Octahedral transition metal complexes

Polyatomic molecules cover such a wide range of different types that it is not possible here to discuss the MOs and electron configurations of more than a very few. The molecules that we shall discuss are those of the general type AFI2, where A is a first-row element, formaldehyde (FI2CO), benzene and some regular octahedral transition metal complexes. 

All the forbidden electronic transitions of regular octahedral transition metal complexes, mentioned in Section 7.3.1.4, are induced by non-totally symmetric vibrations. 

The metal complexes discussed thus far bear little resemblance to the vast majority of common transition-metal complexes. Transition-metal chemistry is dominated by octahedral, square-planar, and tetrahedral coordination geometries, mixed ligand sets, and adherence to the 18-electron rule. The following three sections introduce donor-acceptor interactions that, although not unique to bonding in the d block, make the chemistry of the transition metals so distinctive. 

A persistent feature of qualitative models of transition-metal bonding is the supposed importance of p orbitals in the skeletal hybridization.76 Pauling originally envisioned dsp2 hybrids for square-planar or d2sp3 hybrids for octahedral bonding, both of 50% p character. Moreover, the 18-electron rule for transition-metal complexes seems to require participation of nine metal orbitals, presumably the five d, one s, and three p orbitals of the outermost [( — l)d]5[ s]1[ p]3 quantum shell. 

We first examine the relationships between electron structure and the emission and absorption spectroscopy of metal complexes. Transition metal complexes are characterized by partially filled d orbitals. To a large measure the ordering and occupancy of these orbitals determines emissive properties. Figure 4.2 shows an orbital and state diagram for a representative octahedral MX6 d6 metal complex where M is the metal and X is a ligand that coordinates or binds at one site. The octahedral crystal field of the ligands splits the initially degenerate five atomic d-orbitals by an amount... 

For a recent review on trans-effects in octahedral transition metal complexes, see ... 

The reliable prediction of redox potentials as a function of composition is useful in the synthetic design and application of technetium and other transition metal complexes. A parametric procedure for doing so on the basis of ligand additivity principles has been developed by Lever [28]. Lu etal. [29] used this scheme to correlateTc / ",Tc "/ , andTc hi potentials with the composition of octahedral technetium complexes containing halide, nitrogen, and phosphorus donor ligands. The results are illustrated in Fig. 2 [29], where the observed potentials are plotted according to... 

In the example in Figure 2.24, a clay (a layered double hydroxide [LDH]) was intercalated with a transition metal complex (NH4)2MnBr4. The EXAFS data in Figure 2.24(a) shows the Mn K-edge EXAFS of the pure complex, and we see one coordination sphere of four Br atoms at a distance of 2.49 A, corresponding well to the tetrahedral coordination found in the X-ray crystal structure. However, after intercalation, the complex reacts with the layers in the clay, and the coordination changes to distorted octahedral where Mn is now surrounded by four 0 atoms at a distance of 1.92 A and two Br atoms at a distance of 2.25 A. 

A further simplification of Eq. (24) can be made by considering the symmetry of the crystal field. To see how this happens, we shall consider Vc for octahedral symmetry. This field would be given by six equal charges placed at R along the x, y, and z axes. Since the coordination number six is very common in transition-metal complexes, this type of crystal field occurs often. 

The structure of bis(salicylaldoxime)beryllium has been proposed as being trans octahedral by comparison of the space group and unit cell volume with those of related transition metal complexes it is presumably a dihydrate if it is indeed octahedral in geometry.297 Stability constants have been reported for a range of beryllium /3-ketoamines derived from both salicylaldehyde and acetylacetone precursors. They show strong complexes which are stable to hydrolysis under the conditions used.298,299... 

There is growing evidence that substitution reactions in Co(lll) complexes may not be typical of octahedral transition metal complexes. Early studies of substitution reactions for Cr(III) complexes revealed a rather strong dependence of reaction rate... 

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