Tetrahedral complexes molecular orbitals

A mistake often made by those new to the subject is to say that The Laporte rule is irrelevant for tetrahedral complexes (say) because they lack a centre of symmetry and so the concept of parity is without meaning . This is incorrect because the light operates not upon the nuclear coordninates but upon the electron coordinates which, for pure d ox p wavefunctions, for example, have well-defined parity. The lack of a molecular inversion centre allows the mixing together of pure d and p ox f) orbitals the result is the mixed parity of the orbitals and consequent non-zero transition moments. Furthermore, had the original statement been correct, we would have expected intensities of tetrahedral d-d transitions to be fully allowed, which they are not. 

A quantitative consideration on the origin of the EFG should be based on reliable results from molecular orbital or DPT calculations, as pointed out in detail in Chap. 5. For a qualitative discussion, however, it will suffice to use the easy-to-handle one-electron approximation of the crystal field model. In this framework, it is easy to realize that in nickel(II) complexes of Oh and symmetry and in tetragonally distorted octahedral nickel(II) complexes, no valence electron contribution to the EFG should be expected (cf. Fig. 7.7 and Table 4.2). A temperature-dependent valence electron contribution is to be expected in distorted tetrahedral nickel(n) complexes for tetragonal distortion, e.g., Fzz = (4/7)e(r )3 for com-... 

FIGURE 17.18 A qualitative molecular orbital diagram for a tetrahedral complex. 

The tetrahedral structure of these surface alkyl complexes on MCM-41(5oo) has been highlighted by XANES a sharp, intense pre-edge peak at 4969.6 0.3 eV is characteristic of an electronic transition of titanium, from the Is energetic level to molecular orbitals mixing 3d and 4p of Ti with the orbitals of the Ugands, in a complex where titanium is in a tetrahedral symmetry [28-31]. The same argument can be applied for species obtained from alcoholysis of 2a and 2b, especially using tert-butanol. 

Molecule 14 is a tetradentate ligand consisting of two bpy moieties linked in 3- and -positions by a CH2OCH2 spacer the plain bpy ligand forms complexes of formula [M bpylJ+ of tetrahedral geometry with d10 metal ions, such as Cu1 and Ag1, whose extra stability results from the donation of electron density from filled dn orbitals of the metal to empty molecular orbitals of bpy molecules. 

The permanganate ion, MnO, meets the criteria set forth in the preceding paragraph Manganese is in a formal oxidation state of + 7 and combined with four oxide ions. The molecular orbital diagram for tetrahedral complexes in Fig. 11.52 allows us to identify possible LMCT transitions. In any tetrahedral complex, the four... 

Tetrahedral complexes arc favored by steric requirements, either simple electrostatic repulsions of charged ligands or van dcr Wauls repulsions of large ones. A valence bond (VB) point oT view ascribes tetrahedral structures to p% hybridization From a crystal field (CF) or molecular orbital (MO) viewpoint we have seer that, in general, tetrahedral structures are not stabilized by large LFSE. Tetrahedral complexes are thus favored by large ligands like Cl-. Br. and 1 and small metal ions of three types ... 

Fig. 11.52 Molecular orbital diagram for a tetrahedral ML4 complex, showing possible Kgand-to-metal charge transfer (LMCT) transitions...

In symmetries lower than cubic the (/-orbitals mix with the donor atom s—p hybrid orbitals to varying extents in molecular orbitals of appropriate symmetry. However, the mixing is believed to be small and the ligand field treatment of the problem proceeds upon the basis that the effective d-orbitals still follow the symmetry requirements as (/-orbitals should. There will be separations between the MOs which can be reproduced using the formal parameters appropriate to free-ion d-orbitals. That is, the separations may be parameterized using the crystal field scheme. Of course, the values that appear for the parameters may be quite different to those expected for a free ion (/-orbital set. Nevertheless, the formalism of the CFT approach can be used. For example, for axially distorted octahedral or tetrahedral complexes we expect to be able to parameterize the energies of the MOs which house the (/-orbitals using the parameter set Dq, Ds and Dt as set out in Section 6.2.1.4 or perhaps one of the schemes defined in equations (11) and (12). 

The transition metal ions possess a very stable set of d orbitals, and it is likely that d orbitals are involved in bonding in all transition metal complexes, regardless of structure. The common structures that use d valence orbitals for forming a bonding molecular orbitals are square-planar, tetrahedral, and octahedral. Examples of these structures are given in Figure 8-1. 

Fig. 3. The relative position of molecular orbital energies in regular octahedral MX, and tetrahedral MX, complexes. The arrangement is known from the M.O. interpretation of electron transfer spectra. The relative distances to the two highest empty sets of orbitals is underestimated, it may be much larger.

The molecular orbital theory as applied to octahedral and tetrahedral metal complexes. 

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