Exchange processes involving bridging hydride

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... 

For the reactions with tertiary phosphines, intermediates of the form [HPt(/r-H)( -dppm)2PtH(PR3)]+ have been detected, and equilibrium constants for their formation at low temperature have been determined (110). Exchange of PR3 takes place between the two platinum centers. The elimination of H2 has been shown to be intramolecular, and the rate-determining step is the elimination itself. Rearrangement to a second intermediate without a bridging hydride is believed to occur prior to reductive elimination, but it is not possible to determine with certainty whether the elimination occurs at one platinum center or by means of a concerted dinuclear process. Mechanisms involving each type of elimination process have been proposed (Scheme 8) (110). 

Similar bridging hydrogen exchange processes have been observed for neutral and cationic (zj -allyL/i-IIjMD, complexes 25 (Scheme 3)23.58,63 j ny of these complexes are prepared by the protonation of the corresponding anionic or neutral (/j4-dicnc)MLn complexes 26 (see Section V.C.l). Migration of the metal about a cyclic ) -allyl ligand (i.e. 25a to 25b) is proposed to involve an (/j" -diene)MLn hydride intermediate/transition state 27. 

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