[Fe(Phen)(CN) 4 ] - : A Versatile Building Block for the Design of Heterometallic Systems. Crystal Structures and Magnetic Properties of PPh 4 [Fe(Phen)(CN) 4 ]·2H 2 O and [{Fe(Phen)(CN) 4 } 2 M(H 2 O) 2 ]·4H 2 O [Phen = 1,10-Phenanthroline; M = Mn(II) and Zn(II)]