We explore the molecular properties of adsorbates that dramatically affect growth kinetics and morphology of the {100} face of archerite, also known as potassium dihydrogen phosphate (KH2PO4 or KDP). Aqueous complexes of Al(III), Fe(III), and Cr(III) are known to affect KDP growth, albeit the actual step-pinning complex(es) is unknown. Using in situ atomic force microscopy (AFM), we measured changes in the growth rates of the {100} face of KDP with supersaturation in the presence of trace amounts of [Co(NH3)(6)](3+), [Fe(CN)(6)](3-), eta(1)-[Co(NH3)(5)HPO4](+), beta(2)-[Co(NH3)(4)HPO4](+), eta(2)-[Co(NH3)(4)P2O7H2](+,) and [Rh(H2PO4)(2)(H2O)(4)](+). Unlike in experiments using trivalent-metals, these complexes do not change stoichiometry or structure on the timescale of step motion, so that the actual molecular interactions that affect growth can be Studied. Step velocity and morphology on the {100} face are unaffected by outer-sphere coordination complexes of either charge. Surprisingly, inner-sphere phosphatommine complexes do not affect growth rates regardless of how the phosphate group is coordinated to the metal. However, doping the growth solution with [Rh(H2PO4)(2)(H2O)(4)](+) results in profound step pinning, matching the behavior of KDP surfaces grown in the presence of Rh(III) after an equilibration period. Not only is an inner-sphere phosphate group needed to dock a trivalent metal to the step edge, but compatible hydrogen bonding of the remainder of the inner-sphere ligands with the bulk lattice is also essential. (C) 2004 Elsevier Inc. All rights reserved.