Dynamics of Agostic Complexes of the Type C5R5(C2H4)M(CH2CH2-μ-H)+. Energy Differences between and Ancillary Ligand Control of Agostic and Terminal Hydride Structures

Abstract

The degenerate interconversions of the two enantiomeric forms of agostic C5R5(C2H4)Co(CH2CH2-μ-H)+ (4a, R = CH3; 4b, R = H) have been investigated by 13C NMR spectroscopy at 100.6 MHz in the temperature range-80 to-100 °C. Very substantial line broadening of the olefinic carbon resonances is observed in this temperature range. By estimating 13C chemical shifts for the static species and applying the fast-exchange equation, we determined free energies of activation for this process as ca. 5.3 kcal/mol for both 4a and 4b. Since the rearrangements must proceed via the symmetrical terminal hydrides C5R5Co(C2H4)2(H)+ (5a,b), 5.3 kcal/mol represents a maximum energy difference between the agostic and terminal hydride structures. The analogous rhodium complex has been shown to possess the agostic structure C5Me5(C2H4)Rh-(CH2CH2-μ-H)+ (6), but the barrier for degenerate isomerization via the terminal hydride is < 3.7 kcal/mol, indicating an even smaller difference between the terminal and agostic forms relative to the cobalt species. Since complexes of the type C5R5(L)Rh(C2H4)H+ (L = PR3, P(OR)3) are terminal hydrides, these rhodium systems represent a unique case where agostic and terminal hydride structures are sufficiently close in energy that the stable form observed can be altered by small changes in the ancillary ligand. Show more