Color of transition-metal complexes

Crystal field theory was developed, in part, to explain the colors of transition-metal complexes. It was not completely successful, however. Its failure to predict trends in the optical absorption of a series of related compounds stimulated the development of ligand field and molecular orbital theories and their application in coordination chemistry. The colors of coordination complexes are due to the excitation of the d electrons from filled to empty d orbitals d-d transitions). In octahedral complexes, the electrons are excited from occupied t2g levels to empty Cg levels. The crystal field splitting Ao is measured directly from the optical absorption spectrum of the complex. The wavelength of the strongest absorption is called Amax and it is related to Ao as follows. E = hv, so Ao = hv = Because en-... 

The colors of transition metal complexes are due to the emission of energy in the form of visible light when... 

Scientists have long recognized that many of the magnetic properties and colors of transition-metal complexes are related to the presence of d electrons in the metal cation. In this section we consider a model for bonding in transition-metal complexes, crystal-field theory, that accounts for many of the observed properties of these substances. Because the predictions of crystal-field theory are essentially the same as those obtained with more advanced molecular-orbital theories, crystal-field theory is an excellent place to start in considering the electronic structure of coordination compounds. 

The colors of transition metal complexes will give me a chance to connect matter and color or light. I really want to stress how and why we see the world through a narrow window that is like a tiny sUt in a huge cathedral. 

The colors that we have described arise from d-d transitions, in which an electron is excited from one d-orbital into another. In a charge-transfer transition an electron is excited from a ligand onto the metal atom or vice versa. Charge-transfer transitions are often very intense and are the most common cause of the familiar colors of d-metal complexes, such as the transition responsible for the deep purple of permanganate ions, Mn04 (Fig. 16.33). 

This type of chromogenic sensor utilizes the coordination chemistry of transition metal complexes, which have vacant binding sites to bind specific anions or have pendant arms containing anion receptor units. Transition metal complexes already have their own specific colors due to their different electronic structures. Coordinating directly to anions or binding of anions by the pendant arms results in perturbations of their electronic structures and causes color changes. 

The variety of colors among transition metal complexes has long fascinated the observer. For example, aqueous solutions of octahedral [CofHjOlf,] are pink but those of tetrahedral [CoC ] are blue. The green color of aqueous turns blue when ammonia is added to the solution to give. The reduction... 

Although Pmssian Blue, synthesized in 1704, was the first officially recognized metal coordination complex to be made, discovery of this group of transition metal complex ions is often credited to Taessert, who in 1798 prepared the first known cobalt ammonia salts. His work inspired a revolution in inorganic chemistry. At the turn of the nineteenth century, amidst the flourishing developments of organic chemistry, the striking colors... 

The frequency v), and therefore the wavelength and color, of the light absorbed are related to This, in turn, depends on the crystal field strength of the ligands. So the colors and visible absorption spectra of transition metal complexes, as well as their magnetic properties, provide information about the strengths of the ligand-metal interactions. 

Molecular UV-vis spectroscopy is prevalent in the more advanced chemistry curriculum for undergraduates. It appears in Organic Chemistry in the analysis of organic compounds, and it can also be applied to Physical (or Quantum) Chemistry courses in discussions of molecular orbitals, electronic transitions between these orbitals, and also transition selection rules and microstates. It is also relevant to Inorganic Chemistry, as it is investigated in terms of transition metal complex color, crystal field theory, and molecular orbital diagrams and electronic transitions for a variety of inorganic compounds. 

Visual observation of color changes accompanying oxygenation of transition metal complexes demonstrates that these processes are quite fast and their monitoring requires application of the stopped-flow technique. There have been kinetic studies aimed at elucidating the mechanism of oxygenation, primarily in the case of cobalt(II) complexes. The formation of p-peroxocobalt complexes can be described by the general mechanism ... 

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