X-ray crystal structures are reported for Cr(PCl3)(CO)(5), monoclinic P2(1)/c, Z = 4, a = 6.664(3) Angstrom, b = 12.886(4) Angstrom, c = 12.824(3) Angstrom, beta = 91.71(3)degrees, V = 1100.8(6) Angstrom(3), R (R(w)) = 0.0378 (0.040) using 1623 unique data with I > 2.5 sigma(I) at 180 K;W(PCl3)(CO)(5); monoclinic P2(1)/c, Z = 4, a = 6.774(2) Angstrom, b = 13.022(2) Angstrom, c = 12.975(2) Angstrom, beta = 91.48(2)degrees, V = 1144.2(4) Angstrom(3), R (R(w)) = 0.0449 (0.0493) using 1304 unique data with I > 2.0 sigma(I) at 130 K; Cr(PBr3)(CO)(5), triclinic, P $($) over bar$$ 1, Z = 2, a = 6.573(1) Angstrom, b = 6.688(2) Angstrom, c = 13.394(2) Angstrom, alpha = 88.34(2)degrees, beta = 86.94(1)degrees, gamma = 87.53(3)degrees, V = 587.2(2) Angstrom(3), R (R(w)) = 0.0511 (0.0641) using 2057 unique data with I > 2.0 sigma(I) at 130 K; W(PBr3)(CO)(5), monoclinic P2(1)/c, Z = 4, a = 6.811(1) Angstrom, b = 13.419(2) Angstrom, c = 13.203(4) Angstrom, beta = 92.54(2)degrees, V = 1205(4) Angstrom(3) R (R(w)) = 0.0397 (0.0460) using 1958 unique data with I > 2.0 sigma(I) at 130 K; and Mo(PMe(3))(CO)(5), monoclinic P2(1)/c, Z = 4, a = 7.009(2) Angstrom, b = 11.703(2) Angstrom, c = 15.179(3) Angstrom, beta = 103.25(2)degrees, V = 1211.9(5) Angstrom(3), R (R(w)) = 0.0205 (0.0265) using 1974 unique data with I > 2.0 sigma(I) at 130 K. The compounds Cr(PCl3)(CO)(5), W(PCl3)(CO)(5), and W(PBr3)(CO)(5) are isomorphous. Bond dimensions are interpreted in relation to the pi-acceptor ability of the phosphorus ligand. The disposition of the PY3 moiety is determined with reference to the near planar M(CO)(4) group for each of the complexes studied. The results are rationalized in terms of the steric and electronic properties of the substituents on the phosphorus. The P-Y bond lengths in the PY(3)M(CO)(5) complexes are compared with those of the uncomplexed PY3 ligand to probe the role of P-Y sigma* antibonding orbitals in the pi-acceptor behavior of the phosphorus ligand.