Extraordinarily Large Ferromagnetic Coupling (J≥150 cm ) by Electron Delocalization in a Heterometallic Mo≣Mo-Ni Chain Complex.

The new heterometallic chain compounds Mo Ni(dpa) Cl (1) and [Mo Ni(dpa) Cl ]OTf (2) (dpa=2,2'-dipyridylamine) have been prepared and studied by crystallography and magnetic susceptibility, among other methods. Oxidation of 1 to 2 removes an electron from the multiply bonded Mo unit, consistent with the formulation of 2 containing a (Mo ) ⋅⋅⋅(Ni) core. While 1 contains an S=1, pseudo-octahedral Ni ion, 2 has an S=3/2 ground state, in which the two Ni unpaired electrons, one in a localized δ-orbital and one in a heavily delocalized σ -orbital are joined by an unpaired electron in a Mo-Mo δ-orbital. The S=3/2 ground state is persistent to 300 K, evidencing strong ferromagnetic coupling of the Mo and Ni spins with J≥150 cm . This ferromagnetic interaction occurs via delocalization of a σ -electron across all three metal atoms, forcing ferromagnetic alignment of electrons in orthogonal Ni and Mo δ-symmetry orbitals. We anticipate that this new means of coupling spins can be used as a design principle for the preparation of new compounds with high spin ground states.