Henry Rzepa's blog

In the previous post I pondered the colour of Monastral blue (copper phthalocyanine). Something did not quite fit, and so I speculated that perhaps some oxidation of the pigment might give a new species. This species (Cambridge code FEGJOQ) comprises two parts of copper phthalocyanine, 1 part of the corresponding cation, and 1 part of triodide anion. Looking at the packing of this system, I spotted something I had seen some time ago in NaI₂.Acetone, namely an infinitely long and absolutely straight chain of iodine atoms, a molecular wire if you like.

A different view shows how this wire runs down layers of the phthalocyanine. The iodines are 3.2Å apart, compared to the sum of their van der Waals radii of ~4.0Å.

Four phthalocyanines stack to form a cavity for the iodine wire to run down, and the size of that cavity is perfectly filled by the iodine! One might speculate if a smaller halogen, with lots more space to rattle around it, might not form this structure!