Transition Metals

Question 1

Characteristics of transition metals?

Why do these occur?

Answer 1

-complex formation
-catalytic activity
-formation of coloured ions
-variable oxidation states

-occur due to incomplete d subshell in the atom/ion

Question 2

Define transition metals

Answer 2

elements with an incomplete d-subshell that can form at least one stable ion with an incomplete d-subshell

Question 3

Why is zinc not a transition metal

Answer 3

can only form 2+ ion, which has a complete d subshell

Question 4

Define complex

Answer 4

a central metal ion surrounded by ligands

Question 5

Define ligand

Answer 5

: An atom, ion or molecule which can donatea lone electronpair.

Question 6

Define coordination number

Answer 6

number of co-ordinate bonds formed to a central metal ion.

Question 7

electron configuration of chromium

Answer 7

[Ar] 3d5 4s1

not [Ar] 3d4 4s2

Question 8

electron configuration of copper

Answer 8

[Ar] 3d10 4s1 not [Ar] 3d9 4s2

Question 9

What are monodentate ligands?

Give examples

Answer 9

-ligands that can only form one dative bond to the central metal ion

e.g.
-water (H2O) molecules
-ammonia (NH3) molecules
-chloride (Cl–) ions
-cyanide (CN–) ions

Question 10

What are bidentate ligands?

Give examples

Answer 10

-ligands that can each form two dative bonds to the central metal ion
-due to each ligand having two atoms with lone pairs of electrons

e.g.
-1,2-diaminoethane (H2NCH2CH2NH2)
-also written as ‘en’
-ethanedioate ion (C2O42-)
-also written as ‘ox’

Question 11

What are multidentate ligands?

Give examples

Answer 11

-ligands with more than two atoms with lone pairs of electrons
-so can form more than two dative bonds

e.g.
-EDTA4- (hexadentate ligand as it forms 6 dative covalent bonds to the central metal ion)

Question 12

Complexes with water & ammonia molecules

Answer 12

Water and ammonia molecules are examples of neutral ligands
Both ligands contain a lone pair of electrons which can be used to form a dative covalent bond with the central metal ion
In water, this is the lone pair on the oxygen atom
In ammonia, it is the lone pair on the nitrogen atom
Since water and ammonia are small ligands, 6 of them can usually fit around a central metal ion, each donating a lone pair of electrons, forming 6 dative bonds
Since there are 6 dative bonds, the coordination number for the complex is 6
The overall charge of a complex is the sum of the charge on the central metal ion, and the charges on each of the ligands
A complex with cobalt(II) or chromium(II) as a central metal ion, and water or ammonia molecules as ligands, will have an overall charge of 2+
The central metal ion has a 2+ charge and the ligands are neutral

Question 13

Complexes with hydroxide & chloride ions

Answer 13

Hydroxide and chloride ions are examples of negatively charged ligands
Both ligands contain a lone pair of electrons which can be used to form a dative covalent bond with the central metal ion
Hydroxide ligands are small, so 6 of them can fit around a central metal ion and the complex formed will have a coordination number of 6
Chloride ligands are large ligands, so only 4 of them will fit around a central metal ion
Complexes with 4 chloride ligands will have a coordination number of 4
A complex with cobalt(II) or copper(II) as a central metal ion and chloride ions as ligands, will have an overall charge of 2-
The central metal ion has a charge of 2+
Each chloride ligand has a charge of 1-
There are 4 chloride ligands in the complex, so the overall negative charge is 4-
The overall positive charge is 2+
Therefore, the overall charge of the complex is 2-

A complex with chromium(III) as a central metal ion and hydroxide ions as ligands, will have an overall charge of 3-
The central metal ion has a charge of 3+
Each hydroxide ligand has a charge of 1-
There are 6 hydroxide ligands in the complex, so the overall negative charge is 6-
The overall positive charge is 3+
Therefore, the overall charge on the complex is -3

Question 14

when do comlexes have a linear shape

Answer 14

Central metal atoms or ions with two coordinate bonds form linear complexes
The bond angles in these complexes are 180o
The most common examples are a copper (I) ion, (Cu+), or a silver (I) ion, (Ag+), as the central metal ion with two coordinate bonds formed to two ammonia ligands

The second example is the diamminesilver(I) ion, [Ag(NH₃)₂]⁺, which is present in Tollens’ reagent
Tollens’ reagent is used to test for the aldehyde functional group in organic molecules
In the test, the silver(I) ion is reduced to silver atoms that produce a characteristic silver mirror on the test tube walls

Question 15

when do comlexes have a tetrahedral shape

Answer 15

When there are four coordinate bonds the complexes often have a tetrahedral shape
Complexes with four chloride ions most commonly adopt this geometry
Chloride ligands are large, so only four will fit around the central metal ion
The bond angles in tetrahedral complexes are 109.5o

