The title compounds, [COCl2(C4H4N2)(2)] and [CoBr2(C4H4N2)(2)], show weak f
erromagnetism below 5 K. The relationship between the magnetic properties a
nd the structure is discussed. The crystals of the Cl and Br derivatives ar
e isomorphous. The Co atom is coordinated with two halogen atoms and four N
atoms of pyrimidine molecules at axial and equatorial positions, respectiv
ely, to form the octahedral geometry. The nearest neighbouring Co atoms are
bridged by the pyrimidine molecules to form a three-dimensional network. A
s the pyrimidine molecules are considered to conduct antiferromagnetic inte
ractions, the weak ferromagnetism may result from the chiral structure of t
he crystals.