(1) A simulation of the experimental T curve is possible with D = 8.0(1) cm -1 , E = 0.8(4) cm and principal values of the g-tensor gxx,yy,zz = [1.61(1), 2.12(2), 1.90 (2)]. This set of parameters also reproduced field dependent magnetization curves at temperatures of 1.8, 5, 10 and 20 K (inset in Figure ). Similar to a previous report on molybdenum(III) complexes, the observed g-values lie below 2. 65 To complement the magnetometry results, we performed HFEPR spectroscopy 68 between 180-420 GHz at 5 K on the same pellet of cisfac-[3] 3+ (Figure ). The HFEPR spectra at 5 K show three main features. At 320 GHz, these signals are located at 1.4 T (A, broad), 3.4 T (B, sharp) and 10 T (C, broad) and can be assigned to transitions from mS = -1 /2 to -3 /2 formally forbidden transitions inside the mS = ± 3 /2 doublet in x/y direction, and the Kramers doublet mS = -1 /2 to + 1 /2, respectively (see SI for details). Hence, the ZFS parameter D = 6.7 cm -1 and an approximate value for E of 0.83 cm -1 can be extracted from signals A and B, respectively. These values agree well with spin Hamiltonian simulations (Eq. 1) of the HFEPR spectra, which yield D = 6.7(1) cm -1 , E = 0.8(5) cm -1 and gxx,yy,zz = [1.61(3), 2.12(5), 1.90(3)] (Table ). Since with HFEPR spectroscopy at constant temperature alone the sign of D cannot be determined with certainty, we recorded additional spectra at 320 GHz at variable temperatures (5-40 K, Figure ). All three main signals A-C decrease in intensity with increasing temperature which suggests D > 0 and which is consistent with spin Hamiltonian simulations (see SI for details).