FAST OLEFIN SITE INTERCHANGE OF CYCLOOCTA-1,5-DIENE IN A RHI SQUARE-PLANAR DERIVATIVE

Rapid exchange between non-equivalent sites causes collapse of the olefinic double bonds resonances in the H-1 NMR spectrum of [Rh(COD)(Ph2PPy)Cl] [COD = cycloocta-1,5-diene; (PPh2Py) = 2-(diphenylphosphino)pyridine], 1, below room temperature. A fast equilibrium between the square planar 1 and a pentacoordinated trigonal bipyramid TBP, formed by chelation of the pyridine nitrogen to the metal, accounts for the low activation energy of the process; Rh-cycloocta-1,5-diene pentacoordinates, in fact, are known to undergo readily a Berry pseudorotation that leads to interchange of axial and equatorial sites in the TBP. NMR data for the analogous square planar complex [Rh(COD)(PPh3)Cl], 2, show that in the absence of intramolecular pathways for pentacoordination the activation energy for the double bond interconversion is much higher.