OCCURRENCE AND RECOVERY OF TRANSITION METALS:-  Chemically soft members of the block occur as sulphide minerals and are partially oxidized to obtain the metal; the more electropositive 'hard' metals occur as oxides and are extracted by reduction.
The elements on the left of the 3d series occur in nature primarily as metal oxides or as metal cations in combination with Oxo anions. Of these elements, titanium ores are the most difficult to reduce and the elements is widely produced by heating TiO2 with chlorine and carbon to produce TiCl4, which is then reduced by molten magnesium at about 1000oC. In an inert-gas atmosphere. The oxides of chromium, manganese, and iron are reduced with carbon, a much cheaper reagent. To the right of iron in the 3d series, cobalt, nickel, copper, and zinc occur mainly as sulphides and arsenides, which is consistent with the increasingly soft Lewis acid character of their dipositive ions. Sulphide ores are usually roasted in air either to the metal directly (for ex- nickel) or to an oxide that is subsequently reduced (for ex- zinc). Copper is used in large quantities for electrical conductors; electrolysis is used to refine crude copper to achieve the high purity needed for high electrical conductivity.
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POSITION IN PERIODIC TABLE:-
The transition metals or transition elements traditionally occupy all of the d block of the periodic table. The name transition metal refers to the position in the periodic table of elements. The transition elements represent the successive addition of electrons to the d atomic orbitals of the atoms. In this way, the transition metals represent the transition between group 2 (2A) elements and group 13 (3A) elements.
Transition metals can be more strictly defined as an element whose atom or cation has an incomplete d sub-shell. This definition excludes zinc (Zn), cadmium (Cd), mercury (Hg) and probably Uub from the transition elements, as they have full d10 configurations.
The inner transition metals occupy the f block of the periodic table and again act as a transition between group 2 elements and the transition metals.
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ELECTRONIC CONFIGURATION OF D-BLOCK(TRANSITION) ELEMENTS:-
In transition elements ( leaving few exceptions) the number of electrons in their outermost subshell remains two while their penultimate shell of electrons is being expanded from 8 to 18 electrons due to addition of electrons in d-subshell.
1. First (3d) Transition series(Sc-Zn):-
2. Second (4d) Transition series(Y-Cd):-
3. Third (5d) Transition series(La-Hg):-
4. Fourth (6d) Transition series(Ac-Uub):-
These electronic configurations have the following characteristics.
An inner core of electrons with noble gas configuration.
(n-1)d orbital's are filled up progressively with electrons.
Most of the members have two electrons in the outermost orbital, i.e. ns. Some of the members, i.e., Cr, Cu, Nb, Mo, Ag, Au, etc, have only one electron in ns orbital whereas Pd has no electron in the ns-orbital.
In lanthanum (Z=57), one electron goes to 5d orbital before filling of 4f orbital. (an exception from AUFBAU order).
Fundamental difference in the electronic configuration of representative elements and transition elements:-
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In the representative elements (s and p-block elements), the valence electrons are present only in the outermost shell while in the transition elements, the valence electrons are present in the outermost shell as well as d-orbitals of penultimate shell.
GENERAL CHARACTERISTICS OF TRANSITION ELEMENTS:-
Except for mercury which is a liquid, all transition elements have typically metallic structure and show typical metallic properties such as conductivity, malleability, and ductility, lustre, high tensile strength etc.
Their atomic radii are in between those of s and p block elements. In a series, they decrease with increase in atomic numbers but the decrease is small after midway.
They have high melting and boiling point, high enthalpies of atomisation and high enthalpies of hydration of their ions. These properties depend upon the strength of the metallic bond in them.
Their first ionisation energies are higher than those of s-block elements and less than p-block elements.
They are electropositive in nature.
They show variable oxidation states.
The stability of any oxidation state or the tendency for an transition metal to act as oxidizing or reducing agent depends upon its electrode potential.
A number of these transition metals and their compounds show catalytic properties.
Most of the transition elements form coloured compounds.
Their compounds are generally paramagnetic in nature.
They have great tendency to form complexes.
They form interstitial compounds with elements like H, C, B, and N.
They form alloys.
