The elements of the first transition series

Chapter 24

The metals of the first transition series


The metals of the first transition series show variable valency. The metals arehard Refractoryelectropositivegood conductors of heat and electricityexception : Cu, soft ,ductile metal, relatively noble second only to Ag as a conductor of heat and electricity


Mn and Fe are attacked fairly readily others are generally unreactive at room temperature All react on heating with halogens, sulfur, and other nonmetals The carbides, nitrides, and borides are commonly nonstoichiometric,interstitial,hard, refractory


The Lower Oxidation States


The chemistry of the metals can be classified on the basis of this table ; for example, the d6 series is V‐I, Cro, MnI, FeII, CoIII, and Ni1v

Comparisons of this kind can occasionally emphasize similarities in spectra and magnetic properties differences in properties of the dn species due to differences in the nature of the metal its energy levels, and especially the charge on the ion, often exceed the similarities.


The oxidation states less than II. With the exception of copper, where copper(I) binary compounds and complexes and the Cu+ ion are known, the chemistry of the I, 0, ‐I, and ‐II formal oxidation states is entirely concerned with:

(a) π‐Acid ligands such as CO, NO, PR3, CN‐, and bpy.

(b) Organometallic chemistry in which alkenes, acetylenes, or aromatic systems, such as benzene, are bound to the metal.


The II oxidation state. Binary compounds in this state are usually ionic. The metal oxides are basic have the NaCl structure are often nonstoichiometric, particularly for Ti, V, and Fe The aqua ions [M(H2O)6]2+, except for the unknown Ti2+ ion, are well characterized in solution and in crystalline solids.


V2+, Cr2+, and Fe2+ ions are oxidized by air in acidic solution


halide hydrates do not contain the aqua ion. VCI2 .4H2 O is trans‐VCl2 (H2O)4 and MnCI2.4H2O is a polymer with cis‐MnCI2 (H2O)4 units; the diaqua species of Mn, Fe, Co, Ni, and Cu have a linear polymeric edge‐shared chain structure with trans‐[MCl4(H2O) 2] octahedra.The FeCI2 .6H2O compound,contains trans‐FeCl2 (H2O)4units. The water molecules of [M(H2O)6]2+ can be displaced by ligands such as NH3, en, EDTA4‐, CN‐, and acac .


complexes may be Cationicneutralanionic depending on the charge of the ligandsAddition of OH‐ to the M2+ solutions gives hydroxides Some can be crystalisedE.g. Fe(OH)2 , Ni(OH)2 With HCO3 ,the carbonates of Mn, Fe, Co, Ni, and Cu are precipitated.


The III Oxidation State All of the elements form at least some compounds in this state Exception: Cuonly a few complexes, not stable toward water, are known The fluorides (MF3) and oxides (M20 3) are generally ionic Chlorides, bromides, and iodides , sulfides and similar compoundshave considerable covalent character.


The elements Ti to Co form octahedral ions, [M(H2O)6J 3 +. The C03 + and Mn3+ ions are reduced by water Ti3+ and V3+ ions are oxidized by air. In aqueous solution high acidities are required to prevent hydrolysis


Crystalline chlorides of V, Fe, and Cr are of the type trans‐[VCI2 (H2O)4]+Cl‐∙2H2OThe alums CsTi(SO4)2.12H2O, or KV(SO4)2.12H2O contain the hexaaqua ion .hydrates like Fe(ClO4)3∙10H2O. There are many anionic, cationic, or neutral MIll

complexes, which are mostly octahedral. CrIll and CoIll, hundreds of octahedral complexes that are substitutionally inert are known


Representative octahedral complexes are[TiF6]3‐

[V(CN)6]3‐

Cr(acac)3[Co(NH3)6]3+

The halides (MX3) act as Lewis acids and form adducts, andionic species [VCI4]‐,[CrCI4] ‐


A special feature of the M3+ ions is the formation of basic carboxylates in which an O atom is in the center of a triangle of metal atoms This oxo‐centered unit has been proved for carboxylates ofV, Cr, Mn, Fe, Co, Ru, Rh, and Ir.


