The molecular structures of the rubidium and cesium derivatives of supermesitylphosphane [i.e., (2,4,6-(t)-Bu3C6H2)PH2 = (t)Bu(3)MesPH(2)] as well as several base adducts of these are reported. Sodium hydride, potassium hydride, rubidium metal, and cesium metal react with (t)Bu(3)MesPH(2) in tetrahydrofuran solution at room temperature to produce MPRH salts 1-4 [M = Na (1), K (2), Rb (3), Cs (4); R = (t)Bu(3)Mes] in good yields. X-ray-quality crystals of 2 and 3 were obtained by slow evaporation of solutions of the corresponding MP(H)(t)Bu(3)Mes species dissolved in toluene/thf. Complex 4 was crystallized from hot toluene. On the other hand, slow evaporation of a toluene/tetrahydrofuran solution of CsP(H)(t)Bu(3)Mes (4) produces crystals of the composition {[CsP(H)(t)Bu(3)Mes](2)(mu-THF)(0.9).toluene}(x) (5). Crystallization of 4 in the presence of pyridine yields crystals of {[CsP(H)(t)Bu(3)Mes](2)(mu-pyridine)}(x) (6). Also, crystallization of complexes 3 and 4 from toluene/N-methylimidazole (N-MeIm) gives the isomorphous complexes {[RbP(H)(t)Bu(3)Mes](2)(mu-N-MeIm)}(x) (7) and {[CsP(H)(t)Bu(3)Mes](2)(mu-N-MeIm)}(x) (8), respectively. However, crystallization of 4 from toluene in the presence of bidentate or polydentate bases such as dimethoxyethane or pentamethyldiethylenetriamine does not result in incorporation of these bases into the lattice. Instead, the toluene solvate {[CsP(H)(t)Bu(3)Mes](2)(eta(3)-toluene)(0.5)}(x) (9) is obtained. On the other hand, crystallization of 4 from toluene/ethylenediamine gives the base adduct {[CsP(H)(t)Bu(3)Mes](2)(mu-ethylenediamine)}(x) (10). Complex 3 crystallizes in the triclinic space group P (1) over bar. Crystal data for 3 at 218 K : a = 6.71320(10) Angstrom, b = 10.5022(2) Angstrom, c = 14.9733(3) Angstrom, alpha = 91.3524(13)degrees, beta = 102.5584(13)degrees, gamma = 107.7966(14)degrees; Z = 1; R-1 = 6.55%. Complex 4 crystallizes in the triclinic space group P (1) over bar.Crystal data for 4 at 223 K: a = 7.0730(14) Angstrom; b = 10.395(2) Angstrom; c = 14.933(2) Angstrom; alpha = 81.97(1)degrees; beta = 76.35(2)degrees; gamma = 71.824(14)degrees; Z = 1; R-1 = 4.56%. Complex 5 crystallizes in the monoclinic space group P2(1)/c. Crystal data for 5 at 243 K: a = 15.039(2) Angstrom; b = 16.152(3) Angstrom; c = 20.967(5) Angstrom; beta = 91.53(2)degrees; Z = 4; R-1 = 4.83%. Complex 6 crystallizes in the orthorhombic space group Pbcn. Crystal data for 6 at 298 K: a = 14.686(2) Angstrom; b = 21.295(5) Angstrom; c = 28.767(5) Angstrom; Z = 8; R-1 = 5.61%. Complex 7 crystallizes in the orthorhombic space group Pbcn. Crystal data for 7 at 218 K: a = 14.5533(2) Angstrom; b = 21.4258(5) Angstrom; c 28.5990(5) Angstrom; Z = 8; R-1 = 4.61% . Complex 8 crystallizes in the orthorhombic space group Pbcn. Crystal data for 8 at 219 K: a = 14.6162(2) Angstrom; b = 21.3992(3) Angstrom; c = 28.7037(2) Angstrom; Z = 8; R-1 = 3.57%. Complex 9 crystallizes in the triclinic space group P (1) over bar. Crystal data for 9 at 293 K: a = 11.147(4) Angstrom; b = 14.615(4) Angstrom; c = 14.806(5) Angstrom; alpha = 70.57(3)degrees; beta = 71.85(3)degrees; gamma = 172.93(2)degrees; Z = 2; R-1 = 5.13%. Complex 10 crystallizes in the triclinic space group P (1) over bar.Crystal data for 10 at 173 K: a = 10.5690(4) Angstrom; b = 15.0376(5) Angstrom; c = 15.3643(5) Angstrom; alpha = 111.8630(10)degrees; beta = 100.4120(10)degrees; gamma = 97.4820(2)degrees; Z = 2; R-1 = 4.87%. A common feature of the molecular structures of complexes 2-10 is an infinitely extended polymeric ladder framework in the solid state. Both solution and solid-state NMR data are presented.