Introducing both tetrazine radical and azido bridges afforded two air-stable square complexes [M-4(II)(bpztz(center dot-))(4)(N-3)(4)] (M-II = Zn2+, 1; Co2+, 2; bpztz = 3,6-bis(3,5- dimethylpyrazolyl)- 1,2,4,5-tetrazine), where the metal ions are cobridged by mu(1,1)-azido bridges and tetrazine radicals. Magnetic studies revealed strong antiferromagnetic metal-radical interaction with a coupling constant of -64.7 cm(-1) in the 2J formalism in 2. Remarkably, 2 exhibits slow relaxation of magnetization with an effective barrier for spin reverse of 96 K at zero applied field.