Chromium, molybdenum, and tungsten are transition metals that share several properties. They have high melting points and are relatively inert. Chromium forms protective oxide coatings and is used to make stainless steel. Molybdenum and tungsten can form a variety of oxidation states and complexes, including clusters. Notable compounds include chromates, molybdates, tungstates, and heteropolyacids containing combinations of these elements. The metals have various industrial uses related to their hardness and corrosion resistance.

1. 11
Chromium
Group
COURSE NAME: Chemistry of Transition Elements
COURSE CODE: 4023552-2

2. COURSE NAME: Chemistry of Transition Elements
COURSE CODE: 4023552-2
Introduction and General
Properties of Transition
Elements

3. 1
Chromium Group

4. 2
Oxidation states:
 The ground state electronic configuration for Cr
and Mo is d5s1 with a stable half-filled
d5configuration, whilst W has a d4s2 arrangement.
 From the electronic structures, Cr and Mo might be
expected to have oxidation states from (+I) to
(+VI), and W from (+II) to (+VI) inclusive.

5. 3
General Properties:
• The metals are hard and have very high melting points and low volatility.
• Tungsten is the element with the next highest melting point to carbon.
• Chromium is unreactive or passive at low temperatures because it is
protected by a surface coating of oxide,
• It does dissolve in HCI and H2SO4, but not in HNO3 or aqua regia, which
passivate the metal.
• Molybdenum and tungsten are relatively inert, and are only slightly
attacked by aqueous acids and alkalis

6. 4
Halides:
• Anhydrous Cr(+II) halides can be made either by reducing the tri halides
with hydrogen at 500 oC, or from the metal and HF, HCl, HBr or I2 at 600oC
• The di halides are all readily oxidized in air to the (+III) state unless
protected by an inert atmosphere.
• The (+II) halides of Mo and W do not have simple formulae, and do not
exist as simple ions.
• MoBr2 is really [Mo6Br8]Br4.2H2O, These di halides are based on an
[M6X8]4+ unit, with six electron donors added.
• The structure of the [M6X8]4+unit is an octahedral cluster of six metal atoms,
with eight bridging halogen atoms occupying the eight triangular faces of
the octahedron.

7. 5
Structure of Chromium(II) acetate
Cr2(CH3COOH)4.2H2O
Structure of [Mo6Br8]4+ ion showing the octahedral
M6metal cluster

8. 6
• Cr(+III) is the most important and stable state, related to the high crystal field
stabilization energy from its d3 electronic configuration.
• All the anhydrous MX3 halides are known, and in aqueous solution give violet
[Cr(H2O)6]3+ ions and a number ol other species such as [Cr(H2O)5Cl]2+,
[Cr(H2O)6Cl]3+ and [Cr(H2O)3Cl3].CrCl3 is a solid which forms red-violet flakes.
This is related to the layer lattice structure of the solid.
• Mo(+III) and W(+III) do not exist as simple ions. Mo(+III) compounds are fairly
stable, but slowly oxidize in air and slowly hydrolyse in water.
• With halide ions in solution, octahedral complexes like [MoCl6]3- are formed.
Two solid forms of MoCl3 are known, one with cubic close packing of chlorine
atoms, the other based on hexagonal close packing.
• WCI3 is really W6Cl18 and forms a cluster compound [W6Cl12]6+ structurally like
[Nb6Cl12]2+. W6Br18 also forms a cluster compound, but its structure contains
[W6Br8]6+ and has the same structure as [Mo6Br8]4+.

9. 7
• CrF4 and CrF5 are made by heating the elements at 350 oC and 500 oC
respectively, and CrF6 is made by heating under pressure in a bomb.
• MoCl4 exists in two forms, one like NbC14 comprising chains of octahedral
with the metal atoms displaced in pairs, forming metal-metal bonds, and the
other form without metal-metal bonds.
• MoF5 has a tetrameric structure of four octahedra joined into a ring, like NbF5
and TaF5. MoCl5 is monomeric in solution, but dimerizes to Mo2Cl10 in the
solid.
• The fluorides MoF6 and WF6 are volatile and easily hydrolyzed. They are
diamagnetic and colourless as expected for a d0 configuration. However,
WC16 is black and WBr6 is dark blue.

10. 8
Oxides:
The main oxides are
Cr2O3 is a green solid with a corundum structure. It is formed by burning the
metal in air, by heating CrO3 or by heating ammonium dichromate
(NH4)2Cr2O7 in the well-known volcano experiment used in some fireworks
The oxide is commercially important in the extraction of chromium, as apigment in paint,
rubber and cement, and as a catalyst for a wide variety ofreactions including the
manufacture of polythene and butadiene.
CrO2 is made from CrO3 by hydrothermal reduction, and has an undistorted rutile (TiO2)
structure.

11. 9
• MoO2 and WO2 are both made by reducing the trioxide with hydrogen. They
are brown-violet in colour and are insoluble in non-aqueous solvents.
• CrO3 is a bright orange solid. The colour arises from charge transfer as
Cr(+VI) has a d0 configuration.
• The oxide is prepared by adding H2SO4 to sodium dichromate Na2Cr2O7and
its structure is chains of fused tetrahedral.
• CrO3 is toxic, dissolves readily in water, and is both a very strong acid and an
oxidizing agent. It is widely used for chromium plating, and dissolved in
acetic acid as an oxidant in organic chemistry, though reactions may be
explosive.
• MoO3 and WO3 are formed by heating the metal in air. MoO3 is white as
expected for d0, but on heating it turns yellow due to the formation of defects
in the solid.

