59Co-NMR spectroscopy of the tetrahedral cluster Co4(CO)12:: relaxation and exchange

Selective Co-59-NMR inversion-recovery experiments combined with non-selective experiments on a solution of the cluster Co-4(CO)(12) at various temperatures (from 220 to 330 K) yield information about Co-59 relaxation data and carbonyl ligand exchange. Relaxation data characterise the motional behaviour of Co-4(CO)(12) in solution, but also provide the value of the quadrupole 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 constants 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 measurements show that all four metal centres are involved, which implies that carbonyl scrambling is complete. Experimental and simulated 2D-EXSY Co-59-NMR spectra corroborate this result. Rate constants and activation parameters (Delta G not equal (298 K) = 50.4 +/- 2.0 kJ mol(-1), Delta H-not equal = 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 clusters. (C) 2000 Published by Elsevier Science S.A. All rights reserved.