23.2 Complex formation and shape of complex ions

Question 1

What bonds can all transition metals form and how

Answer 1

. Co-ordinate bonds: When one atom in a covalent bond provides both the electrons.
By accepting electron pairs from other ions or molecules.
So the bonds that form are dative/coordinate bonds.

Question 2

What is a ligand

Give 4 examples

Answer 2

A ion or molecule with a lone pair of electrons, that forms a co-ordinate bond with a transition metal.

H20: :NH3 :Cl- :CN-

Question 3

What is a complex ion

What is its coordination number

Answer 3

When 2, 4 or 6 ligands bond to a transition metal ion.

Its coordination number is the number of co-ordinate bonds to ligands that surround the d block metal ion

Question 4

What is the shape called when 6 ligands bond to a transition metal ion

What are the angles?
What is the coordination number

Answer 4

Octahedral shape
90 degrees
They have six as a coordination number.

Eg [Co(NH2)6]3+

Question 5

What are the two possible shapes for a transition metal ion with 4 ligands.
And what are the angles in these
What would the coordination number be

Answer 5

. Tetrahedral in most cases
Eg Co(Cl4)2-
Angles are 109.5 degrees

However with the metals nickel and platinum, they form square planar
Eg [NiCN4]2-
Angles are 90 degrees

Question 6

What shape are transition metal ions with two ligands attached?
What are the angles
What is the coordination number
What ion does this occur in

Answer 6

Linear shape
180 degrees
Coordination number is 2

This occurs in Ag+ complexes

Question 7

What are aqua ions

Answer 7

. If you dissolve the salt of a transition metal in water, eg copper sulfate, the positively charged metal ion becomes surrounded by water molecules acting as ligands.
Normally there are 6 water molecules in an octahedral arrangement.

The oxygen from the water bonds to the metal ion

Question 8

What are unidentate ligands

Answer 8

. A ligand (particle with a lone pair of electrons that bonds to metals by dative bond) that forms one co-ordinate bond to a metal ion.

Question 9

What are multidentate ligands

Answer 9

• Ligands that form two or more
  co-ordinate bonds to a metal ion.

They have more than one atom with a lone pair of electrons which can bond to a transition metal ion.

Question 10

An example of a bidentate ligand is
Ethene-1,2-diamine, and it binds to chromium 3+ ion.
Describe the structure of the ligand

And forms complex
(Cr(NH2CH2CH2NH2)3 3+

Explain how this occurs

Answer 10

. Each nitrogen has a lone pair of electrons which can form a co-ordinate bond to the metal ion, there are 2 nitrogen’s on each ligand.

. So overall there are 3 ligands that can bind to the chromium ion as each can form 2 coordinate bonds

H2N: - CH2 - CH2 - :NH2

It is a neutral ligand and the chromium ion has a 3+ charge so the complex ion has an overall 3+ charge

Question 11

What is the structure of the bidentate ligand
ethanedioate

Bonds with chromium 3+ also, so what is this complex

Answer 11

(C2O4)2- so the ligand has a 2- charge

[Cr(C2O4)3] 3-
It is overall 3- because each ligand brings the overall charge of the 3+ down to 3-

Question 12

What is the bidentate ligand
Benzene 1,2-diol

What is the formula

Answer 12

Benzene 1,2 diol is a neutral ligand also

Has two oxygens on it, each with a lone pair to form co-ordinate bond

C6H6O2
Is the formula of it
But it is shaped as a hexagon ring

Question 13

What is the multidentate ligand EDTA4-
And how many bonds does it form
What ligand does it act as

Answer 13

. It forms 6 bonds
And it acts as a hexadentate ligand.

It uses lone pairs on 4 oxygen and both nitrogen atoms

Question 14

What are complex ions with multidentate ligands

What are they used for

Answer 14

Called chalates

Can be used to effectively remove d block metal ions from a solution.

Question 15

What is the chelate effect

(when EDTA4- is added to an aqua ion)

Answer 15

. When a hexadentate ligand eg EDTA is added to a solution of a transition metal salt, the EDTA will replace all six water ligands in the aqua ion

[Cu(H20)6]2+(aq) + EDTA4-(aq)

This makes [CuEDTA]2-(aq) + 6H20(l)

In this equation, two species are replaced by seven, as there are 7 products.
This increase in particles causes a significant increase in entropy which drives the reaction to the right.