Question 16

when do comlexes have a square planar shape

Answer 16

Sometimes, complexes with four coordinate bonds may adopt a square planar geometry instead of a tetrahedral one
Cyanide ions (CN-) are the most common ligands to adopt this geometry
An example of a square planar complex is cisplatin
The bond angles in a square planar complex are 90o

Question 17

when do comlexes have a octahedral shape

Answer 17

Octahedral complexes are formed when a central metal atom or ion forms six coordinate bonds
This could be six coordinate bonds with six small, monodentate ligands
Examples of such ligands are water and ammonia molecules and hydroxide and thiocyanate ions
It could be six coordinate bonds with three bidentate ligands
Each bidentate ligand will form two coordinate bonds, meaning six coordinate bonds in total
Examples of these ligands are 1,2-diaminoethane and the ethanedioate ion
It could be six coordinate bonds with one multidentate ligand
The multidentate ligand, for example EDTA4-, forms all six coordinate bonds
The bond angles in an octahedral complex are 90o

Question 18

geometric

Answer 18

Geometrical (cis-trans) isomerism
Even though transition element complexes do not have a double bond, they can still have geometrical isomers
Square planar and octahedral complexes with two pairs of different ligands exhibit cis-trans isomerism (this is a special case of E-Z isomerism)
An example of a square planar complex with two pairs of ligands is the anti-cancer drug cis-platin
Whereas cis-platin has beneficial medical effects by binding to DNA in cancer cells, trans-platin cannot be used in cancer treatment
As long as a complex ion has two ligands attached to it that are different to the rest, then the complex can display geometric isomerism
Examples of octahedral complexes that exhibit geometrical isomerism are the [Cu(NH3)4(H2O)2]2+ and [Ni(H2NCH2CH2NH2)2Cl2]2+ complexes
[Ni(H2NCH2CH2NH2)2Cl2]2+ can also be written as [Ni(en)2Cl2]2+
Like in the square planar complexes, if the two ‘different’ ligands are adjacent (next) to each other then that is the ‘cis’ isomer, and if the two ‘different’ ligands are opposite each other then this is the ‘trans’ isomer
In [Cu(NH3)4(H2O)2]2+, the two water ligands are adjacent to each other in the cis isomer and are opposite each other in the trans isomer

Question 19

optical

Answer 19

Optical isomerism
Octahedral complexes with bidentate ligands also have optical isomers
This means that the two forms are non-superimposable mirror images of each other
They have no plane of symmetry, and one image cannot be placed directly on top of the other
The optical isomers only differ in their ability to rotate the plane of polarised light in opposite directions
Examples of octahedral complexes that have optical isomers are the [Ni(H2NCH2CH2NH2)3]2+and [Ni(H2NCH2CH2NH2)2(H2O)2]2+ complexes
The ligand H2NCH2CH2NH2 can also be written as ‘en’ instead

Question 20

drawing

Answer 20

Drawing stereochemical formulae
Chemists use a convention of wedge drawings to represent three dimensional molecules
The convention is that
a solid line is a bond in the same plane as the paper
a dotted line is a bond receding behind the plane of the paper(this can also be hatched or shaded wedges)
a solid wedge is a bond coming out of the paper

Question 21

What is ligand substitution

Answer 21

when one ligand in a complex is replaced by another
Ligand exchange forms a new complex that is more stable than the original one
The ligands in the original complex can be partially or entirely substituted by others
The complex ion can change its charge or remain the same depending on the ligand involved
There are no changes in coordination number, or the geometry of the complex, if the ligands are of a similar size
But, if the ligands are of a different size, for example water ligands and chloride ligands, then a change in coordination number and the geometry of the complex will occur

Question 22

Complete substitution without change in coordination number in cobalt(II) complexes

Answer 22

The [Co(H2O)6]2+(aq) complex ion is pink in colour
If ammonia solution is added to [Co(H2O)6]2+, a pale yellow / straw coloured solution will be formed
Complete ligand substitution of the water ligands by ammonia ligands has occurred
[Co(H2O)6]2+ (aq)
+ 6NH3 (aq) → [Co(NH3)6 ]2+ (aq) + 6H2O (l)
pink solution yellow solution

If excess concentrated ammonia solution is added to [Co(H2O)6]2+, a brown solution will be formed
The ammonia ligands make the cobalt(II) ion so unstable that it readily gets oxidised in air to cobalt(III), [Co(NH3)6]3+ (aq)
Upon dropwise addition of sodium hydroxide (NaOH) solution to [Co(H2O)6]2+(aq), a blue precipitate is formed
Partial ligand substitution of two water ligands by two hydroxide (OH-) ligands has occurred
[Co(H2O)6]2+ (aq)
+ 2OH- (aq) → Co(OH)2(H2O)4 (s) + 2H2O (l)
pink solution blue precipitate

