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Iron

Chemical element with symbol Fe and atomic number 26

Top 10 Iron related articles

Iron, 26Fe
Iron
Appearancelustrous metallic with a grayish tinge
Standard atomic weight Ar, std(Fe)55.845(2)[1]
Iron in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson


Fe

Ru
manganeseironcobalt
Atomic number (Z)26
Groupgroup 8
Periodperiod 4
Block  d-block
Electron configuration[Ar] 3d6 4s2
Electrons per shell2, 8, 14, 2
Physical properties
Phase at STPsolid
Melting point1811 K ​(1538 °C, ​2800 °F)
Boiling point3134 K ​(2862 °C, ​5182 °F)
Density (near r.t.)7.874 g/cm3
when liquid (at m.p.)6.98 g/cm3
Heat of fusion13.81 kJ/mol
Heat of vaporization340 kJ/mol
Molar heat capacity25.10 J/(mol·K)
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 1728 1890 2091 2346 2679 3132
Atomic properties
Oxidation states−4, −2, −1, 0, +1,[2] +2, +3, +4, +5,[3] +6, +7[4] (an amphoteric oxide)
ElectronegativityPauling scale: 1.83
Ionization energies
  • 1st: 762.5 kJ/mol
  • 2nd: 1561.9 kJ/mol
  • 3rd: 2957 kJ/mol
  • (more)
Atomic radiusempirical: 126 pm
Covalent radiusLow spin: 132±3 pm
High spin: 152±6 pm
Spectral lines of iron
Other properties
Natural occurrenceprimordial
Crystal structurebody-centered cubic (bcc)

a=286.65 pm
Crystal structureface-centered cubic (fcc)

between 1185–1667 K; a=364.680 pm
Speed of sound thin rod5120 m/s (at r.t.) (electrolytic)
Thermal expansion11.8 µm/(m·K) (at 25 °C)
Thermal conductivity80.4 W/(m·K)
Electrical resistivity96.1 nΩ·m (at 20 °C)
Curie point1043 K
Magnetic orderingferromagnetic
Young's modulus211 GPa
Shear modulus82 GPa
Bulk modulus170 GPa
Poisson ratio0.29
Mohs hardness4
Vickers hardness608 MPa
Brinell hardness200–1180 MPa
CAS Number7439-89-6
History
Discoverybefore 5000 BC
Main isotopes of iron
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct
54Fe 5.85% stable
55Fe syn 2.73 y ε 55Mn
56Fe 91.75% stable
57Fe 2.12% stable
58Fe 0.28% stable
59Fe syn 44.6 d β 59Co
60Fe trace 2.6×106 y β 60Co
 Category: Iron
| references

Iron (/ˈərn/) is a chemical element with symbol Fe (from Latin: ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is by mass the most common element on Earth, right in front of oxygen (32.1% and 30.1%, respectively), forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust.

In its metallic state, iron is rare in the Earth's crust, limited mainly to deposition by meteorites. Iron ores, by contrast, are among the most abundant in the Earth's crust, although extracting usable metal from them requires kilns or furnaces capable of reaching 1,500 °C (2,730 °F) or higher, about 500 °C (900 °F) higher than that required to smelt copper. Humans started to master that process in Eurasia by about 2000 BCE, and the use of iron tools and weapons began to displace copper alloys, in some regions, only around 1200 BCE. That event is considered the transition from the Bronze Age to the Iron Age. In the modern world, iron alloys, such as steel, inox, cast iron and special steels are by far the most common industrial metals, because of their mechanical properties and low cost.

Pristine and smooth pure iron surfaces are mirror-like silvery-gray. However, iron reacts readily with oxygen and water to give brown to black hydrated iron oxides, commonly known as rust. Unlike the oxides of some other metals, that form passivating layers, rust occupies more volume than the metal and thus flakes off, exposing fresh surfaces for corrosion.

The body of an adult human contains about 4 grams (0.005% body weight) of iron, mostly in hemoglobin and myoglobin. These two proteins play essential roles in vertebrate metabolism, respectively oxygen transport by blood and oxygen storage in muscles. To maintain the necessary levels, human iron metabolism requires a minimum of iron in the diet. Iron is also the metal at the active site of many important redox enzymes dealing with cellular respiration and oxidation and reduction in plants and animals.[5]

Chemically, the most common oxidation states of iron are iron(II) and iron(III). Iron shares many properties of other transition metals, including the other group 8 elements, ruthenium and osmium. Iron forms compounds in a wide range of oxidation states, −2 to +7. Iron also forms many coordination compounds; some of them, such as ferrocene, ferrioxalate, and Prussian blue, have substantial industrial, medical, or research applications.

