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If the coordination compound of Ni+2 K2[Ni(CN)4] is dissolved in liquid ammonia, the addition of potassium produces yellow K4[Ni(CN)4]. The [Ni(CN)4]4- ion contains nickel in oxidation state 0. It is believed to be tetrahedral.
Carbonyls. Carbonyls Fe(CO)5, Co2(CO)8, and Ni(CO)4 are during in the action of СО on fine powders of metals. СО reaction with Ni proceeds under the mildest conditions: at atmospheric pressure, t=50—80°С. Fe and Co react with СО at 150—200 °С and pressure 10 and 25 МPа, respectively.
All carbonyls are diamagnetics, since СО molecules are strong field ligands whereupon valence d-electrons of a central ion of metal are paired. These unshared electron pairs of the central ion form p-bonds with CO molecules; s-bonds are formed due to unshared electron pairs of CO molecules and free orbital’s of the central ion. If an element has a single electron when all other electrons are paired, it means that the formation of binuclear cluster coordination compound is expected.
 π-bond σ-bond
dsp3-hybridisation
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sp3-hybridisation | ||||||||
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Fe(CO)5 is a yellow liquid, tm. = –20 °С, tb. = +103 °С;
Со2(CO)8 is a оrange crystalline solid, tm.= +51 °С;
Ni(CO)4 is a colorless liquid, tm. = –19 °С, tb. = 43 °С.
Carbonyls are inflammable substances that ignite easily in the air forming metal oxide (for instance, Fe2O3) and СО2. Heating carbonyls at atmospheric pressure without air leads to their decomposition. Products of the reaction in this case are pure metal and CO and thereby this is an industrial process of fine powders of Fe, Co, and Ni of high purity production.
Fe(CO)5 reacts with alkalis in alcoholic solutions:
Fe(CO)5 + 4KOH = K2[Fe(CO)4] + K2CO3 +2H2O.
Сompounds М¢2[Fe(CO)4] are close to salts. Even the acid that corresponds to this salt can be obtained:
Fe(CO)5 + Ba(OH)2 ® BaCO3 + H2[Fe(CO)4].
Carbonyl Со2(CO)8 can be also reduced: carbonyl-hydride is formed under the action of hydrogen:
Со2(CO)8 + Н2 ® 2Н[Со(CO)4].
This acid reacts with alkalis forming salts where oxidation state of cobalt is -1, forming K[Со(CO)4].
Iron. Iron forms the carbonyls Fe(CO)5, Fe2(CO)9 and Fe3(CO)12. In iron pentacarbonyl the iron(0) is 5-coordinated, as shown in the Figure below to give a trigonal bipyramid; Donation of an electron pair by each CO ligand gives the configuration of the nearest noble gas to iron of the coordination compound. Therefore, the [Fe(CO)4]2- ion and some octahedral carbonyl halides Fe(CO)4X2 (X – Cl, Br, I) are known.
Cobalt. Cobalt(0) has an odd number of electrons, and forms no simple carbonyl in this oxidation state. However, carbonyls of formulae Co2(CO)8, Co4(CO)12 and Co6(CO)16 are known. Co2(CO)8 is important catalyst for organic syntheses. In the so-called oxo reaction, where an alkene reacts with carbon monoxide and hydrogen, under pressure, to give an aldehyde, dicobalt octacarbonyl is used as catalyst:
Nickel. Nickel tetracarbonyl Ni(CO)4 was the first metal carbonyl to be discovered, by Mond in 1890. It is obtained by passing of carbon monoxide over nickel metal heated to 320 K. It is a volatile, toxic liquid (b.p. 315 K), and has a tetrahedral structure. It has considerable stability, but inflames in the air; More systematically named butanedione dioxime. there is some double bonding between the nickel and carbon atoms, e.g.
or 
Compounds +2
Oxides FeO, СоО і NiO can be prepared as follows:
Fe2O3 + Н2 
2FeO + H2O,
FeC2O4 FeO + CO + CO2.
