The X-ray crystal structures of cis-Mo(CO)4(Ph2PNH2)2, I, and trans-Mo(CO)4(Ph2PNHMe)2, II, are presented. Complex I crystallizes in the monoclinic space group P21/c(a=13.433(1), b=12.2719(8), c=17.318(2)Å;β=109.79(1)°;V=2686.1(8)Å3;Z=4). Complex II crystallizes in the triclinic space group P-1 (a=6.9986(8), b=10.328(1), c=11.241(2)Å, α=107.58(1)°, β=91.76(1)°, γ=101.28(1)°, V=756.1(4)Å3, Z=1). The molybdenum coordination geometry in each complex is a slightly distorted octahedron. The molybdenum-carbon bond lengths for the carbonyls trans to phosphorus in complex I are shorter than those the carbonyls trans to other carbonyls. The average molybdenum-phosphorus distance in I (2.525(5)Å) is similar to those in other diphenylphosphinamide complexes and longer than the molybdenum-phosphorus distance in II in 2.4585(7)Å). The distance between two nitrogen atoms in cis Mo(CO)4(Ph2PNH2)2 (3.74(3)Å) is significantly larger than the sum of their van der Waals radii (3.10 Å) indicating that the two nitrogens are not hydrogen bonded. © 1993 Plenum Publishing Corporation.