4.2 K Mössbauer spectra of crystals of 1a recorded in parallel applied fields of 1.0 (A), 4.0 (B), and 7.5 T (C). From spectral simulations using an S = 5/2 spin Hamiltonian, we obtained D = 3.25 cm−1, E/D = 0.14(2), g0 = 2.00, A0/gnβn = −19.1(2) T, ΔEQ = −1.02(5) mm s−1, η ≈ 0, and δ = 0.36(3) mm s−1. We have subtracted from the data a spectral simulation for two FeIV═O contaminants (representing 8% of Fe), namely [FeIV(O)(TMC)(NCMe)]2+ (5%) and [FeIV(O)(TMC)(OH)]+ (3%), using the parameters reported in ref 15.