Bridging terminal hydride exchange

A similar rapid exchange of bridging and terminal hydride ligands and the hydrogen atoms in Ru-H-Si 3c-2e bonds is observed in the C-type diruthenium complexes Cp Ru(ju.- T2-HSiR2) 2(Ju,-H)(H) (R = Et, Ph) [see Eq. (13)].38 In these complexes, all the four hydride ligands are equivalent in the H NMR spectrum at 25°C, whereas they separate to three signals... 

A possible exchange process that involves the Cp hydrogens, Zr-H terminal hydride, and Zr-H-B bridge hydrogens is shown in Scheme II. To account for the... 

Other methods involving halide-hydride exchange starting with [BH,] ion are available These are included here because formation of BHB bridge bonds from B—H terminal bonds is interpreted as new B—H bond formation, e.g., BF3 and NaBH, react in diglyme to afford B.H in yields > 90% ... 

Since the averaged Jp, n values are known, evaluation of equation (14) allows the determination of x,. This has been done at different temperatures, and an Arrhenius plot of the log of the exchange rates (K = 1/x, ) vs 1/T provides the activation energy associated to the exchange (17.2 kJ/mol). Now the knowledge of x allows to get an estimation of 5 according to equation (12) from the body of these calculations, the chemical shift separation between bridging and terminal hydrides results to be equal to 2.0 0.1 ppm. 

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