Using dynamic NMR methods the rates of hydrogen exchange following intramolecular ethylene insertion into the metal-hydride bond have been measured for the following group 5 ansa-metallocene complexes: [Me2Si(η5-C5H4) (η5-C5H3-3-R)]Nb (CH2 CH2)H (R = CHMe2, CMe3), rac- and meso-[Me2Si(η5-C5H3-3- CMe3)2]Nb (CH2CH2)H, and [(1,2-SiMe2)2(η5-C5H-3,5- (CHMe2)2) (η5-C5H2-4-CMe3)]Ta (CH2CH2)H. The singly bridged ansa-niobocenes exchange up to 3 orders of magnitude faster than unbridged complexes. However, the doubly bridged ansa-tantalocene complex exchanges at a rate comparable to that previously reported for (η5-C5Me5)2Ta(CH2 CH2)H and much slower than a singly bridged complex, [Me2Si(η5-C5Me4)2] Ta(CH2CH2)H. These "ansa-effects" were investigated by DFT calculations on model complexes. The computed exchange pathway showed the presence of an agostic ethyl intermediate. The calculated barriers for hydrogen exchange of model unbridged, singly bridged, and doubly bridged niobocenes correlate with the experimental results.
