Metal ions, complexed, displacement

Relevant to the conflicting reports of copper versns nickel reqnirements for enzyme activity,biochemical stndies demonstrated the existence of a labile nickel associated with the a snbnnit of ACS/CODH. Very recently, model stndies on a metal-ion captnre of a peptide-backbone, nonlabile [MN2S2] (26) nnit have established the capability of snch a nickel dithiolate to bind exogeneons metals. A qualitative ranking of the binding ability of complex (26) with Zn +, Cn+, and Ni + was established by a metal-ion displacement experiment (Zn + < < Cn+), as shown in Scheme 9. ... 

Reaction. Complexed Metal Ions in Displacement Deposition... 

The MNO 8 complexes, essentially Co porphyrins,71,72 show the predicted strongly bent MNO units. Figure 4 shows formal diagrams of the consensus structures for five-coordinated complexes with MNO ", for n = 6, 7, or 8. All the compounds in Table 3 have a low-spin electronic GS. Note the systematic variation in the MNO angle, M—N(NO) bond length, and metal ion displacements from the porphyrin cores.19... 

At low pH most Mn-+ is adsorbed by a soil FA in the form of outer-sphere complexes, but at pH > 8, or T > 50°C, Mn"+ can enter inner-sphere multiligand complexation sites (McBride, 1982). These results indicate that the type and stability of Mn2+ -HS complexes and, in turn, their ease of exchangeability and bioavailability in natural systems are strongly dependent on pH and temperature. Further, HSs isolated from various sources exhibit a high residual complexing capacity for added Mn that can be bound in water-stable forms, but unlike Fe + and Cu " ", it may be displaced completely by protons or strongly complexed metal ions (Senesi et al., 1991). 

Rates of Reaction. The rates of formation and dissociation of displacement reactions are important in the practical appHcations of chelation. Complexation of many metal ions, particulady the divalent ones, is almost instantaneous, but reaction rates of many higher valence ions are slow enough to measure by ordinary kinetic techniques. Rates with some ions, notably Cr(III) and Co (III), maybe very slow. Systems that equiUbrate rapidly are termed kinetically labile, and those that are slow are called kinetically inert. Inertness may give the appearance of stabiUty, but a complex that is apparentiy stable because of kinetic inertness maybe unstable in the thermodynamic equihbrium sense. 

Titanium tetrachloride and tin tetrachloride can form complexes that are related in character to both those formed by metal ions and those formed by neutral Lewis acids. Complexation can occur with an increase in the coordination number at the Lewis acid or with displacement of a chloride from the metal coordination sphere. 

This reaction will proceed if the metal indicator complex M-In is less stable than the metal-EDTA complex M EDTA. The former dissociates to a limited extent, and during the titration the free metal ions are progressively complexed by the EDTA until ultimately the metal is displaced from the complex M-In to leave the free indicator (In). The stability of the metal-indicator complex may be expressed in terms of the formation constant (or indicator constant) Ku ... 

Stability of the bidentate and multidentate complexes in aqueous solution [16] compared with monodentate complexes. Kinetic studies of gold(III) reactions with ethylenediamine and related ligands show that the initial displacement of one end of the chelate is most often followed by rapid reclosure of the ring, rather than displacement of the second bond to the metal ion [15]. 

A determination of dimethyl sulphoxide by Dizdar and Idjakovic" is based on the fact that it can cause changes in the visible absorption spectra of some metal compounds, especially transition metals, in aqueous solution. In these solutions water and sulphoxide evidently compete for places in the coordination sphere of the metal ions. The authors found the effect to be largest with ammonium ferric sulphate, (NH4)2S04 Fe2(S04)3T2H20, in dilute acid and related the observed increase in absorption at 410 nm with the concentration of dimethyl sulphoxide. Neither sulphide nor sulphone interfered. Toma and coworkers described a method, which may bear a relation to this group displacement in a sphere of coordination. They reacted sulphoxides (also cyanides and carbon monoxide) with excess sodium aquapentacyanoferrate" (the corresponding amminopentacyanoferrate complex was used) with which a 1 1 complex is formed. In the sulphoxide determination they then titrated spectrophotometrically with methylpyrazinium iodide, the cation of which reacts with the unused ferrate" complex to give a deep blue ion combination product (absorption maximum at 658 nm). 

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