Octahedral complexes valence bond theory

Valence bond theory describes the bonding in complexes in terms of two-electron, coordinate covalent bonds resulting from the overlap of filled ligand orbitals with vacant metal hybrid orbitals that point in the direction of the ligands sp (linear), sp3 (tetrahedral), dsp2 (square planar), and d2sp3 or sp3d2 (octahedral). 

More important, the failure of many transition metal aqua ions to fit the correlations of Figure 8.4 highlights the influence of d electron configuration on the reactivity of metal aqua ions in substitution reactions. The importance of d electron configuration was first noted by Taube in 19521 and explained qualitatively in terms of valence bond theory. Taube, with his predilection for simple test tube demonstrations, distinguished labile metal complexes (ones which underwent substitution within the time of mixing) from inert ones, the latter being typically octahedral complexes... 

A more elaborate example than those shown above is the anionic compound SiFg2- (Figure 1.2), which adopts a classical octahedral shape that we will meet also in many metal complexes. Silicon lies below carbon in the Periodic Table, and there are some limited similarities in their chemistry. However, the simple valence bond theory and octet rule that... 

For a high-spin octahedral complex such as [FeFg], the five 3d electrons occupy the five 3d atomic orbitals (as in the free ion shown above) and the two d orbitals required for the sp d hybridization scheme must come from the Ad set. With the ligand electrons included, valence bond theory describes the bonding as follows leaving three empty Ad atomic orbitals (not shown) ... 

In the spirit of valence-bond theory, we must construct six hybrid orbitals that point towards the vertices of the octahedron. We must therefore combine just six of the nine valence orbitals of the metal it turns out that these are the s orbital, the three p orbitals, and two of the d orbitals (this hybridisation pattern will be written d sp, see Chapter 2, 2.1.2.3). Three d orbitals are therefore not involved in the hybridisation, and they stay unchanged (5-4). The link with MO theory is plain these are the three nonbonding orbitals of the octahedral tzg block. In a d complex, these three non-hybridized orbitals are doubly occupied. 

Pauling s valence bond theory is likewise of only limited value in its application to transition metal complexes. In the VBT, the ligand electrons are accommodated in hybrid orbitals localized on the central metal. The orbitals of interest for transition metals are the nd, n -f 1), n + l)p, and n + )d. An octahedral configuration arises from d sp hybridization of the metal orbitals, while dsp hybridization gives the square planar structure and sp hybridization results in a tetrahedral geometry. 

Pyrolysis of one mole of A gives two moles of benzene. From the result of the pyrolysis, benzene is reasonably considered to exist in the compound. However, the IR spectra of the compounds revealed no benzene absorption the compound is not a benzene occlusion complex. From this data, structure [5-37] was tentatively proposed. This structure can be reasonably explained by the valence bond theory, FAN, and molecular orbital theory to be an octahedral complex, as described in Chapter 3. Subsequently, structure [5-38] was assigned to the chloride salt B. From the limited data given above, it is not possible to assign the orientation of two benzene rings on the chromium atom. 

Valence bond theory, while useful for describing the geometries of the complex ions, caimot explain other properties such as color and magnetism. Crystal field theory (CFT), a bonding model for transition metal complexes, accounts for these properties. To illustrate the basic principles of CFT, we examine the central metal atom s d orbitals in an octahedral complex. 

Q7. According to valence bond theory, what is the hybridization of the central metal ion in an octahedral complex ion ... 

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... 

Whether a complex is high- or low-spin depends upon the energy separation of the t2g and eg levels. Nationally, in a fj-bonded octahedral complex, the 12 electrons supplied by the ligands are considered to occupy the aig, and eg orbitals. Occupancy of the and eg levels corresponds to the number of valence electrons of the metal ion, just as in crystal field theory. The molecular orbital model of bonding in octahedral complexes gives much the same results as crystal field theory. It is when we move to complexes with M—L TT-bonding that distinctions between the models emerge. 

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