Transition element
(Under construction)
A transition element is a chemical element whose atomic electron configuration of the ground (lowest energy) state has an incompletely filled d sub-shell. Table I shows the part of the Periodic Table that contains the first three series of transition elements. The symbol "d" stands for an atomic orbital with angular momentum quantum number ℓ = 2. The electron configuration of transition element atoms contains (nd)k, 1 ≤ k ≤ 9, where n is a principal quantum number, n = 3, 4, 5, see Table II.[1] The incomplete electronic d subshell gives rise to some characteristic magnetic properties (paramagnetism and ferromagnetism) and crystals and solutions of transition metal complexes that are brightly colored.
The elements in the fourth transition series (period 7 of the periodic table), are formally transition elements. They are man-made [except for Actinium (Z = 87)] and short-lived, not much is known about their compounds and accordingly they are not discussed in this article.
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Exceptions to the definition
Although the atoms copper (Cu), silver (Ag), and gold (Au) have a filled d sub-shell in their lowest energy state—as Table II shows they have the configuration (nd)10(n+1)s1, with n = 3, 4, and 5, respectively—after ionization (loss of two or more electrons) their cations have an incomplete d sub-shell. Since these cations appear in many complexes, copper, silver, and gold are usually seen as transition elements.
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In the past, the group 12 elements zinc (Zn), cadmium (Cd), and mercury (Hg), that are included in the "d-block" of the periodic table, have been considered as transition elements, but they are nowadays rarely considered as such, because their compounds lack some of the properties that are characteristic for transition elements.
Because scandium (Sc), yttrium (Y), and lanthanum (La) actually do not form compounds analogous to those of the other transition elements and because their chemistry is quite homologous to that of the lanthanoids (previously known as lanthanides), they are often excluded from the group of transition elements. A strict application of the definition would describe also lutetium (Lu) as a transition element as it has a singly occupied 5d orbital in its ground state, but according to IUPAC[2] it is a lanthanoid. Lutetium appears most commonly as a positive ion without d-electrons in the valence shell and without the characteristic properties of a transition element.
Properties
The most striking similarities shared by the transition elements is that they are all metals—which is why they are often called transition metals—and that most of them are hard, strong, and shiny. They have high melting and boiling points, and, being metals, are good conductors of heat and electricity. Many of the elements are technologically important: iron, nickel, cobalt, palladium, platinum, and others are used in heterogeneous catalysis. Much of the current research on the chemistry of transition element complexes is instigated by their industrial importance as catalysts.
The transition elements form many useful alloys, among themselves and with other metallic elements. Most of the transition elements can be dissolved in water and other polar solvents and form complexes in solution, although the "noble" metals platinum, silver, and gold are difficult to dissolve. For obvious reasons the elements copper, silver, and gold are referred to as coinage metals[3]. Note that copper belongs to the class of coinage metals, but is not a noble metal.
The outer s-electrons of the transition metals are easily lost to the bonding partners (the ligands) of the metal. Also one or more d-electrons of the metal are usually lost to its ligands. In other words, most transition element compounds show ionic chemical bonds. Common ligands are: oxide (O2−), halides (F−, Cl−, Br−, I−), hydrates (H2O, OH−), cyanide (CN−), and sulfate (SO42−).
The formal charge of the ionically bound element is known as its oxidation number, or oxidation state. Table III shows the most common oxidation states of the first transition series.[4] Note in this table that the elements exhibit variable oxidation states. The chemistry of the transition series is mainly that of the ions in one of their several oxidation states, and not that of the elemental form itself.
For example, the transition element chromium (Cr) in the ionic water complex chromium hexahydrate, Cr(H2O)63+, is trivalent and is denoted by the oxidation state Cr(III). (This is because water has formal oxidation number zero.) The very commonly occurring Cr(III) cation has electronic structure [Ar](3d)3; it appears, for instance, also in the crystal KCr(SO4)2⋅(H2O)12. The chromium in Cr(CN)64− is divalent, denoted by Cr(II); it has electronic structure [Ar](3d)4. Chromate [CrO4]2− contains Cr(VI), which is isoelectronic with argon. An example of monovalent Cr(I) is the bright-green compound K3[Cr(CN)5NO]⋅H2O, which contains K+, Cr+, NO+, and CN−.
This widely applied classification of transition elements by their oxidation states is not supported by quantum mechanical calculations. Although many theoretical discussions of transition metal complexes assume (often implicitly) ionic bonds, quantum mechanical calculations show that most of the bonds have a good deal of covalent character. Calculations bear out that transfer of more than one full electron to the ligands occurs rarely, let alone six electrons as in Cr(VI). However, in qualitative and semi-quantitative studies, the assumption of ionic bonds with a transition metal cation, provides much insight and yields a systematization of the properties of the transition metal complexes. The covalent character of the bonds is accounted for by the values of the semi-empirical parameters that enter such studies.
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Quantum mechanical description
In a landmark paper[5] Hans Bethe introduced in 1929 a model known as the crystal field model. This model successfully accounts for some magnetic properties and colors of transition metal complexes. It describes a transition metal ion as being in an electrostatic field created by the surrounding charged or dipolar ligands. The electrostatic field has point group symmetry lower than the full rotation symmetry SO(3) of the free atom. Because of the symmetry lowering the (2S+1)(2L+1) different functions that are degenerate in the free atom (a so-called "term" represented by a term symbol) will split up, that is, will obtain different energies. Because the crystal field is non-magnetic, the spin S will be conserved. However, the atomic orbital momentum L will be quenched, i.e., inside the crystal field angular momentum is not conserved, L is no longer a good quantum number.
The crystal field model does not attempt to describe why the configuration of atoms and surrounding ligands is stable. Later the crystal field model was extended by the admixture of ligand orbitals into the atomic orbitals of the central ion. This extended model is known as the ligand field model. The ligand field model aims at predicting correct binding energies as well.
Reference
- ↑ NIST Ground levels and ionization energies for the neutral atoms Retrieved October 1, 2009
- ↑ IUPAC Provisional Recommendations for the Nomenclature of Inorganic Chemistry (online draft of an updated version of the "Red Book" IR 3-6), 2004. Retrieved on 17/9/2009.
- ↑ B. H. Lipshutz and Y. Yamamoto. Introduction, Special issue of Chemical Reviews on Coinage Metals in Organic Synthesis, 2008, vol. 108, pp. 2793–2795 DOI
- ↑ B. Hathaway, An alternative approach to the teaching of systematic transition metal chemistry, Journal of Chemical Education, vol. 56, pp. 390–392 (1979)
- ↑ H. Bethe, Termaufspaltung in Kristallen [Term splitting in cystals], Annalen der Physik, Fünfte Folge, vol. 3, pp. 133–206 (1929) Online. English translation: Selected Works of Hans Bethe, World Scientific, Singapore (1997) Google Books (online)