Arrhenius plots obtained from M ′′ versus T data gave effective barriers to relaxation (U eff ) of 71 and 72 K for 5 and 6, respectively. 1 H NMR spectra in CD 2 Cl 2 show that 5 and 6 are the main species present on dissolution, but there is evidence for some ligand distribution between axial and equatorial sites, by intra-and/or intermolecular exchange processes.