The addition of 2 equiv of (BuLi)-Bu-n to a solution of (BuN)-Bu-t(H)P(mu-(NBu)-Bu-t)(2)PN(H)Bu-t (1) in toluene produces the adduct [LiP((NBu)-Bu-t)(2)](4). 0.25P(4)((NBu)-Bu-t)(6) (2.0.253), but the reverse addition yields only 2. The structure of 2.0.253 has been determined by X-ray crystallography : monoclinic, P2(1)/a, a = 17.649(5) Angstrom, b = 18.272(4) Angstrom, c = 20.295(5) Angstrom, beta = 111.92(2)degrees, V = 6072(3) Angstrom(3), and Z = 4. The tetrameric cluster 2 consists of two P2N2 rings bridged by a tub-shaped, eight-membered Li4N4 ring. The mean P-N distance in the P2N2 rings (1.78 Angstrom) is significantly longer than the corresponding value of 1.65 Angstrom for the P-N bonds linking the P2N2 rings to the Li4N4 unit. The P-4((NBu)-Bu-t)(6) cage 3 in the adduct is formed as the isomer in which two P2N2 rings are bridged by two (NBu)-Bu-t groups. The same tetracyclic isomer of 3 is prepared in 71% yield by the reaction of tBuN(Li)P(mu-(NBu)-Bu-t)(2)PN(Li)Bu-t with ClP(mu-(NBu)-Bu-t)(2)PCl in toluene. Variable temperature P-31{H-1} NMR studies of the 16-atom cluster [LiP((NBu)-Bu-t)(2)](4) (2) at different concentrations in C7D8 indicate that facile dissociation into two molecules of the dimer [LiP((NBu)-Bu-t)(2)](2) occurs above ca. 295 K.