Magnetism in Polyoxometalates: Anisotropic Exchange Interactions in the Co $\rm{^{II}_{3}}$ Moiety of [Co 3 W(D 2 O) 2 (ZnW 9 O 34 ) 2 ] 12− —A Magnetic and Inelastic Neutron Scattering Study

The ground‐state properties of a Co $\rm{^{II}_{3}}$ moiety encapsulated in a polyoxometalate anion were investigated by combining measurements of specific heat, magnetic susceptibility, and low‐temperature magnetization with a detailed inelastic neutron scattering (INS) study on a fully deuterated polycrystalline sample of Na 12 [Co 3 W(D 2 O) 2 (ZnW 9 O 34 ) 2 ]⋅40 D 2 O ( Co 3 ). The ferromagnetic Co 3 O 14 cluster core consists of three octahedrally oxo‐coordinated Co II ions. According to the single‐ion anisotropy and spin–orbit coupling of the octahedral Co II ions, the appropriate exchange Hamiltonian to describe the ground‐state properties of the Co 3 spin cluster is anisotropic and is expressed as ℋ=−2Σ $\mathop{}{}_{\alpha =x,y,z}$ ( J $\rm{_{\alpha }^{12}}$ Ŝ 1 α Ŝ 2 α + J $\rm{_{\alpha }^{23}}$ Ŝ 2 α Ŝ 3 α ), where J α are the components of the exchange interactions between the Co II ions. To reproduce the INS data, different orientations of the two anisotropic J tensors must be considered, and the following conditions had to be introduced: J $_{x}^{12}$ = J $_{y}^{23}$ , J $_{y}^{12}$ = J $_{x}^{23}$ , J $_{z}^{12}$ = J $_{z}^{23}$ . This result was correlated with the molecular symmetry of the complex. The following set of parameters was obtained: J $_{x}^{12}$ = J $_{y}^{23}$ =1.37, J $_{y}^{12}$ = J $_{x}^{23}$ =0.218, and J $_{z}^{12}$ = J $_{z}^{23}$ =1.24 meV. This set also reproduces in a satisfactory manner the specific heat, susceptibility, and magnetization properties of Co 3 .