From the reactions between M(2)(OCH(2)(t)Bu)(6) (2 equiv) and NaP(SiMe(3))(2) (1 equiv) in benzene and 18-crown-6 the phosphinidine clusters [M(4)(mu(3)-PSiMe(3))(OCH(2)(t)Bu)(11)](-), compounds 1 (M = Mo) and 2 (M = W) are formed and are reactive intermediates in the formation of [M(4)(mu(3)-P)(OCH(2)(t)Bu)(10)](-) cluster anions, compounds 3 (M = Mo) and 4 (M = W), respectively, which have been isolated and characterized as their Na(18-crown-6)(+) salts. Crystals of Na(18-crown-6)(+)[Mo-4(mu(3)-P)(OCH(2)(t)Bu)(10)](-). C6H6 were examined by single crystal X-ray crystallography at -110 degrees C in the space group Pn with a 13.000(2) Angstrom, b = 17.501(3) Angstrom, c = 18.703(2) Angstrom, and beta = 93.11(1)degrees with Z = 2 and at -173 degrees C in the space group Cc with a = 22.118(4) Angstrom, b = 17.480(3) Angstrom, c = 23.250(4) Angstrom, and beta = 110.82(1)degrees with Z = 4. At the lower temperature, there is higher symmetry and no disorder of the 18-crown-6 molecule.