Bonding theories ligand field

The compound [Co(NH3)6]Cl3 is yellow-orange, whereas [Co(H20)3F3] is blue. Explain why these materials are so different using the valence bond and ligand field theories. 

The first 12 electrons form six metal-ligand sigma bonds. The J-electrons of the metal enter the Lg- and Cg-orbitals in the same way they did in the crystal field theory. Ligand field theory, however, identifies the t2g-orbitals as nonbonding and the g-orbitals as antibonding. 

Recent progress in ligand field theory. C. K. Jorgensen, Struct. Bonding (Berlin), 1966,1, 3-31 (49). 

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. 

The effects of the bonding electrons upon the d electrons is addressed within the subjects we call crystal-field theory (CFT) or ligand-field theory (LFT). They are concerned with the J-electron properties that we observe in spectral and magnetic measurements. This subject will keep us busy for some while. We shall return to the effects of the d electrons on bonding much later, in Chapter 7. 

The successful rationalization of these transition-metal inverse spinel structures in terms of the relative LFSE s of tetrahedral and octahedral sites is another attractive vindication of ligand-field theory as applied to structure and thermodynamic properties. Once again, however, we must be very careful not to extrapolate this success. Thus, we have a clear prediction that LSFE contributions favour tetrahedral over octahedral coordination, except for d" with n = 0, 5 or 10. We do not expect to rationalize the relative paucity of tetrahedral nickel(ii) species relative to octahedral ones on this basis, however. Many factors contribute to this, the most obvious and important one being the greater stabilization engendered by the formation of six bonds in octahedral species relative to only four bonds in tetrahedral ones. Compared with that, the differences in LSFE s is small beer. Why , one asks, was our rationalization of spinel structures so successful when we neglected to include consideration of the bond count The answer is that cancellations within the extended lattice of the spinels tend to diminish the importance of this term. 

In all these discussions, we separate, as best we might, the effects of the d electrons upon the bonding electrons from the effects of the bonding electrons upon the d electrons. The latter takes us into crystal- and ligand-field theories, the former into the steric roles of d electrons and the geometries of transition-metal complexes. Both sides of the coin are relevant in the energetics of transition-metal chemistry, as is described in later chapters. 

Jorgensen CK (1978) Predictable Quarkonium Chemistry. 34 19-38 Jorgensen CK (1966) Recent Progress in Ligand Field Theory. 1 3-31 Jorgensen CK (1967) Relationship Between Softness, Covalent Bonding, lonicity and Electric Polarizability. 3 106-115... 

The spectrochemical series was established from experimental measurements. The ranking of ligands cannot be fully rationalized using crystal field theory, and more advanced bonding theories are beyond the scope of general chemistry. 

The terms crystal field theory and ligand field theory are not used in a uniform way. As only interactions between adjacent atoms are being considered, without referring to crystal influences, the term crystal field theory does not seem adequate. Some authors consider certain electronic interactions (like n bonds) as part of ligand field theory, although they originate from MO theory. 

The geometry and bond distances of the [Ni(H20)6]2+ cation were thoroughly discussed.709 As expected from ligand field theories, the metal OH2 bond reaches its minimum for Ni11 ions, which... 

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