Octahedral coordination complexes

Role of activation volume in the elucidation of reaction mechanisms in octahedral coordination complexes. G. A. Lawrance and D. R. Stranks, Acc. Chem. Res., 1979,12,403-409 (50). 

This exhausts all possible stmctures of [Cr (NH3)3 CI3 ]. Octahedral coordination complexes of general formula M X3 X2 have two isomers. 

The crystal field energy level diagram for octahedral coordination complexes. The energies of the d orbitals differ because of differing amounts of electron-electron repulsion. The... 

C20-0096. Use orbital sketches to explain why the d, and j2 y2 orbitals have different stabilities in octahedral coordination complexes. 

Do not say octahedrally coordinated complex if you mean an octahedrally coordinated atom in a complex compound. An octahedrally coordinated complex could at best be a complex molecule that is surrounded by six other molecules. 

Fig. 1. Construction of molecular orbitals for octahedral coordination complexes of non w-bonding ligands utilizing metal 3

The treatment of Saika and Slichter has served as model for several subsequent calculations of chemical shifts of nuclei other than hydrogen. The most successful of these calculations have been those of Griffith and Orgel (39) and Freeman et al (35) on the shifts of complexed Co69(III). Shifts for Co59(III) in a variety of octahedrally coordinated complexes are shown in Table I. The range of 14,000 ppm in chemical shifts for these rather similarly constituted complexes is such as to suggest that a para-... 

Berry JF, Bill E, Bothe E, et al. An Octahedral Coordination Complex of Iron(VI). Science. 2006 3I2 I937 41. 

Many of the most commonly encountered transition metal ions accept electron pairs from donors such as CN and NH3 (or lacking these, even from H20) to form octahedral coordination complexes. The hexamminezinc(II) cation depicted below is typical. 

To test your understanding of the MO model for a typical octahedral coordination complex, construct an appropriate, qualitative MO diagram for Oh SHg (a model for known SF6). Hint first calculate the total number of MOs you should end up with from the number of available basis functions (AOs). Second, compare the valence AO functions of S with those of a transition metal (refer to Figure 1.9 and realize that, for a coordinate system with the H atoms on the x, y and z axes, the AO functions of the central atom and the symmetry-adapted linear combinations of ligand functions transform as s, aig p, tiu djey d dy, t2g dx2-y2 dz2, eg in the Oh point group). Now count the number of filled MOs and the number of S-H bonding interactions. 

Let us finally consider implications of these findings for reaction mechanisms in metalloproteins. Therefore, we must take into account that, much like with Sabatier s approach, considerations about thermodynamic stability, which might go as a static phenomenon if it were not for the fact that chemical equilibrium is nothing but the ratio of forward and reverse reaction rates, hence it also is about dynamics and might be compared to other reaction rates, this approach being encouraged by the well-known structure-reaction rate relationships for both (at least benzenoid aromatic) substrates and square-planar or octahedral coordination complexes ... 

The electron distribution in these complexes is not then fundamentally different from that encountered in low-spin d octahedral coordination complexes except that the degeneracy of the t2g nonbonding levels in these latter compounds are lifted in the cyclopentadienyl complexes 36),... 

The second set of hybrid orbitals we constrnct are the d sp hybrids these are six eqnivalent orbitals directed toward the vertices of an octahedron (Fig. 8.31a). They describe the structures and bonding in all of the octahedral coordination complexes discussed in Section 8.6, as well as that in SFg, which we show in Fig-nre 8.31b. 

We apply molecular orbital theory to octahedral coordination complexes just as we did for the simple metal carbonyl in Section 8.2. We begin by constructing the cr MOs from the valence d, s, and p orbitals of the central metal atom and the six ligand orbitals that point along the metal-ligand bond directions in an octahedral complex. In the case of the Cr complexes we use as examples, the relevant... 

Explain why octahedral coordination complexes with three and eight d electrons on the central metal atom are particularly stable. Under what circumstances would you expect complexes with five or six d electrons on the central metal atom to be particularly stable ... 

When heme or hematin are joined to the various proteins in hemoglobin, peroxidase, and catalase, they combine with a larger range of compounds to give stable octahedral coordination complexes than they do by themselves. To illustrate this the chief derivatives of heme and hematin are listed below before proceeding to describe the hemprotein complexes. 

With these analyses in hand, we have basically simplified an octahedral coordination complex into six bonded pairs (for the six classical M-L bonds) and three lone pairs (as in a d electron configuration from the transition metal center). 

The isomer shift values of both components are indicative of iron in the high-spin ferrous state (the ferric state has values between 0.3 and 0.6mms ). Typical values of 1.0-1.4mms have been observed for similar halide complexes. The tetrahedral iron(II) halide complexes generally have lower isomer shift values of around 1.01 mms, when compared to octahedrally coordinated complexes where the isomer shift is about 1.35 mms . One can therefore tentatively conclude that the Fe(II) ions at both sites are in tetrahedral coordination. The large quad-rupole splitting observed indicate a very low symmetry of the environments, but offers no further proof regarding the coordination. 

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