2‐Amino‐5‐(2‐pyridyl)‐thiadiazole as Bidentate Ligand

The amino substituted bidentate chelating ligand 2‐amino‐5‐(2‐pyridyl)‐1,3,4‐thiadiazole (H 2 L ) was used to prepare 3:1‐type coordination compounds of iron(II), cobalt(II) and nickel(II). In the iron(II) perchlorate complex [Fe II (H 2 L ) 3 ](ClO 4 ) 2 ·0.6MeOH·0.9H 2 O a 1:1 mixture of mer and fac isomers is present whereas [Fe II (H 2 L ) 3 ](BF 4 ) 2 ·MeOH·H 2 O, [Co II (H 2 L ) 3 ](ClO 4 ) 2 ·2H 2 O and [Ni II (H 2 L ) 3 ](ClO 4 ) 2 ·MeOH·H 2 O feature merely mer derivatives. Moessbauer spectroscopy and variable temperature magnetic measurements revealed the [Fe II (H 2 L ) 3 ] 2+ complex core to exist in the low‐spin state, whereas the [Co II (H 2 L ) 3 ] 2+ complex core resides in its high‐spin state, even at very low temperatures.