Transition metal

The transition metals are those chemical elements that are located in the central part of the periodic system, in the constant D block, whose main characteristic is the inclusion in its electronic configuration of the orbital. d, partially filled with electrons. This definition can be expanded considering as transition elements those that have electrons housed in the d orbital, this would include zinc, cadmium, and mercury. The IUPAC defines a transition metal as "an element whose atom has an incomplete d subshell (energy subshell) or which can give rise to cations".

They are transition metals, as they have a d¹⁰ configuration. Only a few transient species of these elements are formed, giving rise to ions with a partially complete d subshell. For example mercury(I) only occurs as Hg₂²⁺, which does not form an isolated ion with a partially filled subshell, so the three elements are inconsistent. with the above definition. These form ions with oxidation state 2+, but retain the 4d¹⁰ configuration. Element 112 could also be excluded although its oxidation properties are not observed due to its radioactive nature. This definition corresponds to groups 3 to 12 of the periodic table.

By the broadest definition, the transition metals are the forty chemical elements, from 21 to 30, from 39 to 48, from 71 to 80, and from 103 to 112. The name "transition" It comes from a characteristic that these elements have of being able to be stable by themselves without the need for a reaction with another element. When its last valence shell lacks electrons to be complete, it extracts them from inner shells. With that it is stable, but it would be missing electrons in the shell where it extracted them, so it completes them with other electrons from another shell. And so on; this phenomenon is called "Electronic Transition". This also has to do with the fact that these elements are so stable and difficult to make react with others. The broadest definition is the one that has traditionally been used. However many interesting properties of transition elements as a group result from their partially completed d subshell. Periodic trends in the d block are less predominant than in the rest of the periodic table. Through this the valence does not change because the electrons added to the atom go to internal layers.

Definitions

• The definition of IUPAC defines a transition metal as "an element whose atom has a partially filled sublay d or which can result in cationes with a sublay d incomplete".
• Many scientists describe a "transition detail" as any element in the regular table d block that includes groups 3 to 12 in the periodic table. In actual practice, the series of actinides and lanthanides of the f-block are also considered transitional metals and are called "internal transition meths".
• Cotton and Wilkinson extend the short definition of IUPAC (see above) by specifying which elements are included. In addition to the elements of groups 4 to 11, they add Scandio and Itrio in group 3, which have a sublay d partially filled in metallic status. Lantano and actinio, which consider elements of group 3, are nevertheless classified as lanthanides and actinides respectively.
• The English chemist Charles Rugeley Bury (1890–1968) used for the first time the word transition in this context in 1921, when he referred to a series of transitions of elements during the change of an internal layer of electrons (e.g., n = 3 in the fourth row of electrons). The periodic table of a stable group of 8 to one of 18, or 18 to 32. These elements are now known as block d.

Elements

Elements in groups 4 to 11 are generally recognized as transition metals, justified by their typical chemistry, that is, a wide range of complex ions in various oxidation states, colored complexes, and catalytic properties as an element or as ions (or both). Sc and Y in group 3 are also generally recognized as transition metals. However, the elements La–Lu and Ac–Lr and group 12 attract different definitions from different authors.

Many chemistry textbooks and printed periodic tables classify La and Ac as group 3 elements and transition metals, since their atomic ground state configurations are s²d ¹ as Sc and Y. Elements Ce–Lu are considered as the series " lanthanide " (or "lanthanoid" according to IUPAC) and Th–Lr as the series " actinide ". The two series together are classified as f-block elements or (in older sources) as "internal transition elements". However, this results in a division of the d-block into two rather unequal portions.

Some inorganic chemistry textbooks include "La" with the lanthanides and Ac with the actinides. This classification is based on similarities in chemical behavior (although this similarity mostly only exists between the lanthanides) and defines 15 elements in each of the two series, although they correspond to the filling of an f subshell, which can only hold 14 electrons.

A third classification defines the elements of the f block as La–Yb and Ac–No, while placing Lu and Lr in group 3. This is based on the Aufbau principle (or Madelung's rule) for filling subshells of electrons, in which 4f is filled before 5d (and 5f before 6d), so that the subshell f is actually filled in Yb (and No), while Lu has a [ ]s²f ^{configuration 14}d¹. (Lr is an exception where the d electron is replaced by a p electron, but the energy difference is small enough that in a chemical setting it often shows d occupancy anyway.) La and Ac are, from this point of view, simply considered exceptions to the Aufbau principle with electron configuration [ ]s²f¹⁴d¹ as predicted by the Aufbau principle). The excited states of the free atom and ion can become the ground state in chemical environments, justifying this interpretation; La and Ac have empty lower f subshells that are filled in Lu and Lr, so excitation of f orbitals is possible in La and Ac, but not in Lu or Lr. This justifies the idea that La and Ac simply have irregular configurations (similar to Th as s 2 d 2), and that they are the true beginning of the f-block.

As the third form is the only form that allows simultaneously (1) preservation of the sequence of increasing atomic numbers, (2) an f block 14 elements wide, and (3) avoidance of splitting in the d block, has been suggested by a 2021 IUPAC draft report as the preferred form. Soviet physicists Lev Landau and Evgeny Lifshitz first suggested in 1948 this modification, which treats Lu as a transition element rather than a transition element. of internal transition. After this, it was suggested by many other physicists and chemists, and was generally the classification adopted by those who considered the subject, but textbooks were generally slow to adopt it.

