Cross cyclotetramerization of 1,2,5-thiadiazole-3,4-dicarbonitrile and 4- tert-butylphthalodinitrile in the presence of magnesium butoxide gave a mixture of symmetric and asymmetric porphyrazine magnesium(II) complexes. One of these, (22(3),72(3),122(3)-tri- tert-butyltribenzo[ b,g,l][1,2,5]thiadiazolo[3,4- q]-5,10,15,20-tetraazaporphyrinato)magnesium(II) (MgSBBB), was isolated by column chromatography and was treated with trifluoroacetic acid to obtain the corresponding free ligand (H2SBBB). The effect of the mode of heteroring fusion on the electronic absorption spectra of tribenzoporphyrazines was studied. [ABSTRACT FROM AUTHOR]