Iron Intro articles: 49

Characteristics

Allotropes

Molar volume vs. pressure for α iron at room temperature

At least four allotropes of iron (differing atom arrangements in the solid) are known, conventionally denoted α, γ, δ, and ε.

Low-pressure phase diagram of pure iron

The first three forms are observed at ordinary pressures. As molten iron cools past its freezing point of 1538 °C, it crystallizes into its δ allotrope, which has a body-centered cubic (bcc) crystal structure. As it cools further to 1394 °C, it changes to its γ-iron allotrope, a face-centered cubic (fcc) crystal structure, or austenite. At 912 °C and below, the crystal structure again becomes the bcc α-iron allotrope.[6]

The physical properties of iron at very high pressures and temperatures have also been studied extensively,[7][8] because of their relevance to theories about the cores of the Earth and other planets. Above approximately 10 GPa and temperatures of a few hundred kelvin or less, α-iron changes into another hexagonal close-packed (hcp) structure, which is also known as ε-iron. The higher-temperature γ-phase also changes into ε-iron, but does so at higher pressure.

Some controversial experimental evidence exists for a stable β phase at pressures above 50 GPa and temperatures of at least 1500 K. It is supposed to have an orthorhombic or a double hcp structure.[9] (Confusingly, the term "β-iron" is sometimes also used to refer to α-iron above its Curie point, when it changes from being ferromagnetic to paramagnetic, even though its crystal structure has not changed.[6])

The inner core of the Earth is generally presumed to consist of an iron-nickel alloy with ε (or β) structure.[10]

Melting and boiling points

The melting and boiling points of iron, along with its enthalpy of atomization, are lower than those of the earlier 3d elements from scandium to chromium, showing the lessened contribution of the 3d electrons to metallic bonding as they are attracted more and more into the inert core by the nucleus;[11] however, they are higher than the values for the previous element manganese because that element has a half-filled 3d subshell and consequently its d-electrons are not easily delocalized. This same trend appears for ruthenium but not osmium.[12]

The melting point of iron is experimentally well defined for pressures less than 50 GPa. For greater pressures, published data (as of 2007) still varies by tens of gigapascals and over a thousand kelvin.[13]

Magnetic properties

Magnetization curves of 9 ferromagnetic materials, showing saturation. 1. Sheet steel, 2. Silicon steel, 3. Cast steel, 4. Tungsten steel, 5. Magnet steel, 6. Cast iron, 7. Nickel, 8. Cobalt, 9. Magnetite[14]

Below its Curie point of 770 °C, α-iron changes from paramagnetic to ferromagnetic: the spins of the two unpaired electrons in each atom generally align with the spins of its neighbors, creating an overall magnetic field.[15] This happens because the orbitals of those two electrons (dz2 and dx2y2) do not point toward neighboring atoms in the lattice, and therefore are not involved in metallic bonding.[6]

In the absence of an external source of magnetic field, the atoms get spontaneously partitioned into magnetic domains, about 10 micrometers across,[16] such that the atoms in each domain have parallel spins, but some domains have other orientations. Thus a macroscopic piece of iron will have a nearly zero overall magnetic field.

Application of an external magnetic field causes the domains that are magnetized in the same general direction to grow at the expense of adjacent ones that point in other directions, reinforcing the external field. This effect is exploited in devices that needs to channel magnetic fields, such as electrical transformers, magnetic recording heads, and electric motors. Impurities, lattice defects, or grain and particle boundaries can "pin" the domains in the new positions, so that the effect persists even after the external field is removed — thus turning the iron object into a (permanent) magnet.[15]

Similar behavior is exhibited by some iron compounds, such as the ferrites and the mineral magnetite, a crystalline form of the mixed iron(II,III) oxide Fe
3
O
4
(although the atomic-scale mechanism, ferrimagnetism, is somewhat different). Pieces of magnetite with natural permanent magnetization (lodestones) provided the earliest compasses for navigation. Particles of magnetite were extensively used in magnetic recording media such as core memories, magnetic tapes, floppies, and disks, until they were replaced by cobalt-based materials.

