The complex [(?5-Me3SiC5H4)Fe(10aneP3-H2,C2H3)]+, where Me3SiC5H4 is trimethylsilylcyclopentadienyl and 10-aneP3-H2,C2H3 is 1-vinyl-1,4,8-triphosphacyclodecane, has been prepared by the base-catalyzed template cyclization of 1,3-bis(phosphino)propane (1,3-bpp) and trivinylphosphine (tvp) at the metal center. A related radical-initiated iron-template reaction between 1,2-bis(phosphino)ethane (1,2-bpe) and triallylphosphine (tap) gave [(?5-C5Me5)Fe(Me-10aneP3-H2,C3H5)]+, where C5Me5 is 1,2,3,4,5-pentamethylcyclopentadienyl and Me-10aneP3-H2,C3H5 is the unsymmetric 10-aneP3 derivative 1-allyl-3-methyl-1,4,7-triphosphacyclodecane. This macrocycle is the result of two intramolecular hydrophosphination reactions, one that generates a five-membered (chelate) ring with an exo-methyl group and one a six-membered chelate. The [(?5-CpR)Fe]+ fragment also controls the cyclization of 1,2-bis(diallylphosphino)ethane with phenylphosphine to give a ternary complex containing the symmetrical 11-membered macrocycle 1-phenyl-4,8-diallyl-1,4,8-triphosphacycloundecane, in addition to the unsymmetrical 10-aneP3 derivative.