CASSCF calculations on 2-Sm indicate a ground state with g-values of 0.645 (0.6° to normal of N2O coordination plane), 0.043 and 0.011 (Figure ). Static magnetic data is very well reproduced (Figure and) by CASSCF, which also confirms many low-lying 6 H and 6 F states up to 14,000 cm -1 , as observed in UV-vis-NIR spectroscopy (Table ). The 6 H7/2 states in particular are very low in energy, with the third and fourth Kramer's doublets separated by only 110 cm -1 . This results in significant J-mixing, such that J is a poor quantum number for this complex and the g-values of the lowest three states are changed from those expected in a 6 H5/2 multiplet. We aimed to use CASSCF calculations to provide information on the exchange spectra of 4-Ln, mainly focusing on 4-Yb. However, ab initio calculations on the cations of 4-Ln did not accurately model the low temperature magnetic data for 4-Yb and 4-Tm (Figures and), indicating an incorrect exchange spectrum.