Isotopes

Iron has four stable isotopes: 54Fe (5.845% of natural iron), 56Fe (91.754%), 57Fe (2.119%) and 58Fe (0.282%). 20-30 artificial isotopes have also been created. Of these stable isotopes, only 57Fe has a nuclear spin (−​12). The nuclide 54Fe theoretically can undergo double electron capture to 54Cr, but the process has never been observed and only a lower limit on the half-life of 3.1×1022 years has been established.[17]

60Fe is an extinct radionuclide of long half-life (2.6 million years).[18] It is not found on Earth, but its ultimate decay product is its granddaughter, the stable nuclide 60Ni.[17] Much of the past work on isotopic composition of iron has focused on the nucleosynthesis of 60Fe through studies of meteorites and ore formation. In the last decade, advances in mass spectrometry have allowed the detection and quantification of minute, naturally occurring variations in the ratios of the stable isotopes of iron. Much of this work is driven by the Earth and planetary science communities, although applications to biological and industrial systems are emerging.[19]

In phases of the meteorites Semarkona and Chervony Kut, a correlation between the concentration of 60Ni, the granddaughter of 60Fe, and the abundance of the stable iron isotopes provided evidence for the existence of 60Fe at the time of formation of the Solar System. Possibly the energy released by the decay of 60Fe, along with that released by 26Al, contributed to the remelting and differentiation of asteroids after their formation 4.6 billion years ago. The abundance of 60Ni present in extraterrestrial material may bring further insight into the origin and early history of the Solar System.[20]

The most abundant iron isotope 56Fe is of particular interest to nuclear scientists because it represents the most common endpoint of nucleosynthesis.[21] Since 56Ni (14 alpha particles) is easily produced from lighter nuclei in the alpha process in nuclear reactions in supernovae (see silicon burning process), it is the endpoint of fusion chains inside extremely massive stars, since addition of another alpha particle, resulting in 60Zn, requires a great deal more energy. This 56Ni, which has a half-life of about 6 days, is created in quantity in these stars, but soon decays by two successive positron emissions within supernova decay products in the supernova remnant gas cloud, first to radioactive 56Co, and then to stable 56Fe. As such, iron is the most abundant element in the core of red giants, and is the most abundant metal in iron meteorites and in the dense metal cores of planets such as Earth.[22] It is also very common in the universe, relative to other stable metals of approximately the same atomic weight.[22][23] Iron is the sixth most abundant element in the universe, and the most common refractory element.[24]

Although a further tiny energy gain could be extracted by synthesizing 62Ni, which has a marginally higher binding energy than 56Fe, conditions in stars are unsuitable for this process. Element production in supernovas and distribution on Earth greatly favor iron over nickel, and in any case, 56Fe still has a lower mass per nucleon than 62Ni due to its higher fraction of lighter protons.[25] Hence, elements heavier than iron require a supernova for their formation, involving rapid neutron capture by starting 56Fe nuclei.[22]

In the far future of the universe, assuming that proton decay does not occur, cold fusion occurring via quantum tunnelling would cause the light nuclei in ordinary matter to fuse into 56Fe nuclei. Fission and alpha-particle emission would then make heavy nuclei decay into iron, converting all stellar-mass objects to cold spheres of pure iron.[26]

Iron Characteristics articles: 77

Origin and occurrence in nature

Cosmogenesis

Iron's abundance in rocky planets like Earth is due to its abundant production during the runaway fusion and explosion of type Ia supernovae, which scatters the iron into space.[27][28]

Metallic iron

A polished and chemically etched piece of an iron meteorite, believed to be similar in composition to the Earth's metallic core, showing individual crystals of the iron-nickel alloy (Widmanstatten pattern)

Metallic or native iron is rarely found on the surface of the Earth because it tends to oxidize. However, both the Earth's inner and outer core, that account for 35% of the mass of the whole Earth, are believed to consist largely of an iron alloy, possibly with nickel. Electric currents in the liquid outer core are believed to be the origin of the Earth's magnetic field. The other terrestrial planets (Mercury, Venus, and Mars) as well as the Moon are believed to have a metallic core consisting mostly of iron. The M-type asteroids are also believed to be partly or mostly made of metallic iron alloy.

