Synthesis and structural characterisation of iron(II) and copper(II) diphosphates containing flattened metal oxotetrahedra

The structures of the tetragonal polymorphs of K2MP2O7, M=Cu(II), Fe(II), consist of infinite sheets of stoichiometry [MP2O7]2−, formed from linked pyrophosphate groups and MO4 tetrahedra, separated by potassium ions. In both compounds the unusual tetrahedral coordination of the M(II) centre is strongly flattened as a result of Jahn-Teller (JT) effects for high spin, d6 Fe(II) and p-orbital mixing and second-order JT effects for d9 Cu(II).