All oxides are unstable against oxygen attack:
4FeO + O2 = 2Fe2O3 (excess of O2)
6СоO + O2 = 2Со3O4 (400-900°С)
NiO also absorbs O2 from the air, but this process gives no additional compounds of Ni.
ЕО oxides have basic properties. They are readily soluble in strong acids, don’t react with alkalis and water. СоО is a single exception because it reacts partially with concentrated alkalis.
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Hydroxydes М(ОН)2 can be obtained by double exchange reactions of Me(II) with alkalis. White-greenish Fe(ОН)2 is formed in the absence of O2. Usually precipitate formed contains Fe(ІІ) and Fe(ІІІ). When solutions of alkalis act on solutions of Со(ІІ) salts the blue precipitate of basic salts forms at the beginning The basic salts are gradually transformed into rose Со(ОН)2 under the action of an alkali in excess:
Co(NO3)2 + NaOH 
(CoOH)NO3¯ + NaNO3,
(CoOH)NO3+ NaOH 
Co(OH)2 + NaNO3.
Fe(OH)2, Co(OH)2 and Ni(OH)2 have basic properties:
Ni(OH)2 + 2HCl = NiCl2 + 2H2O.
Iron(II) Compounds
Iron(II) salts are formed when iron is dissolved in dilute acids except for nitric acid. The most important of them is iron(II) sulfate or green vitriol FeSO4 ·7H2O forming light green crystals well soluble in water. Iron(II) sulfate gradually darkens in air and is simultaneously oxidized from its surface, transforming into the yellow-brown basic salt of iron(III).
Iron(II) sulfate is produced by dissolving steel shavings in 20—30% sulfuric acid:
Fe + H2SO4 = FeSO4 + H2↑.
When green vitriol is heated, water is released and a white mass of the anhydrous salt FeSO4 is obtained. At temperatures above 480 °C, the anhydrous salt decomposes with the liberation of sulfur dioxide and trioxide; the latter forms a heavy white vapour of sulfuric acid in humid air:
2FeSO4 = Fe2O3 +SO2↑ + SO3↑.
When a solution of an iron(II) salt reacts with an alkali, a white precipitate of iron(II) hydroxide Fe(OH)2 is formed, which in the air owing to oxidation rapidly acquires a greenish, and then a brown colour, transforming into iron(III) hydroxide Fe(OH)3:
4Fe(OH)2 +O2 + 2H2O = 4Fe(OH)3.
Anhydrous iron(II) oxide FeO can be prepared as a black readily oxidizable powder by reducing iron(III) oxide with carbon monoxide at 500 °C:
Fe2O3 +CO = 2FeO + CO2.
Alkali metal carbonates precipitate white iron(II) carbonate FeCO3 from solutions of iron(II) salts. In water containing CO2, iron(II) carbonate, like calcium carbonate, partly transforms into the more soluble acid salt Fe(HCO3)2. Iron is contained in natural ferruginous mineral waters in the form of this salt.
Iron(II) salts can readily be transformed into iron(III) salts during their reaction with various oxidizing agents—nitric acid, potassium permanganate, or chlorine, for instance:
6FeSO4 + 2HNO3 + 3H2SO4 = 3Fe2(SO4)3 + 2NO↑ + 4H2O.
10FeSO4 + 2KMnO4 + 8H2SO4 = 5Fe2(SO4)3 + K2SO4 + 2MnSO4 + 8H2O.
Owing to their ability of being readily oxidized, iron(II) salts are often used as reducing agents.
Double salts of general formula M2SO4.FeSO4.6H2O (M = alkali metal or ammonium) can be obtained by crystallisation of solutions containing the appropriate proportions of the two simple salts: an acid solution of the salt with M = NH4 (Mohr's salt, ferrous ammonium sulfate) is considerably less quickly oxidised by the air than is the simple iron(II) sulfate solution, and hence is used in volumetric analysis.
Iron(II)sulfide, FeS, may be prepared by heating the elements together, or by precipitation from an iron(II) solution by sulfide ion; it is a black solid which is non-stoichiometric, like the oxide. The yellow sulfide FeS2 (made up essentially of Fe2 + and 
ions) occurs naturally as pyrites.
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