Covalent octahedral complexes

It is interesting to note, as was pointed out to me some years ago by J. L. Hoard, that these considerations lead to an explanation of the difference in stability of cobalt (II) and cobalt (I JI) as compared with iron (II) and iron (III) in covalent octahedral complexes. The formation of covalent complexes does not change the equilibrium between bipositive and tripositive iron very much, as is seen from the values of the oxida-... 

Fig. 17. Structure of (Fe Fe)— an ionic octahedral complex and [Fe+++ (CN)e]— a covalent octahedral complex, the bonds being formed by d p -hybridi-eation.

The role of coordinated ethylene is evidenced by the recent ab initio calculation performed by Espelid and Borve [121-123], who have shown that ethylene may coordinate in two different ways to the reduced Cr(II) species, either as a molecular complex or covalently bound to chromium. At longer Cr-C distances (2.36-2.38 A) an ethylene-chromium zr-complex forms, in which the four d electrons of chromium remain high-spin coupled and the coordination interaction is characterized by donation from ethylene to chromium. Cr(II) species in a pseudo-tetrahedral geometry may adsorb up to two equivalents of ethylene. In the case of a pseudo-octahedral Cr(II) site a third ethylene molecule can also be present. The monoethylene complex on the pseudo-tetrahedral Cr(II) site was also found to undergo a transformation to covalently bound complex, characterized by shorter Cr-C distances (about... 

Fig. 5-12.—The representation of alternative extreme types of electronio structures for the octahedral complex ion [Co(NH ) ]++4 On the left is a representation of the structure with extreme electrostatic bonds. The cobalt atom is represented as having a positive Electric charge, 3+. At the right is represented the structure in which normal covalent bonds are between the cobalt atom and the surrounding nitrogen atoms, as well as between the nitrogen atom and its three attached hydrogen atoms. This structure places the charge 3— on the cobalt atom and 1+ on each nitrogen atom.

The covalent model of hypersensitivity was developed by Choppin and coworkers [70,74] to explain (i) the observed order of sensitivity of the parameters to the environment is 72 > 74 > 7f, >, (ii) the general trend of the hypersensitive transition oscillator strength increases with estimated covalency, (iii) the observed hypersensitivity in octahedral complexes, (iv) the simultaneous occurrence of hypersensitivity and large splittings of... 

Table 2. Relation of /q to covalency parameters for an octahedral complex )...

The same effect occurs in octahedral complexes containing Cu + (d/d ), covalency again requiring the use of 4d orbitals. CnF has a distorted rutile structure in which there are two F ions at 2.27 A and four at 1.93 A from the Cu + (Orgel and Dunitz, 1957). But when the metal ion has 5, 6, 7, 8, or 10 electrons, difluorides have essentially undistorted rutile lattices (Fig. 93). 

At normal levels of iron intake, absorption requires uptake from the intestinal lumen by the mucosa and transfer from the mucosa to the portal blood. Both events are inversely affected by the state of body iron stores. In iron deficiency states, nonferrous metals such as cobalt and manganese, which have an ionic radius similar to that of iron and form octahedral complexes with six-coordinate covalent bonds, also are absorbed at an increased rate. Oral administration of a large dose of iron reduces (or temporarily inhibits) the absorption of a second dose of iron by the absorptive enterocytes even in the presence of systemic iron deficiency. The mechanism of mucosal block, which resists acquiring additional iron by the en-teroeytes with high amounts of intracellular iron, is not yet understood. It probably involves set points established in the enterocytes for iron recently consumed in the diet (dietary regulator). 

A molecular orbital theory for octahedral / complexes is described. It is pointed out that the neglect of covalent bonding in the analysis of optical data for the actinide complexes is not justified, and that its inclusion leads to orbital reductions which are considerably greater than have usually been assumed. 

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