One to five, one at a time: The choice of iron precursors and the amine co-ligand are employed as switches to control the nuclearity in iron phosphate molecular clusters. Using this control, it is possible to isolate selectively either a mono, di, tri, tetra, or pentanuclear cluster, for a given set of reactants. The potential of a mono-organophosphate ester in assembling low to medium nuclearity iron complexes with novel topological architectures has been investigated. Reaction of 2,6-diisopropylphenyl dihydrogen phosphate (dipp-H2) with ferrous acetate under an inert atmosphere resulted in the formation of mononuclear, [Fe||(dipp-H)2(py)4] (1) (py=pyridine), dinuclear [Fe|||2O(dipp-H)4(3,5-dmpz)3(thf)]·(3,5-dmpz)(thf)3 (2) (3,5-dmpz=3,5-dimethylpyrazole, thf=tetrahydrofuran), and trinuclear [Fe|||2Fe||O(dipp-H)6(thf)3]·(collidine) (3) complexes by changing the ancillary amine used in the reaction. Use of a preformed μ3-oxo bridged trinuclear complex, [Fe|||3O(O2CR)6(H2O)3]X (X=Cl and NO3), as the precursor yielded two tetranuclear iron phosphates, [Fe|||4O(dipp)3(py)4(PhCOO)4]·(toluene)3 (4) and [FeIII4O(dipp)3(OAc)4(py)4]·(py)2 (5), having a core structure similar to those found in tetranuclear iron phosphonates. When FeCl3·6H2O was used as the iron precursor, an unprecedented pentanuclear iron phosphate complex, [Fe|||5O(dipp)6(py)4Cl2][pyH] (6) with a novel structure was isolated. Another pentanuclear complex having a similar core structure, [Fe5O(dipp)6(HO2CPh)3(CH3CN)3Cl] (7) was isolated on treatment of dipp-H2 with the triangular complex, [Fe|||3O(O2CPh)6(H2O)3]Cl in the absence of any added amine co-ligand. Compounds 1-7 have been characterized by analytical techniques, spectroscopic studies, single crystal X-ray diffraction studies, and magnetic measurements. The Mössbauer studies carried out at room temperature support the formulation of all the compounds