Revisiting Ir(CO)₃Cl

We return to an old puzzle - the short metal-metal separation and electrical conductivity of the apparently unoxidized one-dimensionally stacked structure of a d⁸ Ir(I) complex, Ir(CO)₃Cl. One would expect neither a short Ir-Ir distance of 2.84 Å, nor metallicity in an unoxidized stacked square-planar d⁸ array. We build up dimer, trimer, one-dimensional polymer and model 3-dimensional structures, in both molecular and extended structure plane wave calculations. The short Ir-Ir separation in the polymer, with a substantial contribution of 6_pz-5d^z2 bonding to it, is obtained without any oxidation. There is computational evidence for an important level crossing in the polymer. The metallicity remains unexplained, but likely arises from partial oxidation. And that remains an outstanding experimental issue.