Transition-metal ions arrays

We are concerned with what happens to the (spectral) d electrons of a transition-metal ion surrounded by a group of ligands which, in the crystal-field model, may be represented by point negative charges. The results depend upon the number and spatial arrangements of these charges. For the moment, and because of the very common occurrence of octahedral coordination, we focus exclusively upon an octahedral array of point charges. 

Coordinative Environment. The coordinative environment of transition metal ions affects the thermodynamic driving force and reaction rate of ligand substitution and electron transfer reactions. FeIIIoH2+(aq) and hematite (a-Fe203) surface structures are shown in Figure 3 for the sake of comparison. Within the lattice of oxide/hydroxide minerals, the inner coordination spheres of metal centers are fully occupied by a regular array of O3- and/or 0H donor groups. At the mineral surface, however, one or more coordinative positions of each metal center are vacant (15). When oxide surfaces are introduced into aqueous solution, H2O and 0H molecules... 

The calculation of the energy of a transition metal ion (/-orbital in the presence of the electric field gradients produced by a regular array of the positive and negative charges of other cations... 

Not mentioned in Table 2 (and often not in the original papers ) is the optical form (chirality) of the amino acids used. All the amino acids, except for glycine (R = H), contain an asymmetric carbon atom (the C atom). In the majority of cases the optical form used, whether l, d or racemic dl, makes little difference to the stability constants, but there are some notable exceptions (vide infra). Examination of the data in Table 2 reveals (i) that the order of stability constants for the divalent transition metal ions follows the Irving-Williams series (ii) that for the divalent transition metal ions, with excess amino acid present at neutral pH, the predominant spedes is the neutral chelated M(aa)2 complex (iii) that the species formed reflect the stereochemical preferences of the metal ions, e.g. for Cu 1 a 2 1 complex readily forms but not a 3 1 ligand metal complex (see Volume 5, Chapter 53). Confirmation of the species proposed from analysis of potentiometric data and information on the mode of bonding in solution has involved the use of an impressive array of spectroscopic techniques, e.g. UV/visible, IR, ESR, NMR, CD and MCD (magnetic circular dichroism). 

Refinement of the data show that the structure of SrjCrNbO, 53 is made up of a partially ordered array of CrOg and NbO comer-sharing octahedra (Fig. 5). The Cr/Nb(l) and Cr/Nb(2) site occupancy values are 63/37 and 37/63 % respectively (Table 1). The large amount of disorder between the Cr and Nb ions leads to AFM type magnetic interactions between identical neighboring transition metal ions. The presence of AFM type interactions leads to a reduction in the observed saturation magnetization moment and contributes to the observed spin-glass type behavior. 

It is convenient to classify extended arrays in terms of a scheme based on the specific nature of the exchange coupling between the magnetic units plus the structural dimensionahty of the array. (This article will only concern interactions between nearest neighbors.) In this scheme, the magnetic units in the arrays may be either radicals or transition metal ions the ions themselves may be of mixed valence and there may be more than one type of metal ion. 

The magnetic properties of individual transition metal ions in compounds and complexes are considered in see Magnetism of Transition Metal Ions). There, the properties of a given ion are assumed to be independent of the presence of any other ions. However, the possibility of minor interactions between ions is taken into account in those cases in which the temperature dependence of the inverse magnetic susceptibihty deviates from the Curie law. This was accomphshed by inclusion of the Weiss constant and the generation of the Curie-Weiss law. The properties of extended arrays of this type may be primarily understood in terms of single ions, and they will not be discussed here. 

The synthesis of paramagnetic materials that have specifically tailored magnetic properties is gaining considerable interest (see Magnetism of Extended Arrays in Inorganic Solids and Magnetism of Transition Metal Ions). For example, metallocenes and their derivatives are known to... 

Alloys Chalcogenides Solid-state Chemistry Hypervalent Compounds Magnetism of Extended Arrays in Inorganic Solids Magnetism of Transition Metal Ions Semiconductors. 

Alternatively, one can treat silicates as closest packed arrays of oxide ions with Si ions fitting into tetrahedral holes and other metal ions fitting into either tetrahedral holes as in phenacite or octahedral holes as in olivine. (See Fig. 7.3 for the alternative ways of describing olivine.) Transition metal ions in these structures behave as they do in complexes Olivine gets its name from the greenish color caused by partial substitution of Fe " for Mg ions in the octahedral holes. The hexa-aquairon(II) ion has a similar green color. The hue of garnets also comes from transition metal ions. 

These oxides contain d and d transition metal ions with a high formal valence state which are linked to oxygens by strong covalent bonds. Their framework is depicted as an array of distorted tetrahedra, pentahedra, or octahedra as in the Wadsley phases" belonging to the (V-0), (Mo-0), (V-Mo-0), (W-0) systems (44,46). 

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