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

Jerald D. Heise, Daniel Raftery, Brian K. Breedlove, John Washington, Clifford P. Kubiak

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16 Citations (Scopus)


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 (Ia) mechanistic model. Free energy calculations show an intrinsic thermodynamic stability associated with the mixed-ligand species 2.

Original languageEnglish
Pages (from-to)4461-4468
Number of pages8
Issue number20
Publication statusPublished - 1998 Sep 28
Externally publishedYes

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Organic Chemistry
  • Inorganic Chemistry


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