Crystal structure and magnetic properties of hydrated and dehydrated nickel(II)-pyromellitate

Ni2(H2O)4PM.2H2O, where PM is benzene-1,2,4,5-tetracarboxylate, forms square-grid layers with Ni-OCO-Ni bridges that are connected to one another by the benzene moiety. It consists of ferromagnetic layers coupled antiferromagnetically to their neighbors with a long-range Neel ordering at 3 K. A critical field of 1300 Oe suffices to reverse all the moments to a ferromagnetic state. On dehydration to Ni2(H2O)2PM the structure is transformed irreversibly to chains connected by benzene and the magnetic behavior is that of a paramagnet with very weak coupling. By the rehydration of the latter to Ni2(H2O)2PM.2H2O, the structure hardly changes but the magnetism recovers showing T N at 3 K