Small metal ions

Immunity the state of a metal whose corrosion rate is low or negligible because its potential is below (less positive than) that of equilibrium with a very small concentration (or activity of its dissolved ions. The metal is thus regarded as thermodynamically stable. Pourbaix has suggested that the small metal ion concentration be 10 mol dm (Atlas of Electrochemical Equilibria in Aqueous Solutions, p. 71, Pergamon/ CEBELCOR, Oxford (1966)). 

Fig. 14. (a) The ax-ax-ax structure adopted by small metal ions, and (b) the ax-eq-ax type of structure adopted by large metal ions in their complexes with cis-inositol (44, 45). 

Fig. 17. Plot of change in complex stability, Alog K, that occurs for the pairs of ligands THEEN and EN (O), and PDTA-amide and EDDA ( ), as a function of metal ion radius (20). The diagram shows how netural oxygen donors stabilize the complexes of large metal ions relative to small metal ions. Alog Ki for THEEN and EN, for example, is log Kx for the THEEN complex of the particular metal ions, minus log Kx for the EN complex. Data from Ref. (11).

Fig. 25. The effect of metal ion size on porphyrin ruffling. Very small metal ions [P(V) with an ideal P-N bond length of 1.84 A and low-spin Ni(II) with an ideal Ni-N length of 1.90 A in (a) and (b)) cause extensive S4 ruffling. Metal ions close to the right size (M-N = 2.035 A) give planar structures [Zn(II) in (c)]. Metal ions that are too large [Pb(II) at (d) with ideal Pb-N of 2.39 A] are extruded from the plane of the porphyrin and cause it to dome. For clarity, substituents on the porphyrins such as phenyl or ethyl groups have been omitted. Modified after Ref. (77).

Tetrahedral complexes arc favored by steric requirements, either simple electrostatic repulsions of charged ligands or van dcr Wauls repulsions of large ones. A valence bond (VB) point oT view ascribes tetrahedral structures to p% hybridization From a crystal field (CF) or molecular orbital (MO) viewpoint we have seer that, in general, tetrahedral structures are not stabilized by large LFSE. Tetrahedral complexes are thus favored by large ligands like Cl-. Br. and 1 and small metal ions of three types ... 

Esr spectra are subject to exchange effects in the same way as nmr spectra. A specific example is provided by electron exchange between sodium naphthalenide and naphthalene. Naphthalene has a set of ten 77-molecular orbitals, similar to the six 7r-molecular orbitals of benzene (Figure 21-5). The ten naphthalene it electrons fill the lower five of these orbitals. In a solvent such as 1,2-dimethoxyethane, which solvates small metal ions well, naphthalene accepts an electron from a sodium atom and forms sodium naphthalenide, a radical anion ... 

One of the more interesting hole size effects arises when the metal ion successfully acts as a template, but is labilised in the macrocyclic complex that is formed. The consequence of this is that the metal ion acts as a transient template. The metal ion may be viewed as pre-organising the reactants to form the macrocyclic products, but then finding itself in an unfavourable environment after the cyclisation. The effect is best observed when a small metal ion is used as a template for a reaction that can only give one product (or at least, only one likely product). What happens to the metal ion when it finds itself in an environment that does not match up to its co-ordination requirements The most useful consequence would be labilisation of the metal ion, with resultant demetallation and formation of the metal-free macrocycle. This would overcome one of the major disad-... 

Figure 3.27 Binding constants for the large metal ion Pb (II) decrease by over six orders of magnitude as the ring size increases in tetraazamacrocycles as a result of the conversion of 5-membered chelate rings (suitable for large metal ions) into six-membered chelate rings (suitable for small metal ions).

Scheme 3.22 Schiff base reactions (a) 1 1 cyclocondensation templated by small metal ions (b) larger macrocycles are obtained with larger templates.

It emerges that each ob conformation reduces the cavity size, i.e., the D3ob3 conformation is most stable with small metal ions and the D3lel3 conformer has the largest cavity size. This is in agreement with the computed Co-N distances of the five stable conformers (see Table 17.16.3). 

The higher the charge density or charge-to-radius ratio, the more likely a metal ion undergoes hydrolysis in aqueous solutions to give hydroxo complexes. Hydroxo complexes may abruptly form polynuclear complexes and precipitate even in solutions more acidic than the pA"a for the first hydroxo complex formation. The first five small metal ions in Table 2 hydrolyze even in acidic solutions and form precipitates. In six-coordination, the charge-to-radius ratio for the first five... 

The coordination number and geometry of complexes (Table 1) is thought to be dictated by the valency (i.e. oxidation state of the central metal ion also see Section 4.5), the number of electrons possessed by the metal to be shared with ligands, the relative sizes of metal ions and ligands, as well as symmetry considerations. Small metal ions and large ligands favor low-coordination numbers and vice versa. 

Many metal ions don t stop at binding one ligand, but can surround themselves with several ligands, often up to four or even six. The maximum number of ligands usually depends on the ionic valencies of metal ion and ligand, on the coordination number of the metal ion, and on steric considerations. The latter are most pronounced for complexes of relatively small metal ions with rather bulky ligands. 

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