Extraordinarily Large Ferromagnetic Coupling (J≥150 cm-1 ) by Electron Delocalization in a Heterometallic Mo≣Mo-Ni Chain Complex.

The new heterometallic chain compounds Mo2 Ni(dpa)4 Cl2 (1) and [Mo2 Ni(dpa)4 Cl2 ]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 Mo2 unit, consistent with the formulation of 2 containing a (Mo2 )5+ ⋅⋅⋅(Ni)2+ core. While 1 contains an S=1, pseudo-octahedral NiII ion, 2 has an S=3/2 ground state, in which the two NiII unpaired electrons, one in a localized δ-orbital and one in a heavily delocalized σnb -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 Mo2 and Ni spins with J≥150 cm-1 . This ferromagnetic interaction occurs via delocalization of a σnb -electron across all three metal atoms, forcing ferromagnetic alignment of electrons in orthogonal Ni and Mo2 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.