What Are Transition Metals?

Transition elements refer to a series of metal elements in the d region of the periodic table, also known as transition metals. In general, this region includes elements of ten groups from 3 to 12, but does not include the internal transition elements of the f region. The term "transition element" was first coined by Mendeleev to refer to Group VIII elements. He believes that from the alkali metal to the manganese group is a "cycle", and the copper group to the halogen is another, then the elements sandwiched between the two cycles must have the nature of transition. Although the word is still used, it has lost its original meaning. One period of the transition metal element is called a transition system, and the elements of the fourth, fifth, and sixth periods belong to the first, second, and third transition systems, respectively.

Most transition metals exist in the earth's crust in the form of oxides or sulfides, and only a few simple substances such as gold and silver can exist stably.

The most typical transition metals are groups 4-10. The copper family can form complexes, but because the d ¹⁰ configuration is too stable, the highest price can only reach +3. The rare earth metals close to the main group have no variable valence state and cannot form complexes. Group 12 elements have only mercury in a variable valence state, and zinc is basically the main group metal. Due to the difference in properties, sometimes copper and zinc elements are not considered as transition metals. At this time, the concept of d-region elements is reduced to groups 3 to 10, and the copper-zinc groups are collectively called ds-region elements. ^[1]

Because the transition metal has an unfilled valence d orbit, based on the rule of eighteen electrons, its properties are significantly different from other elements. ^[1]

In homogeneous catalysis, we are generally not concerned about free ions of transition metals but complexes of transition metals. The complex we are referring to is a central metal ion connected to many attached ions or neutral molecules as a whole forming a distinguishable entity in solution. Synonymous coordination compounds or clusters of coordination atoms are often used instead of the term complex. The ions and molecules surrounding the central ion are usually called "ligands". Typical examples are cl-, Br-, CN-, H ₂ 0, NH ₃ , (C ₆ H ₃ ) ₃ P, C ₂ H ₄ neutral molecules using its lone pair of electrons or nitrate with which it forms a bond. The electron gives the metal center ion. There are also double-ended chelating ligands, like ethylene (di) diamine and acetylacetone anions, which themselves attach to metal ions from two positions, and are called double-chelating ligands. Correspondingly, diethylenetriamine can be used as a trichelate ligand, and triethylenetetramine can be used as a tetrachelate ligand. The number of ligands a metal ion can hold is called the coordination number. Although some metal ions have a characteristic number of coordination, the same central ion may also have different numbers of coordination for different ligands, the complexes [CoCl ₄ ] ^2- and [Co (H ₂ O) ₆ Cobalt in ²⁺ is a typical example.

The structures of a large number of transition metal complexes have been determined by X-ray crystal analysis. The result is the most common arrangement of six-ligands around a metal, more or less a twisted octahedral arrangement centered on the metal. The four ligands are generally metal-centered and are arranged on the tetrahedron or on the corners of the plane square; the five ligands can form a triangular bipyramid or a quadrangular pyramid. It has been proved from the spectrum, ESR spectrum and magnetic susceptibility data that the approximate symmetry of the complex is also maintained in the solution. The term "approximate symmetry" needs further explanation. We use octahedrons, tetrahedrons, and so on, regardless of whether all ligands are the same. Moreover, only atoms near the center of the metal are considered in the symmetric estimates. Therefore, [Ti (H ₂ O) ₆ ] ³⁺ , [Co (NH ₃ ) ₄ Br ₂ ] ³⁺ or [Co (en) ₃ ] ³⁺ are all considered octahedral complexes. (en: usually an abbreviation for double chelated coordination ethylenediamine).