Tetrahedral complexes, molecular

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. 

The formation of dimeric products is unique for the case of boron, because analogous complexes with other elements are all monomeric [95]. This can be attributed to the small covalent radius of the boron atom and its tetrahedral geometry in four-coordinate boron complexes. Molecular modeling shows that bipyramidal-trigonal and octahedral coordination geometries are more favorable for the formation of monomeric complexes with these ligands. 

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

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 ... 

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). 

To enlarge the size of the cavity of the tetrahedral complexes mentioned so far, 4,4 -phenylene and 4,4 -biphenylene spacers were introduced. For instance, when tetramethyl terephthaloyldimalonate was deprotonated with sodium hydride and the doubly negatively charged ditopic, tetradentate ligand (Lpllcl1)2 treated with iron (III) chloride, complex Fe4(Lphc,1)6] with an empty cavity was isolated (not shown). In contrast to racemic (A,A,A,A)/(A,A,A,A)-(16-19), complex (A,A,A,A)- Fe4(Lphen)6] is achiral (meso-form) and has S4-molecular symmetry in the crystal [82-85],... 

The question arose as to which complexes formed octahedral and which bicapped-tetrahedral molecular structures. In the bicapped-tetrahedral complexes 39-42, the number of electronegative ligands attached to silicon is smaller than in the octahedral complexes (30-38). A maximum of one Si-C bond is allowed in the octahedral complexes. When the number is greater than one, bicapped tetrahedral geometries are obtained in the solid-state. 

The polymerization of propylene using complex 14 activated by MAO (Al Zr ratio=500, solvent toluene, 25 °C) yielded 80 g polymer-mol Zrl-hrl with a molecular weight Mw= 115,000 and polydispersity=2.4 [119]. The reaction was carried out in liquid propylene to avoid, as much as possible, the epimerization of the last inserted monomer unit and to allow rational design of the elastomeric polymer. The formation of elastomeric polypropylene is consistent with the proposed equilibrium between ds-octahedral cationic complexes with C2 symmetry inducing the formation of the isotactic domain, and tetrahedral complexes with C2v symmetry responsible for the formation of the atactic domain (Scheme 7). The narrow polydispersity of the polypropylene obtained supports the polymerization mechanism in which the single-site catalyst is responsible for the formation of the elastomeric polymer. 

The d-d bands are usually relatively low in intensity compared to CT bands contrast the palish hues of familiar salts of Cr, Mn, Fe, Co, Ni, and Cu with the intense purple of Mn04. This can be explained within the CF model. The probability of a transition is governed by selection rules see Group Theory). In atomic spectra, transitions between states having the same / quantum number are forbidden if this rule were strictly obeyed, d-d transitions should not be observable. Moreover, if there is a center of symmetry, as in an octahedral or square coplanar complex, d-d transitions are forbidden although they can be observed, albeit relatively weakly. Molecular vibration can disturb the center of synunetry, and Vibronic Coupling lends intensity to d d absorption. A tetrahedral complex has no center of synunetry and the... 

All examples shown in this chapter were for high-symmetric, either octahedral or tetrahedral, complexes. One may therefore wonder if the present considerations still remain valid in cases without symmetry. For instance, will the metal d contributions still be confined to a limited set of molecular orbitals (i.e., the basic ten) in cases where such limitations are not enforced by symmetry That this is indeed the case was already illustrated by the tetrahedral examples (Table 4), where the M M orbitals can in principle be delocaUzed over two bonding ti shells, but in practice significantly contribute only to one of these shells. 

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