Some Problems in the Crystal Chemistry of Platinum Complexes

Abstract

<p>Molecular and crystal structure studies related to various aspects of the chemistry and biochemistry of platinum are presented. The first terminal Pt(II)-OH₂ bond observed in-a single crystal verifies the existence of a stable, four-coordinate aquaplatinum(II) complex. The Pt-OH₂ bond is normal and the cis-i Pt (OH₂) (NH₃)₃ (1-methylcytosine)]²⁺ ion models a proposed initial monofunctional Pt-DNA complex. The molecular structure of trans-[Pt(NO₃)₂(NH₃)₂ in the solid state is shown to be similar to the cis isomer in that it has two monodentate nitrato ligands, but it differs in that the non-bonded oxygen atoms of the two nitrato ligands lie above and below the ligand square plane. Evidence is presented for ionic and tetravalent platinum impurities in freshly prepared solutions of trans-[PtCl₂(NH₃)₂]. The structure of trans-[Pt(NH₃)₂(1-methylcytosine) (9-ethylguanine)]²⁺ offers a possible model for DNA interstrand crosslinking which can explain the thermal stabilization of DNA (with the preservation of base stacking) observed after exposure to low concentrations of trans-[PtCl₂(NH₃)₂.</p> <p>Various problems of crystallographic interest are discussed. The geometry of diethylenetriamine complexes in the solid state is examined, and a convention is proposed for assigning conformations. A survey of dien complexes is used to emphasize the importance of intermolecular hydrogenbonding, as opposed to intramolecular steric interactions, in the configuration of the dien ligand. Various types of disorder in linear chain halogen-bridged mixed-valence platinum compounds are examined. An unusual example of a Pt(II)...Cl-Pt(IV)-Cl chain compound with three-dimensional ordering at the Pt(II) site is presented. The importance of weak intensity data and careful photographic work in these systems is also demonstrated.</p> <p>The structural analyses of aqua, nitrato, and ammine nitrate complexes are aided by the use of the bond valence model. Quantitative bond length and qualitative packing predictions are made, and structural anomalies are explained, using empirical bond valence curves.</p>