The rare iron meteorites are the main form of natural metallic iron on the Earth's surface. Items made of cold-worked meteoritic iron have been found in various archaeological sites dating from a time when iron smelting had not yet been developed; and the Inuit in Greenland have been reported to use iron from the Cape York meteorite for tools and hunting weapons.[29] About 1 in 20 meteorites consist of the unique iron-nickel minerals taenite (35–80% iron) and kamacite (90–95% iron).[30] Native iron is also rarely found in basalts that have formed from magmas that have come into contact with carbon-rich sedimentary rocks, which have reduced the oxygen fugacity sufficiently for iron to crystallize. This is known as Telluric iron and is described from a few localities, such as Disko Island in West Greenland, Yakutia in Russia and Bühl in Germany.[31]

Mantle minerals

Ferropericlase (Mg,Fe)O, a solid solution of periclase (MgO) and wüstite (FeO), makes up about 20% of the volume of the lower mantle of the Earth, which makes it the second most abundant mineral phase in that region after silicate perovskite (Mg,Fe)SiO
3
; it also is the major host for iron in the lower mantle.[32] At the bottom of the transition zone of the mantle, the reaction γ-(Mg,Fe)
2
[SiO
4
] ↔ (Mg,Fe)[SiO
3
] + (Mg,Fe)O
transforms γ-olivine into a mixture of silicate perovskite and ferropericlase and vice versa. In the literature, this mineral phase of the lower mantle is also often called magnesiowüstite.[33] Silicate perovskite may form up to 93% of the lower mantle,[34] and the magnesium iron form, (Mg,Fe)SiO
3
, is considered to be the most abundant mineral in the Earth, making up 38% of its volume.[35]

Earth's crust

Ochre path in Roussillon.

While iron is the most abundant element on Earth, most of this iron is concentrated in the inner and outer cores.[36][37] The fraction of iron that is in Earth's crust only amounts to about 5% of the overall mass of the crust and is thus only the fourth most abundant element in that layer (after oxygen, silicon, and aluminium).[38]

Most of the iron in the crust is combined with various other elements to form many iron minerals. An important class is the iron oxide minerals such as hematite (Fe2O3), magnetite (Fe3O4), and siderite (FeCO3), which are the major ores of iron. Many igneous rocks also contain the sulfide minerals pyrrhotite and pentlandite.[39][40] During weathering, iron tends to leach from sulfide deposits as the sulfate and from silicate deposits as the bicarbonate. Both of these are oxidized in aqueous solution and precipitate in even mildly elevated pH as iron(III) oxide.[41]

Banded iron formation in the McKinley Park, Minnesota

Large deposits of iron are banded iron formations, a type of rock consisting of repeated thin layers of iron oxides alternating with bands of iron-poor shale and chert. The banded iron formations were laid down in the time between 3,700 million years ago and 1,800 million years ago.[42][43]

Materials containing finely ground iron(III) oxides or oxide-hydroxides, such as ochre, have been used as yellow, red, and brown pigments since pre-historical times. They contribute as well to the color of various rocks and clays, including entire geological formations like the Painted Hills in Oregon and the Buntsandstein ("colored sandstone", British Bunter).[44] Through Eisensandstein (a jurassic 'iron sandstone', e.g. from Donzdorf in Germany)[45] and Bath stone in the UK, iron compounds are responsible for the yellowish color of many historical buildings and sculptures.[46] The proverbial red color of the surface of Mars is derived from an iron oxide-rich regolith.[47]

Significant amounts of iron occur in the iron sulfide mineral pyrite (FeS2), but it is difficult to extract iron from it and it is therefore not exploited. In fact, iron is so common that production generally focuses only on ores with very high quantities of it.

According to the International Resource Panel's Metal Stocks in Society report, the global stock of iron in use in society is 2,200 kg per capita. More-developed countries differ in this respect from less-developed countries (7,000–14,000 vs 2,000 kg per capita).[48]

Iron Origin and occurrence in nature articles: 59

Chemistry and compounds

Oxidation
state
Representative compound
−2 (d10) Disodium tetracarbonylferrate (Collman's reagent)
−1 (d9) Fe
2
(CO)2−
8
0 (d8) Iron pentacarbonyl
1 (d7) Cyclopentadienyliron dicarbonyl dimer ("Fp2")
2 (d6) Ferrous sulfate, ferrocene
3 (d5) Ferric chloride, ferrocenium tetrafluoroborate
4 (d4) Fe(diars)
2
Cl2+
2
5 (d3) FeO3−
4
6 (d2) Potassium ferrate
7 (d1) [FeO4] (matrix isolation, 4K)

