Variable-temperature kinetic analysis of a three-site terminal ligand exchange in nickel A-frames by quantitative ³¹P{¹H} EXSY NMR

Variable-temperature two-dimensional phase-sensitive ³¹P{¹H} EXSY NMR spectroscopy was used to determine the kinetics and thermodynamics of terminal ligand redistribution in the nickel A-frames Ni₂(dppm)₂(C=CH₂)X₂ (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, Ni₂(dppm)₂(C=CH₂)-(Cl)(SCN) (2). This type of terminal ligand exchange is characteristic of this class of compounds. One-dimensional ³¹P{¹H} NMR spectra for the mixed-halogen or halogen/pseudo-halogen species show a strong solvent dependence. The observed NMR spectra for 2 range from a broad singlet (DMSO) to a well-resolved AA′BB′ multiplet (benzene) and are dependent on the rate of ligand substitution. Rate constants for the terminal ligand exchange processes in DMSO-d₆ were calculated by three methods from the ³¹P{¹H} EXSY NMR data. From these measurements, a complete thermodynamic characterization of this exchange process was achieved. Entropies of activation calculated from Eyring 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 associative interchange (I_a) mechanistic model. Free energy calculations show an intrinsic thermodynamic stability associated with the mixed-ligand species 2.