hydrido complexes of group V metals seems to reflect the usual tendency for vanadium to behave differently from the other two elements, although generalizations are probably premature in such a new field. Certainly niobium and tantalum form numerous hydrides of similar composition and properties, appearing to have no vanadium counterparts to date, but there have been no systematic investigations involving all three elements under comparable conditions, as is clear from the following discussions. 

The coordinated ethylene is readily expelled at 25 °C by the attack of a conjugated diene and the hydride is transferred to the sterically less crowded diene terminus. Thus the niobium hydrido-olefin complexes serve as convenient reagents for the preparation of... 

The niobium hydrido styrene complex tNb Ti5-C5H4SiMe3)2(H)(Ti2-PhCH=CH2)] has been prepared both from the reaction of [Nb(Ti -C5H4SiMe3)2(H)3] with styrene and from that of [Nb(Ti5-C5H4SiMe3)2Cl2] with [PhCH2CH2MgBr]. The first method gives rise to two isomeric products (endo, where the alkene substituent is central and exo, with the alkene substituent lateral in the equatorial plane), whilst the latter leiuls exclusively to the endo isomer. 

---