The properties of d metals are largely derived from their electronic structure, with the strength of metallic bonding peaking at group 6; the lanthanide contraction is responsible for some of the anomalous behaviour in the 5d series. The d block of periodic table contains the metals most important to modern society. It contains the immensely strong and light titanium, the major components of most steels, the highly electrically conducting copper, the malleable gold and platinum, and the very dense osmium and indium. To a large extent these properties derive from the nature of the metallic bonding that binds atoms together.
Generally speaking, the same band structure is present for all the d-block metals and arise from the overlap of the (n+1)s orbitals to give an s band and the nd orbitals to give a d band. The principal difference between the metals is the number of electrons available to occupy these bands; titanium (3d24s2) has four bonding electrons, vanadium (3d34s2) five, chromium (3d54s1) six, a and so on. The lower, net bonding region of the valence bond is therefore progressively filled with electrons on going to the right across the block, which results in stronger bonding, until around Group 7 (at Mn, Tc, Re) when the electrons begin to populate the upper, net antibonding part of the band. This trend in bonding strength is reflected in the increase in melting point from the low-melting alkali metals (effectively only one bonding electron for each atom, resulting in melting point typically less than 100C) up to manganese, and its decline thereafter to the low melting Group 12 metals. The strength of metallic bonding in tungsten is such that its melting point is exceeded by only one other element, carbon.
The radii of d-metal ions depend on the effective charge of the nucleus, and ionic radii generally decrease on moving to the right as the atomic number increases. The radius metal atoms in solid element are determined by a combination of the strength of the metallic bonding and the size of the ions. Thus, the separation of the centres of the atoms in the solid generally follows a similar pattern to the melting points: they decrease to the middle of the d block, followed by an increase back up to Group 12, with the smallest separations occurring in and near Groups 7 and 8.
The atomic radii of the elements in the 5d series (Hf, Ta, Wâ€¦) are not much bigger than those of their 4d-series congeners (Zr,Nb,Mo,â€¦..). In fact, the atomic radius of Hf is smaller than that of Zr even though it appears in a later period. To understand this anomaly, we need to consider the effect of the lanthanoids (the first row of the f block).
Atomic mass increases with atomic number, and the combination of this increases with the changes in the radii of the metal that in the metal lattice means that the mass densities of the elements reach a peak with iridium.
TRENDS IN CHEMICAL PROPERTIES:-
Many of the d metals display a wide range of oxidation states, which leads to risk and fascinating chemistry. They also form an extensive range of coordination compounds and organometallic compounds.
Variable oxidation state:-
One of the striking features of the transition elements usually exists in several different oxidation states. Furthermore, the oxidation states change in units of one, e.g. Fe3+ and Fe2+, Cu2+, and Cu+.
The oxidation states shown by the transition elements may be related to their electronic structures. Calcium, the s-block element preceding the first row of transition elements, has the electronic structure:
It may be expected that the next ten transition elements would have this electronic arrangement with from one to ten d electrons added in a regular way: 3d1,3d2,3d3â€¦..3d10. This is true except in the cases one of the s electrons moves into the d shell, because of the additional stability when the d orbitals are exactly half filled or completely filled.
Thus Sc have an oxidation number of (+II) if both s electrons are used for bonding and (III) when two s and one d electron are involved. Ti has an oxidation state (+II) when both s electrons are used for bonding, (+III) when two s and one d electrons are used and (+IV) (+IV) and (+V). In the case of Cr, by using the single s electron for bonding, we get an oxidation number of (+I): hence by using varying numbers of d electrons oxidation states of (+II), (+III), (+IV), (+V) and (+VI) are possible.
As may be anticipated from consideration of atomic anionic radii, the 4d and 5d series have higher coordination numbers than their smaller 3d congeners. Note that with the small Fâ€‘ ligand, these 3d elements tends to form 6 cordinate complexes but that the larger 4d and 5d series metals in the same oxidation state tend to form 7, 8, and 9 coordinate complexes. The octacyanomolybdate complex, [Mo(CN)8]3-, illustrates the tendency towards high coordination number with compact ligands.
Structural changes also results from changes in oxidation state. Low oxidation state compounds often exist as ionic solids whereas high oxidation state compounds tends to take on covalent character.