The IV and Higher Oxidation States The IV state is the most important state for Ti main chemistry is that ofTiO2 and TiCl4 and derivatives VCI4, the main VIV chemistry is that of the oxovanadium(IV) or vanadyl ion VO2+. ion can behave like an M2+ ion,forms many complexes cationic, neutral, or anionic, depending on the ligand. For the remaining elements, the IV oxidation state is not very common or well established except in fluorides, fluorocomplex ions, oxo anions, and a few complexes.


The oxidation states V and above are known forV, Cr, Mn, and Fe fluorides,fluorocomplexesoxo anions CrF5

KMn04

K2FeO4

All are powerful oxidizing agents.


TITANIUM Titanium has the electronic structure 3d 24s2

The energy of removal of four electrons is so high that the Ti4+ion may not exist titanium(IV) compounds are covalent some resemblances between Ti1v and SnIV and their radii are similar.TiO2 (rutile) is isomorphous with SnO2 (cassiterite) and is similarly yellow when hot


Titanium tetrachloride, like SnCI4 isa distillable liquidreadily hydrolyzed by waterbehaving as a Lewis acid The bromide and iodide, form crystalline molecular latticesare isomorphous with the corresponding Group IVB halides


Titanium tetrachloride, a colorless liquid (bp 136°C), has a pungent odor,fumes strongly in moist air,and is vigorously, though not violently, hydrolyzed by water.

Partially hydrolyzed species are formed with a deficit of water or on addition of TiCl4 to aqueous HC!.


Titanium oxide has three crystal forms‐rutile,anatase,Brookiteall occur in nature. The dioxide that is used in large quantities as a white pigment in paints is made by vapor phase oxidation of TiCl4 with oxygen. The precipitates obtained by addition of OH‐to TiN solutions are best regarded as hydrous TiO2, not a true hydroxide. This material is amphoteric and dissolves in concentrated NaOH.


VANADIUM Vanadium is widely distributed but there are few concentrated deposits.Occurs in petroleum from Venezuela,is recovered as V2O5 from flue dusts after combustion. Very pure vanadium is rare,is quite reactive toward O2, N2, and C at the elevated temperatures Chief commercial use is in alloy steels and cast iron, gives ductility and shock resistance, commercial production mainly as an iron alloy, ferrovanadium. Vanadium metal is not attacked by air, alkalis, or nonoxidizing acids other than HF at room temperature. It dissolves in HNO3, concentrated H2SO4, and aqua regia.


CHROMIUMThe chief ore is chromite, FeCr2O 4, which is a spinel . It is reduced by C to the carbon‐containing alloy ferrochromium. FeCr2O 4 +4C Fe∙2Cr+ 4 COWhen pure Cr is required, the chromite is first treated with molten NaOH and O2 to convert the CrIll to CrO4

2‐. The melt is dissolved in water and sodium dichromate is precipitated. This precipitate is then reduced


Chromium is resistant to corrosion, hence its use as an electroplated protective coating. It dissolvesfairlyreadily inHCI, H2SO4,andHCIO4,but it is passivated by HNO3.


Acid solutions of dichromate are strong oxidants:

In alkaline solution, the chromate ion is much less oxidizing:


MANGANESE Manganese is relatively abundant,occurs in substantial deposits, mainly oxides, hydrous oxides, or the carbonate. From them, the metal can be obtained by reduction with AI. Manganese is quite electropositive, and readily dissolves in dilute, nonoxidizing acids.


The most common compound of MnlV is manganese dioxide, a gray to black solid found in nature as pyrolusiteManganese dioxide is inert to most acids except when heated, but it does not dissolve to give MnlV in solution; instead, it functions as an oxidizing agent, the exact manner of this depending on the acid. With HCI, chlorine is evolved.


Solutions of permanganate are intrinsically unstable, decomposing slowly but observably in acid solution.

In basic solution, permanganate is a powerful oxidant.


The addition ofKMn04 to concentrated H2SO4 gives stoichiometrically:

the dangerous explosive oil, Mn2O7.This can be extracted into CCl4 or chlorofluorocarbons in which it is reasonably stable and safe.


IRON Iron is the second most abundant metal, after AI, and the fourth most abundant element in the earth's crust. The core of the earth is believed to consist mainly of Fe and Ni. The major ores areFe2O3 (hematite), Fe3O4 (magnetite), FeO(OH) (limonite),FeCO3 (siderite).


