In both cases 2-Tb and 3-Tb equation ( ) was sufficient to produce good fits of our experimental data (Fig. ). The best fit parameters obtained were τ0 = 1.17 ). However, the structural changes alone appear comparably small to cause a difference of the energy barriers of more than a factor of two. We believe that the potassium ion also significantly changes the electronic structure of within the aromatic stannole system, resulting in the strong performance difference between 2-Tb and 3-Tb. Oe (Fig. ). At room temperature we found a χT value of 13.61 cm 3 K mol -1 which is in reasonably good agreement to the expected value of 14.17 K and 30 K. The relaxation times were obtained following the approach described by Blackmore et al, for a more detailed description see SI. The combined τ vs. T data have been fit using equation ( ). The best fit yielded the parameters: τ0 = 9.05 x 10 -10 s, Ueff = 1375 K (955 cm -1 ), C = 1.17 x 10 -9 K -n s -1 , n = 4.90 and τQTM = 325.69 s. Deriving the blocking temperature TB with τ = 100 s, gives a blocking temperature of 54.05 K, which is in perfect agreement to the observed trend in the T-dependent ZFC and FC susceptibilities (Fig. ). Hysteresis measurements with 50 Oe/s reveal a vastly open loop at 2 K, with a coercive field of 1.86 T.