12. 10
Chromates, Molybdates and Tungstates:
• The oxides CrO3, MoO3 and WO3 are strongly acidic, and dissolve in aqueous
NaOH forming discrete tetrahedral chromate CrO4
2-, molybdate MoO4
2- and
tungstate WO4
2-ions.
CrO3 + 2NaOH → 2Na+ + CrO4
2- + H2O
• These compounds exist both in solution and as solids, but chromates are strong
oxidizing agents. Molybdates and tungstates can be reduced (for example the
blue oxides) but lack strong oxidizing properties.
• On acidifying, chromates form dichromates in which two tetrahedral units join
together.

13. 11
There are some suggestions that tri chromates and tetra chromates
could also be formed, but the dichromate ion exists in strongly acidic
solutions. In very concentrated acid, CrO3 is precioitated.

14. 12
• Na2Cr2O7 is the most important chromium compound, and is produced in the
extraction of chromium. It is used in the chrome tanning of leather and
various lead chromes, for'anodizing' aluminum, and as an oxidizing agent
• All the polyanions contain MoO6 or WO6 octahedral which are joined
together in a variety of ways by sharing corners or edges, but not faces.
The polyacids ol Mo and W are divided into two main types:
(l) Isopolyacids, where the anions which condense together are all of the same
type-for example all MoO6 groups or WO6 groups.
(2) Heteropolyacids, where .two or more different types of anion
condensetogether-for example molybdate or tungstate groups with phosphate
silicate or borate groups.

15. 13
The iso poly acids of Mo and W are not completely understood, and are
verydifficult to study because the extent of hydration and protonation of the
variousspecies in solution is not known and the fact that a solid can be
crystallized from solution does not prove that the ion has that structure or
even exists insolution.

16. 14
• Heteropoly ions are formed if a molybdate or tungstate
solution is acidified in the presence of phosphate, silicate
or metal ions.
• The second anion provides a center round which the
MoO6 or WO6 octahedra condense, by sharing oxygen
atoms with other octahedra and with the central group.
• The central groups are often oxyanions such as PO4
3-,
SiO4
4-, BO4
3-, but other elements includingAl, Ge, Sn,
As, Sb, Se, Te, I and many of the transition elements will
serve asthe second group.
• A well-known example of heteropolyacid formation is
the test for phosphates.

17. 15
A phosphate solution is warmed with ammonium molybdate and nitric acid. and a yellow
precipitate of ammonium phosphomolybdate
(NH4)3[PO4.Mo12O36] is formed.
The structures of a number of heteropolyacids have been established. In the
hetercpoivacids, for example l2-phosphotungstic acid, twelve WO6 octahedra surround a
PO4 tetrahedron.
This ion may be considered as fourgroups of three WO6 octahedra.
The 6-heteropolyacids accommodate larger central atoms, which have a coordination
number of six.
The arrangement of six MoO6 octahedra as shown leaves a
central cavity large enough to accept the octahedron from the
hetero atom, and has been found in K6[TeMo6O24].

18. 16
Tungsten Bronzes:
• Alkali metal tungstates are reduced by eating with hydrogen or tungstenand give
blue, purple, red or yellow tungsten bronzes. These are semi-metalli chave a
luster and conduct electricity, but are very inert. Their formula is MxWO3, where
M is Na or K and x is always less than one.
• This produces a defective lattice and some of the sites which should be occupied
by alkali metals are vacant. It might be thought that for each Na+ removed
fromNaWO3 one tungsten would change from W(+V) to W(+VI).
• The propertiesof the tungsten bronzes are better explained by assuming that all
the tungstenatoms are in the (+VI) state.
• The valency electrons from the alkali metals are free to move throughout
the lattice as in a metal; hence the electrical conductivity. Lithium also
forms bronzes, but these do not conduct electricity.

19. 17
Oxyhalides:
Oxyhalides of the type MO2Cl2 may be formed by dissolving the trioxide instrong acid, or
in some cases by the action of strong acids on salts such as dichromates, or by direct
addition of the halogens to the dioxide.
• Chromyl chloride CrO2Cl2 is a deep red liquid and its formation is used
inqualitative analysis to confirm the presence of chloride ions.
•
• Chromyl and molybdenyl chlorides are covalent acid chlorides and are
readily decomposed by water.
• Tungstenyl chloride hydrolyses less readily.
• A number of double salts are known, such as chrome alum K2SO4.Cr2(SO4)3.
24H2O, which crystallizes from mixed .solutions of Cr2(SO4)3 and K2SO4.

20. 18
Extraction and Uses:
• The elements have even atomic numbers, and are relatively abundant
Chromium occurs as the ore chromite FeCr2O4.
• Chromium is used in many ferrous alloys, including stainless steel, and for
these purposes, ferrochromiuman alloy containing Fe, Cr and C is produced
This is then added to other alloys as required.
When pure chromium is needed chromite is heated strongly with Na2CO3
and oxidized
The chromate is removed by dissolving in water, acidified to give the
dichromate, then precipitated and reduced with carbon

21. 19
• The commercial uses of Na2Cr2O7, CrO3 and Cr2O3 have been
mentionedpreviously.
• Molybdenum occurs as molybdenite MoS2, which is converted to MoO3 and then
reduced with hydrogen.
• Tungsten occurs in the form of tungstates, themost common being wolframite
FeWO4.MnWO4.
• Tungsten is extracted from its ores by fusion with sodium carbonate.
• The sodium tungstate is leached out and converted first to the hydrated oxide by
acidification and then to the anhydrous oxide by heating.
• Tungsten is obtained by reduction with hydrogen.
• Molybdenum and tungsten alloyed with steel make very hard alloys, whichare used
to make 'cutting steel' and machine tools.
• Molybdenum di sulphide MoS2 has a layer lattice and is an excellent lubricant.
The Cr2O3 is reduced with Al