Pure iron is quite reactive. In moist air it is rather rapidly oxidized to give a hydrous iron(III) oxide (rust) The metal dissolves readily in dilute mineral acids With nonoxidizing acids and in the absence of air, Fell is obtained Very strongly oxidizing media, such as concentrated HNO3or acids containing dichromate, passivate iron Iron is attacked by hot concentrated NaOH


The addition of OH‐to Fe2+ solutions gives the pale green hydroxide, which is very readily oxidized by air to give red‐brown hydrous iron (III) oxide. The iron (II) ion, [Fe(H2O)6]2+, gives many crystalline salts. Mohr's salt, (NH4)2[Fe(H2O)6] (SO4)2, is reasonably stable toward air and loss of water, is commonly used to prepare standard solutions of Fe2+ for volumetric analysis and as a calibration substance in magnetic measurements.


COBALT The trends toward decreased stability of the very high oxidation states and the increased stability of the II state relative to the III stateseries Ti, V, Cr, Mn, and Fe, persist with Co.The highest oxidation state is now IV, and only a few such compounds are known. Cobalt(III) is relatively unstable in simple compounds, Addition of OH‐to C02+ hydroxide, blue or pink depending on the conditions;weakly amphoteric dissolving in very concentrated OH‐ to give a blue solution containing the [CO(OH)4]2‐ ion.


Cobalt always occurs in association with Ni and will usually occur also with As. The chief sources of Co are "speisses," which are residues in the smelting of arsenical ores of Ni, Cu, and Pb. Cobalt is relatively unreactive, although it dissolves slowly in dilute mineral acids.


NICKEL Nickel occurs in combination with arsenic, antimony, and sulfur e.g. millerite (NiS) garnierite, a magnesium‐nickel silicate Nickel is also found alloyed with iron in meteors; the interior of the earth considerable quantities.The ore is roasted in air to give NiO, Reduced to Ni with C. Nickel is usually purified by electrodeposition


Nickel is quite resistant to attack by air or waterat ordinary temperatures when compact electroplated as a protective coating.It dissolves readily in dilute mineral acids. The metal or high Ni alloys are used to handle F2 and other corrosive fluorides. Nickel absorbs considerable amounts of hydrogen when finely divided and special forms ofNi (e.g., Raney nickel) are used for catalytic reductions. The Chemistry of Nickel(II), d8

The binary compounds, such as NiO and NiCI2, need no special comment. Nickel(II) forms a large number of complexes with coordination numbers six, five, and four.


COPPER single s electron outside the filled 3d shell. little in common with the alkalis formal stoichiometries in the +1 oxidation state.Filled d shell less effective than noble gas shell in shielding the s electron from nuclear charge,first ionization potential of Cu is higher than in the alkalid shell electrons involved in metallic bonding,heat of sublimation and melting point of copper are also much higher than those of the alkalis.


Most CuI compounds are fairly readily oxidized to Cull, but further oxidation to CuIII is difficult. There is a well‐defined aqueous chemistry of Cu2+

large number of salts of various anions,

many are water soluble,

exist in addition to a wealth of complexes.


Binary Compounds Black crystalline CuO is obtained by pyrolysis of the nitrate or other oxo salts; above 800 °c it decomposes to Cu2O. hydroxide is obtained as a blue bulky precipitate on addition of NaOH to Cu2+ solutions soluble in strong acidsconcentrated NaOHdeep blue anionshalides are the yellow chloride and the almost black bromideinfinite parallel bands of square CuX4 units sharing edges


Catalytic Properties of Copper Compounds Copper compounds catalyze reactions, heterogeneously, homogeneously, in the vapor phase, in organic solvents, and in aqueous solutions.Many of these reactions, particularly if in aqueous solutions, involve oxidation‐reduction systems and CUI‐CUII redox cycle.Molecular oxygen oxidant, e.g. copper‐catalyzed oxidations of ascorbic acidCu+ + O2 =CuO2

+

CuO2+ + H+ = Cu2 + + HO2 CU+ + HO2= Cu2+ + HO2

‐

H+ + HO2‐= H2O 2

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The elements of the first transition series