For this reason, chelate complexes with multidentate ligands are favoured over complexes with monodentate ligands

Question 16

What does the polydentate porphyrin do in haemoglobin

Answer 16

Haemoglobin carries oxygen around body from the lungs.

Molecule consists of Fe2+ ion with a coordinate number 6.
Four of the coordination sites are taken up by a ring system celled a porphyrin which acts as a tetradentate ligand.

This complex is called a haem.

Question 17

Describe the structure of porphyrin in haemoglobin

How is carbon monoxide affecting the Fe2+

Answer 17

Fe2+ ion binds to 4 :N , a globin, and an O2 or H2O
It first binds to 4 :N but below the plane of this ring is a fifth nitrogen atom acting as a ligand, and it is part of a complex called globin.

The Fe2+ to O2 bond is weak as :O2 is not a very good ligand, allowing the molecule to easily give up to its cells.

However, when a better ligand than oxygen binds, it binds irreversably to the fe2+ so destroys the oxygen carrying site.
This explains the poisonous effect of carbon monoxide

Question 18

What ligands have a negative charge

Answer 18

:Cl- ions
Eg in [CoCl4]2- ion, with 4 ligands it is a tetrahedral .

Question 19

Compare the complex ions of [Co(NH3)6]3+ ion and the [CoCl4]2- ion.

Answer 19

• [Co(NH3)6]3+ ion has 6 ligands and is an octahedral shape.
  An octahedron has six points but eight faces.
  Metal ion Co3+ has a charge of +3 and the ligands are all neutral. So complex ion has an overall charge of +3

In [CoCl4]2- ion, the metal ion is Co2+ so has a 2+ charge, and there are four 1- ions, the overall charge is -2.

It is a tetrahedral shape, and so is different from other cobalt complex. The :Cl- ions are larger ligands than the ammonia molecules so fewer ligands can fit around the central metal ion.

Question 20

What shape complex is [Ag(NH3)2]+:

What is the name of the solution, where is it used
Eg aldehydes and ketones

What is the name of the test.

Answer 20

Linear complex

The solution containing it is called Tollens’ reagent. It is used in organic chemistry to distinguish aldehydes from ketones.
The aldehydes reduce the [Ag(NH3)2]+ to Ag (metallic silver) while ketones do not.
The silver forms a mirror on the surface of the test tube, giving the name of the test.

Called the silver mirror test

Question 21

What is an isomer

What two types of isomers can transition metal complexes form

Answer 21

Compounds with the same molecular formula but with different arrangements of their atoms in space.

Geometrical isomers
Optical isomers

Question 22

Describe the two types of geometrical isomers

What ligands does this occur in
What shape complexes

Do different geometrical isomers have different chemical properties in cisplatin for example

Answer 22

Cis-Trans
Occurs for monodentate ligands

Occurs in octahedral and square planar complexes

Yes, they will eg cisplatin is an anti cancer drug which is a cis isomer, whilst the trans isomer of it does nothing

Question 23

Describe the structure of cisplatin

Answer 23

Formula [Pt(Cl)2(NH3)2]

It is the square planar complex of platinum
Anti cancer drug

It is chiral so only the cis isomer works to cure disease, the trans one doesn’t affect cancer

Question 24

Describe cis isomers of mono-dentate ligands

Answer 24

Eg in an octahedral complex ion, the same ligands are next to each other

Same with a square planar complex

Question 25

Describe trans isomers of
mono-dentate ligands

Answer 25

Eg in octahedral, the same ligands go opposite each other

Same with in square planar

Question 26

When does optical isomerism occur

Answer 26

When there are two or more bi-dentate ligands in a complex

The two isomers are non-superimposable mirror images of each other
so are non identical in mirror images

Question 27

Do the properties change in optical isomers

Answer 27

They are said to be chiral, so they have identical chemical properties, but can be distinguished by their effect on polarised light.

One isomer will rotate the plane of polarisation of polarised light clockwise, and the other anticlockwise