Transition metals ionic radii

Prediction of Structural Changes. Crystal field theory is frequently applied to account for and even predict structural and chemical changes. A well-known example is the variation of first row transition metal ionic radii in an octahedral environment as illustrated in Figure 6-38 [50], The dashed line connects the points for Ca, Mn, and Zn, i.e., atoms with spherically symmetrical distribution of d electrons. Since the shielding of one d electron by another is imperfect, a contraction in the ionic radius is expected along this series. This in itself would account only for a steady decrease in the radii, whereas the ionic radii of all the other atoms are smaller than interpolation from the Ca-Mn-Zn curve would suggest. As is well known, the... 

Shannon and Prewitt base their effective ionic radii on the assumption that the ionic radius of (CN 6) is 140 pm and that of (CN 6) is 133 pm. Also taken into consideration is the coordination number (CN) and electronic spin state (HS and LS, high spin and low spin) of first-row transition metal ions. These radii are empirical and include effects of covalence in specific metal-oxygen or metal-fiuorine bonds. Older crystal ionic radii were based on the radius of (CN 6) equal to 119 pm these radii are 14-18 percent larger than the effective ionic radii. 

Figure 8-16. Correlation of ionic radius and LFSE with log values for divalent transition-metal complexes of 1,2-diaminoethane.

The dominant features which control the stoichiometry of transition-metal complexes relate to the relative sizes of the metal ions and the ligands, rather than the niceties of electronic configuration. You will recall that the structures of simple ionic solids may be predicted with reasonable accuracy on the basis of radius-ratio rules in which the relative ionic sizes of the cations and anions in the lattice determine the structure adopted. Similar effects are important in determining coordination numbers in transition-metal compounds. In short, it is possible to pack more small ligands than large ligands about a metal ion of a given size. 

Derived from the German word meaning devil s copper, nickel is found predominantly in two isotopic forms, Ni (68% natural abundance) and Ni (26%). Ni exists in four oxidation states, 0, I, II, III, and IV. Ni(II), which is the most common oxidation state, has an ionic radius of —65 pm in the four-coordinate state and —80 pm in the octahedral low-spin state. The Ni(II) aqua cation exhibits a pAa of 9.9. It forms tight complexes with histidine (log Af = 15.9) and, among the first-row transition metals, is second only to Cu(II) in its ability to complex with acidic amino acids (log K( = 6-7 (7). Although Ni(II) is most common, the paramagnetic Ni(I) and Ni(III) states are also attainable. Ni(I), a (P metal, can exist only in the S = state, whereas Ni(lll), a cT ion, can be either S = or S =. ... 

At the first step, the insertion of MMA to the lanthanide-alkyl bond gave the enolate complex. The Michael addition of MMA to the enolate complex via the 8-membered transition state results in stereoselective C-C bond formation, giving a new chelating enolate complex with two MMA units one of them is enolate and the other is coordinated to Sm via its carbonyl group. The successive insertion of MMA afforded a syndiotactic polymer. The activity of the polymerization increased with an increase in the ionic radius of the metal (Sm > Y > Yb > Lu). Furthermore, these complexes become precursors for the block co-polymerization of ethylene with polar monomers such as MMA and lactones [215, 217]. 

Unlike the other alkaline earth and transition metal ions, essentially on account of its small ionic radius and consequent high electron density, Mg2+ tends to bind the smaller water molecules rather than bulkier ligands in the inner coordination sphere. Many Mg2+-binding sites in proteins have only 3, 4 or even less direct binding contacts to the protein, leaving several sites in the inner coordination sphere occupied by water, or in the phosphoryl transferases, by nucleoside di- or triphosphates. 

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