Large isotropic exchange interactions from E.P.R. on doublet-triplet organometallic molecular pairs in diamagnetic host-lattices

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Abstract
In order to study exchange interactions between representative paramagnetic metal-π-complexes of the ferrocene type, bis-(benzene)-vanadium(S = 1/2) - nickelocene(S = 1) and cyclopentadienyl-cycloheptatrienyl-vanadium(S = 1/2) - nickelocene(S = 1) doublet-triplet pairs in a diamagnetic ruthenocene lattice have been investigated by low temperature X-band E.P.R. Single crystal and powder spectra of bi-doped ruthenocene (∼1 per cent S = 1/2 and ∼4–12 per cent S = 1) samples clearly revealed the existence of several different ferro- or antiferromagnetically coupled molecules in neighbouring lattice sites (called ‘pairs’), and from these experiments surprisingly large isotropic exchange constants J in the range of -4·7 to 7·9 cm-1 (ℋex = J S D · S T) could be determined. Thereby a large axial zero-field splitting of 33·6 cm-1 on nickelocene triplet allowed a pair ground state description in terms of an effective spin hamiltonian with S′ = 1/2, I′ = 7/2. Due to a strong dependence of the new effective g- and A-tensors on J, (S = 1/2) - nickelocene doublet-triplet molecular pairs have turned out to be unique systems to measure exchange interactions with E.P.R.