Co-59-NMR spectroscopy of the tetrahedral cluster Co-4(CO)(12): relaxationand exchange

Selective Co-59-NMR inversion-recovery experiments combined with non-select ive experiments on a solution of the cluster Co-4(CO)(12) at various temper atures (from 220 to 330 K) yield information about Co-59 relaxation data an d carbonyl ligand exchange. Relaxation data characterise the motional behav iour of Co-4(CO)(12) in solution, but also provide the value of the quadrup ole coupling constant of the basal cobalt nucleus (chi = 100 MHz) and thus complete previous solid state NMR results. Simulations based on relaxation data confirm that the difference between the two quadrupole coupling consta nts accounts for the anomalous features observed in C-13-NMR spectra via Co -59-C-13 scalar coupling. As far as the ligand exchange is concerned, the m easurements show that all four metal centres are involved, which implies th at carbonyl scrambling is complete. Experimental and simulated 2D-EXSY Co-5 9-NMR spectra corroborate this result. Rate constants and activation parame ters (Delta G not equal (298 K) = 50.4 +/- 2.0 kJ mol(-1), Delta H-not equa l = 46.5 +/- 2.5 kJ mol(-1); Delta S-not equal -13 +/- 9 J K-1 mol(-1)) are found in the range of values reported for other tetranuclear carbonyl clus ters. (C) 2000 Published by Elsevier Science S.A. All rights reserved.