Synthesis, crystal structures, and chemical bonding are reported for four binary phosphides with different degrees of phosphorus oligomerization, ranging from isolated P atoms to infinite phosphorus chains. Ba3P2 = Ba4P2.67 square(0.33) (square = vacancy) crystallizes in the anti-Th3P4 structure type with the cubic space group 1 (4) over bar 3d (no. 220), Z = 6, a = 9.7520(7) angstrom. In the Ba3P2 crystal structure, isolated P3 anions form distorted octahedra around the Ba2' cations. fl-BasP4 crystallizes in the EusAs4 structure type with the orthorhombic space group Cmce (no. 64), Z = 4, a = 16.521(2) angstrom, b = 8.3422(9) angstrom, c = 8.4216(9) angstrom. In the crystal structure of beta-Ba5P4, one-half of the phosphorus atoms are condensed into P-2(4-) dumbbells. SrP2 and BaP2 are isostructural and crystallize in the monoclinic space group P2(1)/c (no. 14), Z = 6, a = 6.120(2)16.368(1) angstrom, b = 11.818(3)/12.133(2) angstrom, c = 7.441(2)17.687(2) angstrom, beta = 126.681(4)/126.766(2)degrees for SrP2/BaP2. In the crystal structures of SrP2 and BaP2, all phosphorus atoms are condensed into infinity P-1(1-) cis trans helical chains. Electronic structure calculations, chemical bonding analysis via the recently developed solid-state adaptive natural density partitioning (SSAdNDP) method, and UV vis spectroscopy reveal that SrP2 and BaP2 are electron-balanced semiconductors.