Role of coordination geometry in dictating the barrier to hydride migration in d⁶ square-pyramidal iridium and rhodium pincer complexes

Syntheses of the olefin hydride complexes [(POCOP)M(H)(olefin)][BAr f 4 ] (6a-M, M = Ir or Rh, olefin = C 2 H 4 ; 6b-M, M = Ir or Rh, olefin = C 3 H 6 ; POCOP = 2,6-bis(di-tert-butylphosphinito)benzene; BAr f = tetrakis(3,5- trifluoromethylphenyl)borate) are reported. A single-crystal X-ray structure determination of 6b-Ir shows a square-pyramidal coordination geometry for Ir, with the hydride ligand occupying the apical position. Dynamic NMR techniques were used to characterize these complexes. The rates of site exchange between the hydride and the olefinic hydrogens yielded δG ‡ = 15.6 (6a-Ir), 16.8 (6b-Ir), 12.0 (6a-Rh), and 13.7 (6b-Rh) kcal/mol. The NMR exchange data also established that hydride migration in the propylene complexes yields exclusively the primary alkyl intermediate arising from 1,2-insertion. Unexpectedly, no averaging of the top and bottom faces of the square-pyramidal complexes is observed in the NMR spectra at high temperatures, indicating that the barrier