The addition of 18-crown-6 ether (1,4,7,10,13,16-hexaoxacyclooctadecane) to tetranuclear [(thf)K(OPAryl2)]4 and [(thf)4Ca(OPAryl2)2] yields the corresponding mononuclear complexes [(18C6)K(OPAryl2)] [Aryl = Ph (1a), Mes (1b)] and [(18C6)Ca(OPAryl2)2] [Aryl = Ph (2a), Mes (2b)]. The metathesis reaction of [(thf)K(OPAryl2)]4 with CaI2 yields the calciate [K2Ca(thf)x{OPMes2}4]. The addition of dimesitylphosphane oxide and crystallization from a hexane solution gives [K2Ca{OPMes2}4{Mes2P(O)H}] (3). The complexes [(thf)K(OPMes2)]4, [(thf)4Ca(OPMes2)2], 1b, 2b, and the calciate 3 are tested as catalysts in the hydrophosphorylation of isopropylisocyanate with dimesitylphosphane oxide, quantitatively yielding N-isopropyl(dimesitylphosphoryl)formamide. The potassium complexes are more efficient catalysts than the calcium congeners, and coordination of 18-crown-6 decelerates the catalytic conversion.