The radical ligand 5-(3-[N-tert-Butyl-N-aminoxyl]phenyl)pyrimidine (RL) forms cyclic M₂(RL)₂ coordination complexes with M(hfac)₂, M = Mn, Co, Cu, hfac = hexafluoroacetylacetonate.  The coordination complexes were all submitted to variable temperature x-ray crystallography and magnetic analysis in order to examine structure-property relationships for molecular magnetism. While the cyclic dimer complexes are isostructural, the magnetic behavior is quite different.  The Mn(II) and Co(II) complexes show strong antiferromagnetic exchange between the metal ion and the radical ligand within the dimers.  The slight ferromagnetic (FM) rise in the ChiT(T) data for the Mn(II) complex is attributed to weak FM exchange between dimer units.  The dip in the Co(II) complex ChiT(T) graph below 100 K indicates that the paramagnetic moment nulls out at low temperature.  The Cu(II) complex shows a consistent monotonic AFM decrease from room temperature down to around 100 K, and there is a thermally reversible Jahn-Teller distortion of 0.2-0.3 Å in the Cu(II)-nitroxide bond lengths over this temperature range that corresponds to an axial to equatorial coordination change.  The coordination change is accompanied by a startling solid state, reversible thermochromic change from yellow brown at room temperature to bright green at 77 K.