The rise in χ(T ) below T = 10 K indicates the presence of small quantities of paramagnetic spins, which we determined to be 1.1(1) spin % from fitting of the Curie-like tail (Fig. ). This Curie-like tail may be caused free-spins at chain-ends or Cr 3+ formed due to surface oxidation (Fig. ). Indeed, measurement of the magnetic susceptibility of CrCl 2 (pym) after air exposure showed a large increase in the paramagnetic contribution, 15.0(2) spin % (Fig. ), and X-ray photoelectron spectroscopy (XPS) of this air-exposed sample primarily detected oxidised Cr (Fig. ), with Cr 3+ , Cr 6+ and metallic Cr present, as well as O 1s peaks consistent with the formation of Cr(OH) 3 .