Molecular and Crystal Structure of Two Cyclobuten-3,4-dione Derivatives: The Dithallium Salt of 1,2-Dicyanimino- (T12CMCB) and the l,2-Diiodocyclobuten-3,4-dione (I2 CB)

stituents which may facilitate the formation of stable radicals and acceptor species [1 ], In such a context we considered the 1,2-dicyanimino derivative [C4 0 2(NCN)2]2_, henceforth CMCB2because such a group should have at least some of the required properties [2], The very heavy T1 cation had to be accepted, because it was the only one which gave suitable crystals. Owing to the very scanty information [3] available on the cyclobutene-3,4-dione moiety as to both geometry and force field, we were compelled to con­ sider model compounds containing this grouping. Due to the generally electron-attractive character of the substituents referred to in the previous para­ graph, the most logical model compounds appeared to be the 1,2-dihalogeno derivatives [4], Out of these, we first considered the determination of the geometry of the only compound then known.