The reaction of Ph2P(Se)N(SiMe3)2 with potassium tert-butoxide in a 1:1 molar ratio in THF at room temperature produces (K[Ph2P(Se)NSiMe3].THF)2 (5) in excellent yields. An X-ray structural determination of 5 reveals a dimeric step-shaped structure involving two four-membered NPSeK rings fused to a central K2Se2 ring. The P-Se bond lengths are ca. 2.18 angstrom while the P-N bond distances are 1.556(13) and 1.594(12) angstrom. The value of 1J(P-31-Se-77) for 5 is 604 Hz and the stretching frequency nu(P=Se) occurs at 546 cm-1. Crystals of 5 are monoclinic, space group P2(1)/c, with a = 9.643(2) angstrom, b = 16.500(5) angstrom, c = 29.366(7) angstrom, B = 91.34(2)-degrees, V = 4671(1) angstrom3, and Z = 8, with final R and R(w) values of 0.076 and 0.081, respectively. The reactions of 5 with I2 or Ph2P(S)Cl occur at the selenium sites to give (Me3SiN)Ph2PSeSePPh2(NSiMe3) and (Me3SiN)Ph2PSePPh2(S), respectively, which were identified by P-31 and Se-77 NMR spectroscopy.