The 1:1 mole ratio reactions of boron trihalides (BX(3)) with tris(trimethylsilyl)phosphine [P(SiMe(3))(3)] produced 1:1 Lewis acid/base adducts [X(3)B . P(SiMe(3))(3), X = Cl (1), Br (2), I (5)]. Analogous 1:1 mole ratio reactions of these boron trihalides with lithium bis(trimethylsilyl)phosphide [LiP(SiMe(3))(2)] produced dimeric boron-phosphorus ring compounds {[X(2)BP(SiMe(3))(2)](2), X = Br (3), Cl (4)}. X-ray crystallographic studies were successfully conducted on compounds 1-4. Compound 1 crystallized in the orthorhombic space group Pbca, with a = 13.420(3) Angstrom, b = 17.044(5) Angstrom, c = 21.731(7) Angstrom, V = 4970.6(25) Angstrom(3), and D-calc = 1.229 g cm(-3) for Z = 8; the B-P bond length was 2.022(9) Angstrom, Compound 2 crystallized in the orthorhombic space group Pbca, with a = 13.581(6) Angstrom, b = 17.106(7) Angstrom, c = 22.021(9) Angstrom, V = 5116(4) Angstrom(3), and D-calc = 1.540 g cm(-3) for Z = 8; the B-P bond length was 2.00(2) Angstrom. Compound 3 crystallized in the monoclinic space group P2(1)/n, with a = 9.063(5) Angstrom, b = 16.391(8) Angstrom, c = 9.331(4) Angstrom, V = 1379.2(12) Angstrom(3), and D-calc = 1.676 g cm(-3) for Z = 2; the B-P bond length was 2.023(10) Angstrom. Compound 4 crystallized in the monoclinic space group P2(1)/n, with a = 9.143(5) Angstrom, b = 16.021(8) Angstrom, c = 9.170(4) Angstrom, V = 1342.2(11) Angstrom(3), and D-calc = 1.282 g cm(-3) for Z = 2; the B-P bond length was 2.025(3) Angstrom. Thermal decomposition studies were performed on compounds 1-4, yielding colored powders with boron : phosphorus ratios greater than 1:1 and significant C and H contamination indicated by elemental analyses.