Zinc, cadmium, and mercury are sometimes excluded from the transition metals, since they have the electron configuration [ ]d¹⁰s² , no incomplete d shell. In the +2 oxidation state, ions have the electron configuration [ ]…d¹⁰. Although these elements can exist in other oxidation states, including the +1 oxidation state, as in the diatomic ion Hg2+
2, they still have a full d shell in these oxidation states. The group 12 elements Zn, Cd and Hg can therefore, under certain criteria, be classified as post-transition metals in this case. However, it is often convenient to include these elements in a discussion of transition elements. For example, when analyzing the crystal field stabilization energy of the first row transition elements, it is convenient to also include the elements calcium and zinc, since both Ca2+
and Zn2+
have a value of zero, against which the value of other transition metal ions can be compared. Another example occurs in the Irving-Williams series of complex stability constants.

The recent (though disputed and so far not independently reproduced) synthesis of mercury(IV) fluoride (HgF
4) has been taken by some to reinforce the view that group 12 elements should be considered transition metals, but some authors still consider this compound to be exceptional. It is hoped that Copernicium can use its d electrons for chemistry, as its 6d subshell is destabilized by strong relativistic effects due to its very high atomic number, and as such is expected to behave similarly to a transition metal. when it shows oxidation states higher than +2 (which are not definitely known for the lighter group 12 elements).

Although meitnerium, darmstadtium, and roentgenium lie within the d-block and are expected to behave as transition metals analogously to their lighter congeners such as iridium, platinum, and gold, this has not yet been established. experimentally confirmed. It is not clear whether copernicium behaves more like mercury or has properties more similar to the noble gas radon.

Properties

Almost all the elements are typical metals, of high hardness, with high melting and boiling points, good conductors of both heat and electricity. Many of the properties of transition metals are due to the ability of electrons in the d orbital to localize within the metal lattice. In metals, the more electrons that share a nucleus, the stronger the metal. They have a great versatility of oxidation states, being able to reach a positive charge as high as that of their group, and sometimes even negative (as in some coordination complexes).

• Their combinations are strongly colored and paramagnetic.
• Their normal potentials are usually less negative than that of representative metals, with the so-called noble metals among them.
• They can form alloys among them.
• They're generally good catalysts.
• They are solid at room temperature (except mercury)
• They form ionic complexes.

Variable oxidation states

Unlike group 1 and group 2 metals, transition element ions can have multiple stable oxidation states since they can lose d electrons without great energy sacrifice. Manganese, for example, has two 4s electrons and five 3d electrons that can be removed. The loss of all these electrons leads to a +7 oxidation state. Osmium and ruthenium are commonly found alone in a very stable +8 oxidation state which is one of the highest for isolated compounds.

Certain patterns in oxidation states arise through the periods of transition elements:

• The number of oxidation states increases for each ion to the Mn, from which it begins to decrease. The latest transition metals have a greater attraction between protons and electrons (since there are more than each present), which would require more energy to remove electrons.
• When the elements are in low oxidation states, they can be found as simple ions. However, the transition metals in high oxidation states are generally covalently attached to electro-negative elements such as oxygen or fluoride forming polyatomic ions such as chromate, vanadate, or permanganate.

Other properties regarding the stability of oxidation states:

• Ions in high oxidation states tend to be good oxidant agents, while elements in low oxidation states tend to be good reducing agents.
• Ions 2+ over the period start as strong reducers and become more stable.
• 3+ ions start stable and become more oxidizing throughout the period.

Catalytic activity

Transition metals make good homogeneous and heterogeneous catalysts, for example iron is the catalyst for the Haber process and both nickel and platinum are used for the hydrogenation of alkenes. This is because they are able to react under numerous oxidation states and as a consequence to form new compounds providing an alternative reaction pathway with a lower activation energy.

Colored compounds

From left to right, aqueous solution of: Co(NO₃)₂ (red); K2Cr2O7 (orange); K2CrO4 (yellow); NiCl₂ (green); CuSO4 (blue); KMnO4 (violet).

Due to their structure, transition metals form many colored ions and complexes. Colors can change between different ions of the same element. For example, MnO₄⁻ (Mn in the 7+ oxidation state) is a violet compound, while Mn²⁺ is pale pink..

Ligand coordination can play its part in determining the color in a transition compound due to changes in the energy of the d orbitals. The ligands remove the degeneracy of the orbitals and divide them into high and low energy groups. The difference in energy between the high and low energy orbitals will determine the color of the light that is absorbed, since electromagnetic radiation is absorbed if it has an energy that corresponds to this difference. When an ion with ligands absorbs light, some electrons are promoted to a higher energy orbital. If the absorbed light is of different frequency, different colors are observed.

The color of a complex depends on:

• the nature of metal ion, particularly the number of electrons in orbitals d
• the order of the ligands around the metal ion (for example, different geometric isomers can show different colors)
• the nature of the ligands surrounding the metal ion. If the ligands are stronger, the difference in energy between groups 3 is greater.d.

The complex formed by the d-block element zinc (although not strictly a transition element) is colorless, because the 3d orbitals are full and the electrons are not able to move to the top group.