NOBLE CHARACTER:- 
With the exception of Group 12, metals on the bottom right of the d block are the resistant to oxidation. This resistance is largely due to strong intermetallic bonding and high ionization energies. It is most evident for silver, gold, and the 4d and 5d series metals in Groups 8-10.
The later are referred to as the platinum metals because they occur together in platinum-bearing ores. In recognition of their traditional use, copper, silver, and gold are referred to as the coinage metals. Gold occurs as the metal ; silver, gold, and the platinum metals are also recovered in the electrolytic refining of copper. The prices of individual platinum metals vary widely because they are recovered together but their consumption is not proportional to their abundance. Rhodium by far the most expensive metal in this group because it is widely used in industrial catalytic processes and in automotive catalytic converters. Rhodium is almost twenty times more costly than the less catalytic useful metal palladium even though they occur in similar abundance.
Binary metal halides of the d-block elements occur for all the elements with nearly all oxidation states represented. As we should expect, the more strongly oxidizing halogens bring the out the higher oxidation states, with the corollary that the low oxidation state binary halides are more stable as iodides and bromides.
Of all groups, only the members of Group 11 (Cu, Ag, Au) have simple mono-halides. For Cu, these salts are highly insoluble in water and dissolve only when complexed by other ligands . Silver (I) halides are sparingly soluble and photosensitive, and decomposing to the metal. The only mono halide of Au that exists is the chloride; it is oxidised by water.
Higher halides exist for most of the d block, and covalent character becomes more prevalent with high oxidation state, especially for the lower halogens.
APPLICATIONS OF TRANSITION METALS:-
1. Application of transition metals for the synthesis of 18F-labelled radiotracers:-
Palladium is used as catalyst in synthesis of 18F- labelled radiotracers.
2. Application of Transition Metal Complex Formation in gas Chromatography:-
Metal complexation may be used for four purposes in gas
i) To help the separation of certain compounds present in the sample. In this case complexation is performed by using a stationary phase containing a metal.
ii) To utilize GC for the calculation of stability constants or other physico-chemical data.
iii) To analyse the metals themselves, by making organic volatile complexes and analysing them by GC.
iv) To increase sensitivity for inorganic and organic compounds by forming metal complexes and utilize e.g. an electron capture detector which has an increased sensitivity for such compounds.
3) Transition metals are used in organic synthesis.
4) Transition Metals - Real-life application:-
At one time, gold was used in coins, and nations gauged the value of their currency in terms of the gold reserves they possessed. Gold is as popular as ever for jewellery and other decorative objects, of course, but for the most part, it is too soft to have many other commercial purposes. One of the few applications for gold, a good conductor of electricity, is in some electronic components. Also, the radioactive gold-198 isotope is sometimes implanted in tissues as a means of treating forms of cancer.
Like gold, silver has been a part of human life from earliest history. Usually it is considered less valuable, though some societies have actually placed a higher value on silver because it is harder and more durable than gold. Its uses are much more varied than those of gold, both because of its durability and the fact that it is less expensive. Alloyed with copper, which adds strength to it, it makes sterling silver, used in coins, silverware, and jewellery.
Copper is an extremely efficient conductor of heat and electricity, and because it is much less expensive than the other two, pure copper is widely used for electrical wiring. Because of its ability to conduct heat, copper is also applied in materials used for making heaters, as well as for cookware. Due to the high conductivity of copper, a heated copper pan has a uniform temperature, but copper pots must be coated with tin because too much copper in food is toxic.
Copper is also like its two close relatives in that it resists corrosion, and this makes it ideal for plumbing. Its use in making coins resulted from its anti-corrosive qualities, combined with its beauty: like gold, copper has a distinctive color. This aesthetic quality led to the use of copper in decorative applications as well: many old buildings used copper roofs, and the Statue of Liberty is covered in 300 thick copper plates.
Together with copper, zinc appeared in another alloy that, like bronze, helped define the ancient world: brass. Just as a penny is not really copper but zinc, "tin" roofs are usually made of galvanized steel. Highly resistant to corrosion, galvanized steel finds application in everything from industrial equipment to garbage cans. Zinc oxide is applied in textiles, batteries, paints, and rubber products, while luminous zinc sulfide appears in television screens, movie screens, clock dials, and fluorescent light bulbs.
