Inert metal ions

Substitution-inert metal ions as probes of biological function. J. I. Legg, Coord. Chem. Rev., 1978, 25,103-132 (84). 

A picket fence porphyrin system. Finally, examples of pendant donor groups attached to a porphyrin ring are known. A novel example of such a molecule is meso-a,a,a,a-tetrakis(o-nicotinamidophenyl)porphyrin (111) which is capable of binding two metal ions such that each has a square-planar environment with the square planes orientated coaxially to each other (Gunter et al., 1980). When a kinetically-inert metal ion such... 

Before we consider substitution processes in detail, the nature of the metal ion in solution will be briefly reviewed.A metal ion has a primary, highly structured, solvation sheath which comprises solvent molecules near to the metal ion. These have lost their translational degrees of freedom and move as one entity with the metal ion in solution. There is a secondary solvation shell around the metal ion, but the solvent molecules here have essentially bulk dielectric properties. The (primary) solvation number n in M(S)"+ of many of the labile and inert metal ions has been determined, directly by x-ray or neutron diffraction of concentrated solutions, from spectral and other considerations and by examining the exchange process... 

Metal-substitution studies, especially those in which Co11 replaces Zn11, have proved to be an important tool in the study of zinc metalloenzymes the Con-substituted species often have activities approaching those of the Zn11 enzyme in its in vivo reaction. These modification procedures have been the subject of a recent review,1265 which, however, focusses in particular on the use of substitution-inert metal ions. A recent innovation in metal-substitution techniques is the replacement of Znn in zinc metalloenzymes by 113Cdn, which can then be examined by Cd NMR. The results of a recent such investigation1266 indicate that the u3Cdn can serve as an... 

Monodentate coordination in solution is more readily proved with the kinetically inert metal ions. Thus, coordination through the amino N donor atom has been established with Cr111, Coin, Ir111, Ptu and Rhin. Monodentate coordination through the weaker field carboxyl O donor atom is not so common but has been observed with Co111 and more recently with Pt11. With the kinetically labile metal ions, monodentate coordination is sometimes observed at pH 3-5 as a minor species or can be obtained by means of blocking either the N or O donor atoms by substitution. 

In more recent years attention has turned from studying the equilibria of binary metal-amino acid complexes to that of ternary complex formation in aqueous media, particularly to complexes of the type (aa)—M11—L, where L is some other ligand or a different amino acid to (aa), and M11 is a kinetically labile metal ion. Ternary complexes involving kinetically inert metal ions, e.g. Co,w and Pt", are more well known since they can be separated from mixtures and studied in isolation. Such is not the case with the labile systems. Because of the facile nature of their equilibria they must be studied in situ (claims regarding the separation of labile species by chromatographic procedures... 

Macropolycyclic polyamine ligand (a form of cryptand leading to extremely inert metal ion complexes)... 

It needs to be noted that supramolecular systems may also form under kinetic rather than thermodynamic control. This situation will tend to be more likely for larger supramolecular assemblies incorporating many intermolecular contacts, especially when moderately rigid components are involved. It may also tend to occur when metal ions, and especially kinetically inert metal ions, are incorporated in the framework of the resulting supramolecular entity or when, for example, an intermediate product in the assembly process precipitates out of solution because of its low solubility. 

It has been recognized for many decades that there is an intimate relationship between coordinated ligand type and physical properties. In particular, the rate at which a donor ligand can be displaced by another in the coordination sphere of an inert metal ion is markedly dependent on the type of ligand involved. If a variety of donors are bound to a metal ion, one particular site may be far more likely to undergo ligand exchange or substitution than others this selectivity is important in the efficient operation of many metalloenzymes and in the operation of certain catalysts. Even with simple octahedral corn-... 

It is therefore the purpose of this review to deal with traditional and simple inert metal-ion complexes, mainly octahedral, in which the rate of displacement of at least one ligand permits it to be defined as... 

Other transition metal complexes with cis dichloro ligands have been tested for antitumor activity. The palladium analog cis-[PdCl2(NH3)2] is inactive, prohahly due to the high kinetic lability of Pd(II) compared to Pt(II), and isomerization is facile this process is blocked in the chelated en complex, which is active (16). More inert metal ions such as Rh(III) and Ru(III) also have active analogs (Table VI), and it is likely that some ruthenium complexes will soon enter clinical trials (31-33). 

The electrochemistry of metalloproteins has developed markedly (4-6) over the past 15 years. It has mainly been concerned with the electrochemistry at solid electrodes gold, upon which are adsorbed redox-inactive promoters, i.e., molecules that bind both to the electrode and the protein and edge-plane graphite, with or without redox-inert metal ions in solution. 

The surface characteristics of the diamagnetic solid FeS2 have been reviewed in Luther (1987). The Fe(II) ion is a low-spin, inert metal ion (d6, tlg). Briefly, the surface of a FeS2 crystal can be viewed as a unit of FeS2. This unit can be further divided to Fe2+ and S2In the Lewis acid-base sense, the fragment would be... 

Circular dichroism (CD) has played an important role in our studies on the modification of enzymes and hormones with Co(III). The objective of these studies has been to incorporate selectively substitution inert metal ions at specifically modified sites in proteins as probes of biological function. Significant information concerning the catalytic mechanism of carboxypeptidase A (CPA) (1) has been obtained from a site specific modification of tyrosine 248 with Co(III) (2). The method developed for CPA has been extended to other enzymes and hormones in order to devg op an improved method for incorporating stable radioisotopes t Co) into proteins. The substitution-inertness of Co(III) provides the necessary stability in these derivatives (3). 

The DNA-binding affinities of metallo-MPE were obtained with inert metal ions. The affinity constants of Ni -MPE and Mg -MPE are 2.4 X 10 and 1.5 X 10 M, respectively, similar to the affinity constant of ethidium bromide (125). As expected for an intercalator, Mg -MPE unwinds DNA (11 3° per bound molecule), and the binding site size is two base pairs. 

Ionization isomerism is best characteized in systems involving substitutionally inert metal ions where kinetic, rather than thermodynamic, factors control isomer formation. Numerous cases are known of cobalt(III) compounds in which exchange of a ligand between inner and outer coordination spheres results in a spectacular colour change. Thus, tra/is-[CoCl2(en)2]N02 (en = 1,2-ethanediamine) is green and frans-[CoCl(N02)(en)2]Cl is orange tmns-[CoCl(NCS)-(en)2]NCS is blue and frans-[Co(NCS)2(en)2]Cl is deep red. 

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