A series of phosphorus compounds containing carboxyl groups that serve as mimics for amino acid residues was synthesized. The series was composed of the phosphonium salts 1A, 1B, and 2, the anionic phosphines 3A and 3B, and the anionic phosphine oxide 4. X-ray structural analysis revealed that P-O coordination occurred in the presence of extensive hydrogen bonding and led to pseudo or regular trigonal bipyramidal geometries. P-31 chemical shifts indicated retention of the basic coordination geometries in solution. The two forms observed for 1 and 3 revealed the influence of hydrogen bonding on the P-O donor interactions while 2 and 4 showed the influence of molecular packing effects in competition with hydrogen bonding interactions. The results suggest that phosphoryl transfer enzyme mechanisms should benefit by taking into account P-O donor interactions by residues at active sites that can be manipulated by hydrogen bonding and molecular packing effects in enhancing nucleophilic attack at phosphorus centers.