A new synthesis for 2-chloro-1,3,5-trimethyl-4,6-dioxo-1,3,5,2λ 3 -triazaphosphinane (1) is described. Chlorine-fluorine exchange in 1 by means of sodium fluoride in acetonitrile in the presence of catalytic amounts of a crown ether furnished 2-fluoro-1,3,5-trimethyl-4,6-dioxo-1,3,5,2λ 3 -triazaphosphinane (2). The synthesis of the bromine and iodine analogues, 3 and 4, by metathesis of 1 with the appropriate trimethylsilyl halides, Me 3 SiBr and Me 3 SiI, respectively, is also described. The syntheses of an iron(0)tetracarbonyl complex (5) and of a dichloroplatinum( II) complex (6) are reported. A single crystal X-ray diffraction study was conducted on 6. [P2 1 /c, a = 1161.3(3), b = 1423.2(3), c = 1247.7(3) pm, β = 109.42(2)°, R = 0.028], There are deviations from ideal square planar geometry at platinum. The heterocycles display a flattened boat conformation. A Staudinger reaction of 1 with Me 3 SiN 3 led to the formation of a substituted cyclotriphosphazene.