Question 23

cause of incomplete ligand substitution

Answer 23

the energetics of the reaction and stability of the product are not favourable
Copper(II)ions illustrate this behaviour with ammonia
Different sized ligands can also lead to incomplete substitution

Incomplete substitution in copper(II) complexes
When a transition element ion is in solution, the most common arrangement is a hexaaqua complex ion (i.e. it has six water ligands attached to it)
For example, Cu2+(aq) is [Cu(H2O)6]2+(aq)
The [Cu(H2O)6]2+ (aq) complex ion is pale blue in colour
Upon dropwise addition of sodium hydroxide (NaOH) solution, a light blue precipitate is formed
Partial ligand substitution of two water ligands by two hydroxide ligands has occurred
[Cu(H2O)6]2+ (aq) + 2OH- (aq) → Cu(OH)2(H2O)4 (s) + 2H2O (l)
blue solution light blue precipitate
Upon addition of excess concentrated ammonia (NH3) solution, the pale blue precipitate dissolves to form a deep blue solution
Again, partial ligand substitution has occurred
Cu(OH)2(H2O)4 (s) + 4NH3 (aq) → [Cu(NH3)4(H2O)2 ]2+ (aq) + 2H2O (l) + 2OH- (aq)
light blue precipitate deep blue solution
If you were to add concentrated ammonia (NH3) solution dropwise to the [Cu(H2O)6]2+ (aq), rather than sodium hydroxide (NaOH) solution, the same light blue precipitate would form
Again, the pale blue precipitate will dissolve to form a deep blue solution, if excess ammonia solution is then added

Change in co-ordination number

The water ligands in [Cu(H2O)6]2+ can also be substituted by chloride ligands, upon addition of concentrated hydrochloric acid (HCl)
The complete substitution of the water ligands causes the blue solution to turn yellow
[Cu(H2O)6]2+ (aq) + 4Cl- (aq) → [CuCl4 ]2- (aq) + 6H2O (l)
blue solution yellow solution

The coordination number has changed from 6 to 4, because the chloride ligands are larger than the water ligands, so only 4 will fit around the central metal ion
This is a reversible reaction, and some of the [Cu(H2O)6]2+ complex ion will still be present in the solution
The mixture of blue and yellow solutions in the reaction mixture will give it a green colour
Adding water to the solution will cause the chloride ligands to be displaced by the water molecules, and the [Cu(H2O)6]2+ (aq) ion and blue solution will return

Incomplete substitution in cobalt(II) complexes
The water ligands in [Co[H2O)6]2+ can also be substituted by chloride ligands, upon addition of concentrated hydrochloric acid
The complete substitution of the water ligands causes the pink solution to turn blue
[Co(H2O)6]2+ (aq) + 4Cl- (aq) → [CoCl4 ]2- (aq) + 6H2O (l)
pink solution blue solution
Like with [Cu(H2O)6]2+ above, the coordination number has changed from 6 to 4, because the chloride ligands are larger than the water ligands, so only 4 will fit around the central metal ion
Adding water to the solution will cause the chloride ligands to be displaced by the water molecules, and the [Co(H2O)6]2+ (aq) ion and pink solution will return

Question 24

The Haem Complex

Answer 24

Haemoglobin is one of nature’s complexes using a transition metal ion
The haem molecule is a complex with iron(II) at its centre
Oxygen atoms form a dative covalent bond with the Fe(II) which enables oxygen molecules to be transported around the body in the blood

Oxygen molecules are not very good ligands and bond weakly to the iron(II)
The weak bonds allows them to break off easily and be transported into cells

Carbon monoxide is toxic because it is a better ligand than oxygen and binds strongly and irreversibly to the iron(II) preventing oxygen from being carried to the cells
If oxygen attached to the haemoglobin (oxyhaemoglobin) is replaced by carbon monoxide (carboxyhaemoglobin), a darker red colour is produced in the haem complex
A sign of carbon monoxide poisoning
The condition anaemia occurs when a person does not have enough haemoglobin in their blood due to a loss of blood or deficiency in iron
Deficiency in iron can be restored by taking iron sulfate tables in the diet

Question 25

chelate effect

Answer 25

The replacement of monodentate ligands with bidentate and multidentate ligands in complex ions is called the chelate effect
It is an energetically favourable reaction, meaning that ΔGꝋ is negative
The driving force behind the reaction is entropy
The Gibbs equation reminds us of the link between enthalpy and entropy:
ΔGꝋ = ΔHreactionꝋ – TΔSsystemꝋ

Reactions in solution between aqueous ions usually come with relatively small enthalpy changes
However, the entropy changes are always positive in chelation because the reactions produce a net increase in the number of particles
A small enthalpy change and relative large positive entropy change generally ensures that the overall free energy change is negative
For example, when EDTA chelates with aqueous cobalt(II) two reactants becomes seven product species
[Co(H2O)6 ]2+ (aq) + EDTA4- (aq) → [CoEDTA]2- (aq) + 6H2O (l)