PdCl(NO) – an iconic compound with corrugated Pd$_4$Cl$_4$ octagons built up by Pd$_2$Cl$_2$(NO)$_2$ moieties

During outstanding research in the late 1950s in Munich, which led to the development of the Wacker process for converting ethene into acetaldehyde by catalysis of PdCl$_2$, black insoluble nitrosyl-palladium chloride (PdCl(NO)) was obtained. More than sixty years after its first synthesis, its crystal structure was now determined by X-ray diffraction. PdCl(NO) (mP16, P2$_1$/c, a=10.2684(5), b=4.0737(2), c=7.8456(4) Å, β=111.125(1)°, wR2=0.0572) consists of distorted Pd$_4$Cl$_4$ octagons in chair arrangement to which four distorted Pd$_2$Cl$_2$ squares are annulated on every second edge. In this arrangement each of the two Pd atoms of the squares are connected to one N−O group, bonded alternatively up and down to the Pd atoms with a Pd−N−O angle of 129°. Such a square has the composition of the dimer which was found in the mass spectrum at 343.6 m/e. The octagons with four squares are interconnected to corrugated layers in the b-c-plane as a two-dimensional polymer.