Iron shows the characteristic chemical properties of the transition metals, namely the ability to form variable oxidation states differing by steps of one and a very large coordination and organometallic chemistry: indeed, it was the discovery of an iron compound, ferrocene, that revolutionalized the latter field in the 1950s.[49] Iron is sometimes considered as a prototype for the entire block of transition metals, due to its abundance and the immense role it has played in the technological progress of humanity.[50] Its 26 electrons are arranged in the configuration [Ar]3d64s2, of which the 3d and 4s electrons are relatively close in energy, and thus it can lose a variable number of electrons and there is no clear point where further ionization becomes unprofitable.[12]

Iron forms compounds mainly in the oxidation states +2 (iron(II), "ferrous") and +3 (iron(III), "ferric"). Iron also occurs in higher oxidation states, e.g. the purple potassium ferrate (K2FeO4), which contains iron in its +6 oxidation state. Although iron(VIII) oxide (FeO4) has been claimed, the report could not be reproduced and such a species from the removal of all electrons of the element beyond the preceding inert gas configuration (at least with iron in its +8 oxidation state) has been found to be improbable computationally.[51] However, one form of anionic [FeO4] with iron in its +7 oxidation state, along with an iron(V)-peroxo isomer, has been detected by infrared spectroscopy at 4 K after cocondensation of laser-ablated Fe atoms with a mixture of O2/Ar.[52] Iron(IV) is a common intermediate in many biochemical oxidation reactions.[53][54] Numerous organoiron compounds contain formal oxidation states of +1, 0, −1, or even −2. The oxidation states and other bonding properties are often assessed using the technique of Mössbauer spectroscopy.[55] Many mixed valence compounds contain both iron(II) and iron(III) centers, such as magnetite and Prussian blue (Fe
4
(Fe[CN]
6
)
3
).[54] The latter is used as the traditional "blue" in blueprints.[56]

Iron is the first of the transition metals that cannot reach its group oxidation state of +8, although its heavier congeners ruthenium and osmium can, with ruthenium having more difficulty than osmium.[6] Ruthenium exhibits an aqueous cationic chemistry in its low oxidation states similar to that of iron, but osmium does not, favoring high oxidation states in which it forms anionic complexes.[6] In the second half of the 3d transition series, vertical similarities down the groups compete with the horizontal similarities of iron with its neighbors cobalt and nickel in the periodic table, which are also ferromagnetic at room temperature and share similar chemistry. As such, iron, cobalt, and nickel are sometimes grouped together as the iron triad.[50]

Unlike many other metals, iron does not form amalgams with mercury. As a result, mercury is traded in standardized 76 pound flasks (34 kg) made of iron.[57]

Iron is by far the most reactive element in its group; it is pyrophoric when finely divided and dissolves easily in dilute acids, giving Fe2+. However, it does not react with concentrated nitric acid and other oxidizing acids due to the formation of an impervious oxide layer, which can nevertheless react with hydrochloric acid.[6]

Binary compounds

Oxides and hydroxides

Ferrous or iron(II) oxide, FeO
Ferric or iron(III) oxide Fe
2
O
3
Ferrosoferric or iron(II,III) oxide Fe
3
O
4

Iron forms various oxide and hydroxide compounds; the most common are iron(II,III) oxide (Fe3O4), and iron(III) oxide (Fe2O3). Iron(II) oxide also exists, though it is unstable at room temperature. Despite their names, they are actually all non-stoichiometric compounds whose compositions may vary.[58] These oxides are the principal ores for the production of iron (see bloomery and blast furnace). They are also used in the production of ferrites, useful magnetic storage media in computers, and pigments. The best known sulfide is iron pyrite (FeS2), also known as fool's gold owing to its golden luster.[54] It is not an iron(IV) compound, but is actually an iron(II) polysulfide containing Fe2+ and S2−
2
ions in a distorted sodium chloride structure.[58]

Halides

Hydrated iron(III) chloride (ferric chloride)

The binary ferrous and ferric halides are well-known. The ferrous halides typically arise from treating iron metal with the corresponding hydrohalic acid to give the corresponding hydrated salts.[54]

Fe + 2 HX → FeX2 + H2 (X = F, Cl, Br, I)

Iron reacts with fluorine, chlorine, and bromine to give the corresponding ferric halides, ferric chloride being the most common.[59]

2 Fe + 3 X2 → 2 FeX3 (X = F, Cl, Br)

Ferric iodide is an exception, being thermodynamically unstable due to the oxidizing power of Fe3+ and the high reducing power of I:[59]

