Some organometallic solids comprising linear chains of alternating
m
8
= 1/2 metallocenium donors, D, and cyanocarbon acceptors, A, i.e. ••• D +*A~*D+*A- * •••, exhibit cooperative magnetic phenomena, i.e. ferro-, antiferro-, ferri-, and meta-magnetism. For [Fe
II
(C
5
Me
5
)
2
]+*[T C N E ]- (Me = methyl; T C N E = tetracyanoethylene) bulk ferromagnetic behaviour is observed below the Curie temperature of 4.8 K. Replacement of Fe
III
with Fe
II
, Ni
III
and Cr
III
leads to complexes with dia-, antiferro- and ferrimagnetic behaviour, respectively. These results are consistent with a model of configuration mixing of the lowest charge transfer excited state with the ground state developed earlier to understand the magnetic coupling of such systems. The model predicts the magnetic coupling as a function of electron configuration and direction of charge transfer (retro or reverse) and is a useful guide to designing new organic and /or organometallic complexes with cooperative magnetic coupling. To test the model and identify new materials with ferromagnetic coupling new TCNE-based electron transfer salts were prepared.