Ligand field-molecular orbital

Crystal field and ligand field molecular orbitals... 

Instead of the usual valence bonds, the metal atoms in chlorophylls and cytochromes should be presented in terms of the six coordinate bonds described by ligand-field (molecular-orbital) theory. Fe has one coordinate bond to each of the four N s in the porphyrin ring (the two solid lines and the two broken lines emanating from Fe in Fig. 5-16 also see Fig. 5-2), one to... 

Figure 7.41 Perturbation of crystal field molecular orbitals (MOs) by ligand MOs...

One always begins with the monomer. What are its frontier levels The classical crystal field or molecular orbital picture of a square planar complex figure 2) leads to a 4 below 1 splitting of the d block. For 16 electrons we have xz, yz, and occupied and x - y empty. Competing with the ligand-field-destabilized orbital for being the lowest unoccupied molecular orbital (LUMP) of the molecule is the metal z. These two orbitals can be manipulated in understandable ways n-acceptors push z down, R-donors push it up. Better o-donors push - y up. 

For Iran sition metals th c splittin g of th c d orbitals in a ligand field is most readily done using HHT. In all other sem i-ctn pirical meth -ods, the orbital energies depend on the electron occupation. HyperCh em s m oiccii lar orbital calcii latiori s give orbital cri ergy spacings that differ from simple crystal field theory prediction s. The total molecular wavcfunction is an antisymmetrized product of the occupied molecular orbitals. The virtual set of orbitals arc the residue of SCT calculations, in that they are deemed least suitable to describe the molecular wavefunction, ... 

One way that molecular mechanics methods have been adapted to transition metal applications is by including one orbital-based term in the force field to describe the metal center. These terms are typically based on semiempirical methods or even some variation of ligand field theory. 

Color from Transition-Metal Compounds and Impurities. The energy levels of the excited states of the unpaked electrons of transition-metal ions in crystals are controlled by the field of the surrounding cations or cationic groups. Erom a purely ionic point of view, this is explained by the electrostatic interactions of crystal field theory ligand field theory is a more advanced approach also incorporating molecular orbital concepts. 

Unlike reactive diatomic chalcogen-nitrogen species NE (E = S, Se) (Section 5.2.1), the prototypical chalcogenonitrosyls HNE (E = S, Se) have not been characterized spectroscopically, although HNS has been trapped as a bridging ligand in the complex (HNS)Fc2(CO)6 (Section 7.4). Ab initio molecular orbital calculations at the self-consistent field level, with inclusion of electron correlation, reveal that HNS is ca. 23 kcal mof more stable than the isomer NSH. There is no low-lying barrier that would allow thermal isomerization of HNS to occur in preference to dissociation into H -1- NS. The most common form of HNS is the cyclic tetramer (HNS)4 (Section 6.2.1). 

Frontier Molecular Orbital theory is closely related to various schemes of qualitative orbital theory where interactions between fragment MOs are considered. Ligand field theory, as commonly used in systems involving coordination to metal atoms, can be considered as a special case where only the d-orbitals on the metal and selected orbitals of the ligands are considered. 

There are two major theories of bonding in d-metal complexes. Crystal field theory was first devised to explain the colors of solids, particularly ruby, which owes its color to Cr3+ ions, and then adapted to individual complexes. Crystal field theory is simple to apply and enables us to make useful predictions with very little labor. However, it does not account for all the properties of complexes. A more sophisticated approach, ligand field theory (Section 16.12), is based on molecular orbital theory. 

FIGURE 16.36 I1ie tear-shaped objects are representations of the six ligand atomic orbitals that are used to build the molecular orbitals of an octahedral complex in ligand field theory. They might represent s- or p-orbitals on the ligands or hybrids of the two. 

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