VARIABLE-TEMPERATURE KINETIC-ANALYSIS OF A 3-SITE TERMINAL LIGAND-EXCHANGE IN NICKEL A-FRAMES BY QUANTITATIVE P-31(H-1) EXSY NMR

Variable-temperature two-dimensional phase-sensitive P-31{H-1} EXSY NM R spectroscopy was used to determine the kinetics and thermodynamics o f terminal ligand redistribution in the nickel A-frames Ni-2(dppm)(2)( C=CH2)X-2 (X = SCN (1), Cl (3); dppm = bis(diphenylphosphino)methane). Solutions containing 1:1 mixtures of 1 and 3 undergo terminal ligand exchange via a two-step process involving a mixed-ligand intermediate, Ni-2(dppm)(2)(C=CH2)(Cl)(SCN) (2). This type of terminal ligand excha nge is characteristic of this class of compounds. One-dimensional P-31 {H-1} NMR spectra for the mixed-halogen or halogen/pseudo-halogen spec ies show a strong solvent dependence. The observed NMR spectra for 2 r ange from a broad singlet (DMSO) to a well-resolved AA'BB' multiplet ( benzene) and are dependent on the rate of ligand substitution. Rate co nstants for the terminal ligand exchange processes in DMSO-d(6) were c alculated by three methods from the P-31{H-1} EXSY NMR data. From thes e measurements, a complete thermodynamic characterization of this exch ange process was achieved. Entropies of activation calculated from Eyr ing plots range from -11.4 to -19.0 cal/(K mol). This implies ordering of the system during the transition state and is consistent with an a ssociative interchange (I-a) mechanistic model. Free energy calculatio ns show an intrinsic thermodynamic stability associated with the mixed -ligand species 2.