Iron triad trends

ü The elements of iron triad are found in nature mostly as sulfides form strong complexes.

ü All elements also.

These elements belong to the d block. They have electron configurations: 4s²3d^x with x=6,7, and 8 for Fe, Co and Ni, respectively.Therefore, oxidation state +2 is the most common.

In the chemistry of the triad, we observe the continuing tendency for the higher oxidation states to decrease in stability along the first transition series; unlike cobalt and iron, the +3 state of nickel is rare and relatively unimportant and the +2 state is the only important one.

Iron is characterized by two series of compounds: iron(II) and iron(III). The former corresponds to iron(II) oxide FeO, and the latter to iron(III) oxide Fe₂O₃. In addition, salts of ferric acid H₃FeO₄ are known in which the oxidation state of iron is +6.

The simple compounds of cobalt(III) are strongly oxidizing whereas crystalline field of ligands in complexes of Co(III) makes them relatively stable. Cobalt (II) compounds are the most stable.

History Of Discovery

Iron (Ferrum). Cobalt (Cebafttim). Nickel (Niccolum)

IRON

The abundance of iron ores from which it may be readily obtained led to its use in a very remote period. Iron has been known and used since prehistoric times. In China the usage of steel goes back to 2550 B.C. Some vedic poets wrote that their prehistoric ancestors already knew iron.

The earlier sources of the ores appear to have been in India. A remarkable Iron pillar, dating to about A.D. 400, remains standing today at the center of the Quwwat-ul-Islam Masjid, the first Moslem mosque in Delhi, India (Fig. 3). This pillar is a classical example of massive production of high class iron and is the biggest hand-forged block of iron from antiquity. According to information from various Roman and Greek texts, metals like iron, tin, copper and brass were imported from India.

COBALT

The word Cobalt derived from the German "kobold" = a goblin, gnome, evil spirit (Mittelhochdeutsch "kobe" [hut, shed] + "holt" [goblin, from "hold" = gracious, friendly]). In some mining regions there were specific prayers to protect the miners from those kobolds, who by German superstition were delighted in destroying the work of miners, causing them endless trouble. The word then became also the term silver miners used for worthless rock, containing arsenic and sulfur (cobaltite, CoAsS). Paracelsus (Theophrastus Bombastus von Hohenheim, 1493—1541) vaguely mentioned cobalt in his Book of Minerals as a troublesome and worthless mineral found in large quantity in mines on the borders of Saxony and Bohemia. Georgius Agricola (1494—1555) used the term cobalt, to denote substances which, although resembling metallic ores, gave no metal on smelting.

In 1735, Georg Brandt (1694—1768) pointed out that the primary cause to the blue colour of those glass and smalts was due to the presence of a metal or semi-metal, that he called cobalt rex. About 1741 he wrote: "As there are six kinds of metals, so I have also shown with reliable experiments... that there are also six kinds of half-metals: a new half-metal, namely Cobalt regulus in addition to Mercury, Bismuth, Zinc, and the reguluses of Antimony and Arsenic. " He gave six ways to distinguish Bismuth and Cobalt which were typically found in the same ores. Cobalt was not considered an element until Antoine Lavoisier (1743—1794) redefined the term.

NICKEL

Naturally occurring nickel-copper alloys, called paktong, were used in China over 2000 years ago. In 235 BC, coins in China were made from nickel. Saxon miners were familiar with the reddish-coloured ore, a combination of arsenic and nickel (niccolite, NiAs), which superficially resembles Cu₂O.

These miners attributed their inability to extract copper from this source to the work of the devil and named the ore Kupfernickel (in Swedish kopparnickel).

Originally it was a term of abuse used by the miners in the Erzgebirge (Ore Mountains), who searching for Silver found this "inferior" metal. The word is derived from Kupfer = Copper and Nickel = demon, goblin, rascal (a pet form of the name Nikolaus [Nicholas], hence Old Nick "the devil"). Thus kupfernickel — Latinized as Cuprum Nicolai — can be translated as "old Nick's Copper" (or "Devil's Copper").

In 1751 the Swedish mineralogist Axel Fredrik Cronstedt (1722—1765) isolated an impure metal from ores, probably Gersdorffite (NiAsS). He identified in it a new half-metal. A few years later, he found out that his half-metal was identical with the metallic component of Kupfernickel, sent to him from Germany. For the new half-metal he chose to retain the name Kupfernickel, or shortly Nickel, until it became sure it was a new metal. Like all metals, Nickel was not considered an element until Antoine Lavoisier (1743—1794) proposed his new chemistry.

	Fe	Co	Ni
Content in the Earth crust, % (mass./mol)	4.0 2.0	4×10^–3 1.5×10^–3	0.01 3.0×10^–3
Common minerals	2Fe2O3·3H2O limonite FeCO3 siderite Fe2O3 hematite Fe3O4 magnetite FeS2 pyrite	smaltite CoAs2 cobaltite CoAs	gersdorfite NiAsS niccolite NiAs

Iron is the most abundant metal on the globe after aluminium, forming 4% by of the Earth's crust (mass). It is found in the form of a variety of compounds: oxides, sulfides, and silicates. It occurs in the free state only in meteorites.

The most important iron ores include magnetite Fe₃O₄, red iron ore or hematite Fe₂O₃, brown iron ore or l imonite 2Fe₂O₃·3H₂O, and spathic iron ore or siderite FeCO₃. Iron pyrites, or simply pyrite FeS₂ encountered in large amounts is rarely used in metallurgy because iron of a very low quality due to the high sulfur content is produced from it. But pyrite nevertheless has an important application—it is the starting material in the production of sulfuric acid.

In Russia, deposits of iron ores are found in the Urals, where whole mountains (for example Magnitnaya, Kachkanar, and Vysokaya) are formed of magnetite of an excellent quality. Rich deposits in Ukraine are in the Krivoi Rog Region and on the Kerch Peninsula, the former being red iron ore, and the latter brown ore.

Figure. A rock containing three crystals of pyrite (FeS₂). The crystal structure of pyrite is simple cubic, and this is reflected in the cubic symmetry of its natural crystal facets

Cobalt is rare in nature: its content in the Earth's crust is about 0.004% (mass). It is most often combined with arsenic as the minerals smaltite CoAs₂ and cobaltite or cobalt glance CoAs.Cobalt is relatively uncommon but widely distributed; it occurs biologically in vitamin B12 (a complex of cobalt(III) in which cobalt is bonded octahedrally to nitrogen atoms and the carbon atom of a CN^– group).

Nickel is more abundant than cobalt {about 0.01% (mass) of the Earth's crust} but only a few deposits are economically useful for extraction. Like cobalt, nickel occurs in nature together with arsenic or sulfur. These include the minerals niccolite or arsenical nickel NiAs and gersdorfite NiAsS.

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