Treatment of hexachlorocyclotriphosphazene (I) with L-glutamic acid diethyl ester (II) in toluene in the presence of Et3N provides the ethyl glutamate derivative of cyclotriphosphazene (III), which is then subjected to saponification with LiOH in H2O to furnish compound (IV) (alternatively, the same procedure can be followed by performing the reaction between (I) and L-glutamic acid dimethyl ester (V) to afford the methyl glutamate derivative of cyclotriphosphazene (VI), which is saponified under the same conditions as (III)). Finally, the target product is obtained by reaction of intermediate (IV) with platinum derivative (VII) by means of Ag2SO4 in water, followed by treatment with BaCl2.H2O and precipitation in acetone.