Tuning the coordination geometries and magnetic dynamics of [Ln(hfac)4](-) through alkali metal counterions.

Four lanthanide compounds with formulas [Cs{Dy(hfac)4}] (1), [Cs{Er(hfac)4}] (2), [K{Dy(hfac)4}] (3), and [K{Er(hfac)4}] (4) (hfac = hexafluoroacetylacetone) are reported. Compounds 1 and 2 crystallize in the orthorhombic Pbcn space group, while 3 and 4 are in the triclinic P1̅ space group. All display chain structures in which the mononuclear Ln(hfac)4 anions are linked by alkali metal ions. However, the coordination geometries around the Ln atoms are quite different depending on the cation. They adopt a distorted dodecahedron with pseudo-D2d symmetry in the cesium compounds 1 and 2, while a distorted square-antiprism with pseudo-D4d symmetry is adopted in the potassium compounds 3 and 4. The latter compounds show distinct field-induced slow magnetization relaxation. The energy barriers are 23.95 and 20.21 K for compounds 3 and 4, respectively.