The reactivity of the lithium triphosphacyclohexadienyl complex Li[1,3,5-MeP3C3But3] toward a range of group 13 and 14 halide complexes has been investigated. The heterocycle reacts with MX (M = Ga, In, or Tl; X = Cl or I) to give the diphosphacyclopentadienyl (i.e., diphospholyl) complexes [M(?5-1,3-P2C3But3)] in good yield via phosphinidene, PMe, elimination reactions. One complex, M = Tl, has been structurally characterized and found to exist as a one-dimensional polymer in the solid state. Similarly, the reactions of Li[MeP3C3But3] with MCl2 (M = Sn or Pb) have given the tetraphosphametallocenes [M(?5-1,3-P2C3But3)2], which have been structurally characterized. These exhibit fluxional behavior in solution, which has been examined by variable-temperature NMR studies. The monomeric guanidinato?tin chloride complexes [LSnCl] (L = Cy2NC(NAr)2- or (cis-2,6-Me2C5H8N)C(NAr)2-, Cy = cyclohexyl, Ar = C6H3Pri2-2,6) have been prepared, structurally characterized, and treated with Li[MeP3C3But3]. Again, this has yielded diphospholyl complexes [LSn(?1-1,3-P2C3But3)] via phosphinidene elimination processes. In contrast, the reactions of Ph3ECl, E = Sn or Pb, do not proceed via phosphinidene elimination reactions, but instead by triphosphacyclohexadienyl rearrangement processes that eventually lead to complexes [Ph3M(?2-P,P-MeP3C3But3)], containing five-coordinate metal centers that are P,P-chelated by an anionic bicyclic ligand. In the case of the tin complex, a reaction intermediate has been isolated and shown to contain the first structurally characterized example of a 1,2-diphosphabicyclo[1.1.0]butane fragment. A mechanism for the formation of this intermediate has been proposed. ; doi: 10.1021/om0605581