The crystal structure of the title compound was determined at 250 K in space groupI4 bar 2m,a= 20.661(5) Å ,c= 6.764(7) Å ,Z= 8. The individual dithiadiazole rings form two sets of contrarotatory 4-member pinwheels clustered around a 4-fold rotation-inversion axis located halfway along the unit cell edges, describing an infinite channel lined with sulfur atoms but in which there are short intra-stack contacts through only one S atom of each CN2S2group. The double-layer stacking occurs in order to accommodate the bulk of thetBu group, and the spacing between layers is very regular, with short and long S cdot cdot cdot S contacts of 3.48(2) and 3.61(2) Å and considerable thermal motion in thecdirection. The title compound and its SbF6-salt are oxidized at + 0.81 V (in CH2Cl2) and at + 0.61 V (in CH3CN), while a reduction process is observed only in CH2Cl2at -0.73 V vs. SCE. Magnetic susceptibility data between 5 and 400 K demonstrate at very low temperature that the sample follows the Curie-Weiss law, θ = 0 K, and χ0= -156 ppm emu mol-1. The free-spin concentration atT= 0 K is approx 1.3%, due to paramagnetic defects in an essentially diamagnetic structure. The diamagnetism starts to lift above 210 K; above 260 K, a strong antiferromagnetic exchange is operative. These results are consistent with the lifting of the Peierls distortion in this structure, starting above approx 200 K. The crystal structure of the parent diamidine 5-tert-butyl-1,3-[(Me3Si)2NCNSiMe3]2C6H3was determined inC2/cwitha= 10.0788(3),b= 21.328(5),c= 20.876(5) Å , &bgr; = 99.41(2)°,Z= 4. The two amidine functional groups are equivalent by crystal symmetry.Key words: dithiadiazole, diradical, magnetic susceptibility, crystal structure, bulky substituent.