Zinc phosphide is used as a rodent poison. Like several other transition metals, zinc is a part of many living things, yet it can be toxic in large quantities or specific compounds. For a human being, inhaling zinc oxide causes involuntary shaking. On the other hand, humans and many animals require zinc in their diets for the digestion of proteins. Furthermore, it is believed that zinc contributes to the healing of wounds and to the storage of insulin in the pancreas.
Today cadmium is used in batteries, and for electroplating of other metals to protect them against corrosion. Because the cost of cadmium is high due to the difficulty of separating it from zinc, cadmium electroplating is applied only in specialized situations. Cadmium also appears in the control rods of nuclear power plants, where its ready absorption of neutrons aids in controlling the rate at which nuclear fission occurs.
Cadmium is highly toxic, and is believed to be the cause behind the outbreak of itai-itai ("ouch-ouch") disease in Japan in 1955. People ingested rice oil contaminated with cadmium, and experienced a number of painful side effects associated with cadmium poisoning: nausea, vomiting, choking, diarrhea, abdominal pain, headaches, and difficulty breathing.
Mercury, of course, is widely used in thermometers, an application for which it is extremely well-suited. In particular, it expands at a uniform rate when heated, and thus a mercury thermometer. At temperatures close to absolute zero, mercury loses its resistance to the flow of electric current, and therefore it presents a promising area of research with regard to superconductivity.
The ways in which iron is used are almost too obvious (and too numerous) to mention. If iron and steel suddenly ceased to exist, there could be no skyscrapers, no wide-span bridges, no ocean liners or trains or heavy machinery or automobile frames. Furthermore, alloys of steel with other transition metals, such as tungsten and niobium, possess exceptionally great strength, and find application in everything from hand tools to nuclear reactors. Then, of course, there are magnets and electromagnets, which can only be made of iron and/or one of the other magnetic elements, cobalt and nickel.
In the human body, iron is a key part of hemoglobin, the molecule in blood that transports oxygen from the lungs to the cells. If a person fails to get sufficient quantities of iron-present in foods such as red meat and spinach-the result is anemia, characterized by a loss of skin color, weakness, fainting, and heart palpitations. Plants, too, need iron, and without the appropriate amounts are likely to lose their color, weaken, and die.
The element, which makes up less than 0.002% of Earth's crust, is found today primarily in ores extracted from mines in Canada, Zaire, and Morocco. One of the most important uses of cobalt is in a highly magnetic alloy known as alnico, which also contains iron, nickel, and aluminum. Combined with tungsten and chromium, cobalt makes satellite, a very hard alloy used in drill bits. Cobalt is also applied in jet engines and turbines.
Today, nickel is applied, not surprisingly, in the American five-cent piece-that is, the "nickel"-made from an alloy of nickel and copper. Its anti-corrosive nature also provides a number of other applications for nickel: alloyed with steel, for instance, it makes a protective layer for other metals.
Today, platinum is used in everything from thermometers to parts for rocket engines, both of which take advantage of its ability to with stand high temperatures.
IRIDIUM AND OSMIUM:-
Tennant discovered a second element in 1804, also from the residue left over from the acid process for extracting platinum. This one had a distinctive smell when heated, so he named it osmium after osme, Greek for "odor." In 1898, Austrian chemist Karl Auer, Baron von Welsbach (1858-1929), developed a light bulb using osmium as a filament, the material that is heated. Though osmium proved too expensive for commercial use, Auer's creation paved the way for the use of another transition metal, tungsten, in making long-lasting filaments. Osmium, which is very hard and resistant to wear, is also used in electrical devices, fountain-pen tips, and phonograph needles.
5) Transition metals are applied in the synthesis of metal hydride.Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â M=C=O + OH-Â ====> Â M-H + CO2
Â Â Â Â Here metal carbonyl group reacts with hydroxide to give metal hydride and carbon dioxide.Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â
6) Transition metal used in the complexes in fluorescence cell imaging.
Transition metal complexes have often been proposed as useful fluorophores for cell imaging due to their attractive photo physical attributes, but until very recently their actual applications have been scarce and largely limited to ruthenium complexes in DNA and oxygen sensing.