Trinuclear complexes M[Co(en) 2 (SO 3 ) 2 ] 2 (H 2 O) 2 ·4H 2 O (M=Mn, Fe, Co, Ni, Cu, Zn): their utilization for stoichiometric spinel oxides and the crystal structure of Ni[Co(en) 2 (SO 3 ) 2 ] 2 (H 2 O) 2 ·4H 2 O