2 I + 2 Fe3+ → I2 + 2 Fe2+ (E0 = +0.23 V)

Ferric iodide, a black solid, is not stable in ordinary conditions, but can be prepared through the reaction of iron pentacarbonyl with iodine and carbon monoxide in the presence of hexane and light at the temperature of −20 °C, with oxygen and water excluded.[59]

Solution chemistry

Comparison of colors of solutions of ferrate (left) and permanganate (right)

The standard reduction potentials in acidic aqueous solution for some common iron ions are given below:[6]

Fe2+ + 2 e ⇌ Fe E0 = −0.447 V
Fe3+ + 3 e ⇌ Fe E0 = −0.037 V
FeO2−
4
+ 8 H+ + 3 e
⇌ Fe3+ + 4 H2O E0 = +2.20 V

The red-purple tetrahedral ferrate(VI) anion is such a strong oxidizing agent that it oxidizes nitrogen and ammonia at room temperature, and even water itself in acidic or neutral solutions:[59]

4 FeO2−
4
+ 10 H
2
O
→ 4 Fe3+
+ 20 OH
+ 3 O2

The Fe3+ ion has a large simple cationic chemistry, although the pale-violet hexaquo ion [Fe(H
2
O)
6
]3+
is very readily hydrolyzed when pH increases above 0 as follows:[60]

[Fe(H
2
O)
6
]3+
[Fe(H
2
O)
5
(OH)]2+ + H+
K = 10−3.05 mol dm−3
[Fe(H
2
O)
5
(OH)]2+
[Fe(H
2
O)
4
(OH)
2
]+ + H+
K = 10−3.26 mol dm−3
2[Fe(H
2
O)
6
]3+
[Fe(H
2
O)
4
(OH)]4+
2
+ 2H+ + 2H
2
O
K = 10−2.91 mol dm−3
Blue-green iron(II) sulfate heptahydrate

As pH rises above 0 the above yellow hydrolyzed species form and as it rises above 2–3, reddish-brown hydrous iron(III) oxide precipitates out of solution. Although Fe3+ has an d5 configuration, its absorption spectrum is not like that of Mn2+ with its weak, spin-forbidden d–d bands, because Fe3+ has higher positive charge and is more polarizing, lowering the energy of its ligand-to-metal charge transfer absorptions. Thus, all the above complexes are rather strongly colored, with the single exception of the hexaquo ion – and even that has a spectrum dominated by charge transfer in the near ultraviolet region.[60] On the other hand, the pale green iron(II) hexaquo ion [Fe(H
2
O)
6
]2+
does not undergo appreciable hydrolysis. Carbon dioxide is not evolved when carbonate anions are added, which instead results in white iron(II) carbonate being precipitated out. In excess carbon dioxide this forms the slightly soluble bicarbonate, which occurs commonly in groundwater, but it oxidises quickly in air to form iron(III) oxide that accounts for the brown deposits present in a sizeable number of streams.[61]

Coordination compounds

Due to its electronic structure, iron has a very large coordination and organometallic chemistry.

The two enantiomorphs of the ferrioxalate ion

Many coordination compounds of iron are known. A typical six-coordinate anion is hexachloroferrate(III), [FeCl6]3−, found in the mixed salt tetrakis(methylammonium) hexachloroferrate(III) chloride.[62][63] Complexes with multiple bidentate ligands have geometric isomers. For example, the trans-chlorohydridobis(bis-1,2-(diphenylphosphino)ethane)iron(II) complex is used as a starting material for compounds with the Fe(dppe)
2
moiety.[64][65] The ferrioxalate ion with three oxalate ligands (shown at right) displays helical chirality with its two non-superposable geometries labelled Λ (lambda) for the left-handed screw axis and Δ (delta) for the right-handed screw axis, in line with IUPAC conventions.[60] Potassium ferrioxalate is used in chemical actinometry and along with its sodium salt undergoes photoreduction applied in old-style photographic processes. The dihydrate of iron(II) oxalate has a polymeric structure with co-planar oxalate ions bridging between iron centres with the water of crystallisation located forming the caps of each octahedron, as illustrated below.[66]

Crystal structure of iron(II) oxalate dihydrate, showing iron (gray), oxygen (red), carbon (black), and hydrogen (white) atoms.
Blood-red positive thiocyanate test for iron(III)

