Transition metal complexes coordination color

Transition Metal Complexes Related to the Simon test is a family of color-producing reactions based on transition metal complexes (coordination complexes) and tightly associated ion pairs. Coordination complexes arise from a Lewis acid-base interaction between a metal cation, such as cobalt, and an atom with unshared electrons, such as water or, in the case of drugs, basic nitrogen found in alkaloids and amines. Metals that have been used in these reagents include copper, vanadium, bismuth, and cobalt Cobalt, as part of two common reagents (cobalt thiocyanate and Dilli-Koppanyi) is perhaps the most versatile. Cobalt has an electron structure of 3d 4s, while ttie cation has a 3d (2 ) or 3d (3 ") structure. 

Transition metal complexes, catalysts based on, 20 193 Transition-metal compounds cause of color, 7 326t, 328-331 Transition-metal coordination compounds, 7 582-584... 

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. 

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-... 

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... 

We discuss color and magnetism in coordination compounds, emphasizing the visible portion of the electromagnetic spectrum and the notion of complementary colors. We then see that many transition-metal complexes are paramagnetic because they contain unpaired electrons. 

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. 

Although CFT can explain the magnetism of coordination compounds and predict that they are colored, it is not a quantitative theory. It cannot predict, for instance, the exact color of a transition metal complex because there is no way of rationalizing the relative magnitude of a priori. Even for a metal ion having... 

A satisfactory theory of bonding in coordination compounds must account for properties such as color and magnetism, as well as stereochemistry and bond strength. No single theory as yet does aU this for us. Rather, several different approaches have been applied to transition metal complexes. We will consider only one of them here—crystal field theory— because it accounts for both the color and magnetic properties of many coordination compounds. 

In Chapters 2 and 3, we considered the history, nomenclature, and structures of coordination compounds. In these earlier discussions, we introduced the metal-ligand (M-L) coordinate-covalent bond in which the ligand shares a pair of electrons with the metal atom or ion. Now we are in a position to consider the nature of the M-L bond in greater detail. Is it primarily an ionic interaction between ligand electrons and a positively charged metal cation Or should the M-L bond be more properly described as predominantly covalent in character Whatever the character of the bond, the description of M-L interactions must account for (1) the stability of transition metal complexes, (2) their electronic and magnetic characteristics, and (3) the variety of striking colors displayed by these compounds. 

Color is a spectacular property of coordination complexes. For example, the hexaaqua cations of 3 transition metals display colors ranging from orange through violet (see photo at right). The origin of these colors lies in the d orbital energy differences and can be understood using crystal field theory. 

Quinoxaline-2,3-dithione, as reported earlier,1 is useful for its coordinating properties with transition metals. The metal complexes of the dithione with Cu, Ni, Zn, Pd, and Pt have been prepared,171 and the spectral properties of the Ni and Pd complexes examined.171,172 UV data indicate that quinoxaline-2,3-dithione (153) is present as such, rather than as 2,3-dimercaptoquinoxaline the highly colored nature of its complexes is attributed to charge transfer.171... 

The weak electron transitions responsible for the color of coordination complexes are correlated with the t2g to eg transition of energy A. The origin of the strong transitions responsible for the deep colors of some complexes of metals in high oxidation states, like the permanganate and the chromate ions, has a different explanation not covered here. 

Tphe bright colors of the coordination complexes of transition metal elements, including the platinum group metals, were of great assistance to pioneer workers with these materials. Thus, chemical changes could be followed visually it was frequently very easy from their colors to demonstrate the existence of isomers upon which Alfred Werner was able to base his monumental theory of coordination. Such early studies were limited to a simple qualitative visual evaluation of the color. 

Heteroatom-stabilized carbene complexes of type 1, first discovered by E.O. Fischer in 1964 [1], nowadays belong to the best investigated classes of transition metal compounds. Such complexes are coordinatively saturated, intensely colored solids = 350-400 nm), which exhibit a sufficient stability for normal preparative use. Especially chromium carbene complexes (2) enjoy increasing importance in organic synthesis, and it must be added that thermal reactions such as benzannulations (i.e. the Ddtz reaction), cyclopropanations and additions to a,j8-unsatu-rated complexes clearly predominate [2J. 

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