Ions, ligand field

CrOl ) reasonably stable the Cr(I) oxidation stale is practically unknown. For both Cu2 and Cr3 (as well as many other transition metal ions) ligand field effects in their complexes (see Chapter II) are much more important in determining stable oxidation states than are electron configurations. 

The selectivity of peptide motifs for certain metals comes from the coordinating contribution from amino acid side chains, the common coordination number of the metal, hardness/softness of the metal ion, ligand field stabilisation effects and the hardness/softness of any coordinating side chains of the amino acid sequence. An example of the influence of side chains and the importance of the position of the side chain comes from the tripeptides Gly-Gly-His, also known as copper binding peptide. The side chain imidazole ring of the His residue has a very efficient nitrogen donor (the imidazole N), which can form a tetradentate chelate ring for coordination as in Scheme 10.3. 

Unlike the case with the d orbitals in the transition elements, spectra and magnetism associated with 4/ orbitals in Ln3+ compounds are very similar to those found in free gas-phase ions. Ligand field and chemical bonding effects associated with incomplete 4/orbitals are very small and hardly detectable in chemical trends. 

Transition metals readily form complexes, such as [Fe(CN)6], the ferrocyanide ion, Ni(CO)4, nickel tetracarbonyl, and [CuC ], the copper tetrachloride ion. MO theory applied to such species has tended to be developed independently. It is for this reason that the terms crystal field theory and ligand field theory have arisen which tend to disguise the fact that they are both aspects of MO theory. 

When Cr202 is introduced as an impurity into the a-Al202 lattice, as occurs in the semiprecious mineral mby, the color is red rather than the normal green. This color anomaly is the result of ligand field splitting of the Cr(III) ion (51,52). Chromium (ITT) also colors other minerals (53). 

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. 

Color from Color Centers. This mechanism is best approached from band theory, although ligand field theory can also be used. Consider a vacancy, for example a missing CF ion in a KCl crystal produced by irradiation, designated an F-center. An electron can become trapped at the vacancy and this forms a trapped energy level system inside the band gap just as in Figure 18. The electron can produce color by being excited into an absorption band such as the E transition, which is 2.2 eV in KCl and leads to a violet color. In the alkaU haUdes E, = 0.257/where E is in and dis the... 

Simple ligand-field arguments, which will be elaborated when M ions of the Ni, Pd, Pt triad are discussed on p. 1157, indicate that the configuration favours a 4-coordinate, square-planar stereochemistry. In the present group, however, the configuration is associated with a lower oxidation state and the requirements of the 18-electron rule, which favour 5-coordination, arc also to be considered. The upshot is that most Co complexes are 5-coordinate, like [Co(CNR)5j, and square-planar Co is apparently unknown. On the other hand, complexes of Rh and Iri are predominantly square planar, although 5-coordination docs also occur. 

A considerable number of rhodium(III) complexes exist. Their stability and inertness are as expected for a low-spin d6 ion any substitution leads to a considerable loss of ligand-field stabilization. 

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. 

Is there a correlation between the ligand field strength of the halide ions F, Cl, Br, and 1 and the electronegativity of the halogen If so, can this correlation be explained by ligand field theory Justify your answer. 

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