Iron(III) complexes are quite similar to those of chromium(III) with the exception of iron(III)'s preference for O-donor instead of N-donor ligands. The latter tend to be rather more unstable than iron(II) complexes and often dissociate in water. Many Fe–O complexes show intense colors and are used as tests for phenols or enols. For example, in the ferric chloride test, used to determine the presence of phenols, iron(III) chloride reacts with a phenol to form a deep violet complex:[60]

3 ArOH + FeCl3 → Fe(OAr)3 + 3 HCl (Ar = aryl)

Among the halide and pseudohalide complexes, fluoro complexes of iron(III) are the most stable, with the colorless [FeF5(H2O)]2− being the most stable in aqueous solution. Chloro complexes are less stable and favor tetrahedral coordination as in [FeCl4]; [FeBr4] and [FeI4] are reduced easily to iron(II). Thiocyanate is a common test for the presence of iron(III) as it forms the blood-red [Fe(SCN)(H2O)5]2+. Like manganese(II), most iron(III) complexes are high-spin, the exceptions being those with ligands that are high in the spectrochemical series such as cyanide. An example of a low-spin iron(III) complex is [Fe(CN)6]3−. The cyanide ligands may easily be detached in [Fe(CN)6]3−, and hence this complex is poisonous, unlike the iron(II) complex [Fe(CN)6]4− found in Prussian blue,[60] which does not release hydrogen cyanide except when dilute acids are added.[61] Iron shows a great variety of electronic spin states, including every possible spin quantum number value for a d-block element from 0 (diamagnetic) to ​52 (5 unpaired electrons). This value is always half the number of unpaired electrons. Complexes with zero to two unpaired electrons are considered low-spin and those with four or five are considered high-spin.[58]

Iron(II) complexes are less stable than iron(III) complexes but the preference for O-donor ligands is less marked, so that for example [Fe(NH
3
)
6
]2+
is known while [Fe(NH
3
)
6
]3+
is not. They have a tendency to be oxidized to iron(III) but this can be moderated by low pH and the specific ligands used.[61]

Organometallic compounds

Iron penta-
carbonyl

Organoiron chemistry is the study of organometallic compounds of iron, where carbon atoms are covalently bound to the metal atom. They are many and varied, including cyanide complexes, carbonyl complexes, sandwich and half-sandwich compounds.

Prussian blue

Prussian blue or "ferric ferrocyanide", Fe4[Fe(CN)6]3, is an old and well-known iron-cyanide complex, extensively used as pigment and in several other applications. Its formation can be used as a simple wet chemistry test to distinguish between aqueous solutions of Fe2+ and Fe3+ as they react (respectively) with potassium ferricyanide and potassium ferrocyanide to form Prussian blue.[54]

Another old example of organoiron compound is iron pentacarbonyl, Fe(CO)5, in which a neutral iron atom is bound to the carbon atoms of five carbon monoxide molecules. The compound can be used to make carbonyl iron powder, a highly reactive form of metallic iron. Thermolysis of iron pentacarbonyl gives triiron dodecacarbonyl, Fe
3
(CO)
12
, a with a cluster of three iron atoms at its core. Collman's reagent, disodium tetracarbonylferrate, is a useful reagent for organic chemistry; it contains iron in the −2 oxidation state. Cyclopentadienyliron dicarbonyl dimer contains iron in the rare +1 oxidation state.[67]

Structural formula of ferrocene and a powdered sample

A landmark in this field was the discovery in 1951 of the remarkably stable sandwich compound ferrocene Fe(C
5
H
5
)
2
, by Paulson and Kealy[68] and independently by Miller and others,[69] whose surprising molecular structure was determined only a year later by Woodward and Wilkinson[70] and Fischer.[71] Ferrocene is still one of the most important tools and models in this class.[72]

Iron-centered organometallic species are used as catalysts. The Knölker complex, for example, is a transfer hydrogenation catalyst for ketones.[73]

Industrial uses

The iron compounds produced on the largest scale in industry are iron(II) sulfate (FeSO4·7H2O) and iron(III) chloride (FeCl3). The former is one of the most readily available sources of iron(II), but is less stable to aerial oxidation than Mohr's salt ((NH
4
)
2
Fe(SO
4
)
2
·6H2O
). Iron(II) compounds tend to be oxidized to iron(III) compounds in the air.[54]

Iron Chemistry and compounds articles: 81