Spin alignment in high-spin carbenes of heteroatomic pi-conjugation

2,6-, 2,4-, And 3,5-pyridine-bis (phenylmethylene) (3,5-PY), in which heter ocycle plays a ferromagnetic or antiferromagnetic linker between carbenic u nits, have been studied by random orientation ESR spectroscopy. The electro nic structures of isomers are important for the understanding of spin align ment vs. topology of the heteroatomic pi-electron networks. In 2,6-PY and 2 ,4-PY, the perturbing nitrogen atoms are in active positions, giving rise t o a larger influence on their spin structure than in 3,5-PY. A simple appli cation of molecular Hund's rule is invalidated.