Addition of pnictogen atoms to chromium(II): synthesis, structure and magnetic properties of a chromium(IV) phosphide and a chromium(III) arsenide
Chromium(II) chloride reacts with lithium pentamethylcyclopentadienide (LiCp*, Cp* = C5Me5) and LiE(SiMe3)2 (E = P or As) to give the heterocubane chromium phosphide [(η5-Cp*Cr)(μ3-P)]4 (1) or the chromium arsenide cage [(η5-Cp*Cr)3(μ3-As)2] (2), respectively. The crystal and molecular structures of both compounds have been determined by X-ray crystallography. The reactions represent unusual oxidative additions of pnictogen atoms to chromium(II), which in the case of 1 results in the formation of the unusual chromium(IV) formal oxidation state, and in the case of 2 provides access to chromium(III). Compound 1 is also a rare example of a transition metal μ3-phosphide-ligated cubane. Magnetic susceptibility and magnetization measurements, with support from DFT calculations, indicate S = 2 and S = 9/2 ground states for 1 and 2, respectively, which can be rationalized by considering the electronic structure in terms of chromium–chromium bonding.