transition metals 15

What’s a transition metal

D block elements that can form one or more stable ion with a partially filled d orbital

Why transition metals show variable oxidation number

Because they have electrons of similar energy in both their 3d and 4s orbitals. This allows the metals to form stable ions by losing different number of electrons.

Ligand

Molecules that donate a lone pair of electrons to a metal to form a dative covalent bind

Complex ion

A central metal ion surrounded by ligands

Coordination number

The number of coordinate bonds to the central metal cation from the surrounding ligands

4 properties of transition metals

1) they show variable oxidation numbers in their compounds.

2) they form coloured ions in solution.

3) they act as catalysts, both as the elements or as their compounds.

4) they form complex ions involving monodentate, bidentate, and multidentate ligands.

Monodentate ligands

Ligands that form one dative covalent bond with the central metal ion (e.g. H2O, NH3).

Ligands with one donor atom with a lone pair.

E.g. H2O is still monodentate even though it has 2 lone pairs because the 2nd lone pair can’t be used as H2O has a bent shape, so one lone pair is directed away from the central metal ion

Bidentate ligands

Ligands that form 2 dative covalent bonds with the metal ion (e.g. C2O4)

Multidentate ligands

Ligands that form more than 2 dative covalent bonds with the central metal ion (e.g. EDTA)

How complex ions form

Transition metal cations can act as complex ions in aqueous solutions, and in compounds. Molecules with a lone pair of electrons can act as ligands to form dative covalent binds with the metal ions

Explain the shapes of complex ions

1) it depends in the coordination number of the ion. The ligands are positioned as far away as possible to minimise repulsion.

2) complex ions can also display geometric (cis z/trans e) and optical (non superimposable) isomerism

Why cis- platin is used for cancer treatments while trans- platin isn’t

Trans- platin is toxic

Why do complexes with six- fold coordination have an octahedral shape

Although the monodentate ligands have multiple lone pairs each (e.g. H2O), each only uses one lone pair to form a coordinate bond. The electron pair donor in each legend is an element in period 2, so all the ligands are of approximately equal size.

2 reasons why may some complex ions have no colour

Ions that have completely filled, or empty 3d energy levels are not coloured.

If there’s a large difference in energy levels between the split 3d lower energy level and higher energy level, then high frequency (low wavelength) radiation is absorbed, which is in the UV region of the spectrum instead of the visible light region.

Explain why Cu 2+ is blue

When water ligands are attached to the Cu2+ ion, the energy level splits into two levels with different energies. The lower energy level contains 6 electrons, the higher energy level contains 3.

If one e- in the lower energy level absorbs energy from the visible spectrum, it can move to the higher energy level (excitation/ promotion).

When it moves to a higher energy level, the bigger the energy difference between the two energy levels, the more energy the electron absorbs. The amount of energy gained is directly proportional to the frequency of light absorbed, and inversely proportional to the wavelength of light.

With the Cu2+ ion, the small difference in energy levels means that low frequency (high wavelength) radiation is absorbed from the red end of the spectrum. So blue light is transmitted.

3 things that can change to result in a colour change of transition metals that ions

Oxidation number, ligand, coordination number

4 types of reactions with transition metals

1) redox reactions= Change in oxidation number of the metal centre.

2) Acid- base reactions= where ligands may act as an acid or a base to react with another substance. Some complex ions are amphoteric (can react with both acid and alkali. The ability of a ppt to redissolve makes it amphoteric).

3) Ligand exchange reactions= ligands in the complex ion are replaced by a different ligand. When a multidentate ligand displaces monodentate ligands, there will be an increase in entropy (order to disorder) because there are more products forming. It leads to the formation of a more stable ion. E.g. CO replacing O2 in haemoglobin then can’t be exchanged again cus CO is a strong field ligand.

4) Coordination number change reactions= coordination number if the complex ion changes as the number of ligands change.

When the electrode systems are layer out most negative at the top, most positive at the bottom, where would the best reducing agent and best oxidising agent be

Best reducing agent= top right,

Best oxidising agent= bottom left

How to calculate E cell

Most negative of the two electrode systems on the top,

Combine the two to make an ionic equation,

E cell= RHS - LHS (LHS is the more negative)

Why can transition metals act as great catalysts

Their variable oxidation states

What’s a heterogeneous catalyst

A catalyst that is in a different state from the reactants, which work by adsorbing reactants at active sites in their surface.

The catalyst is a solid and usually used as small particles (e.g. powders) cus it increases the surface area.

Reactants can be g / l / aq

How do heterogeneous catalysts work

Surface Adsorption Theory with 4 steps:

1) adsorption, in which one or more reactants become attached to the surface of the catalyst.

2) reaction, following the weakening of bonds in the adsorbed reactants.

3) desorption, in which the reaction products become detached from the surface of the catalyst.

4) catalyst regeneration, the surface is now free and can be used again

2 examples of heterogeneous catalysts reactions

Contact process for making sulphuric acid,

Catalytic converter

What’s a homogeneous catalyst

A catalyst that is in the same state as the reactants, so the catalysed reaction often proceeds via intermediate species.

Both catalyst and reactants are either all gasses or all aqueous

2 examples of homogeneous catalyst reactions

Fe in the reaction between I- and S2O8 2-.

Autocatalysis of Mn 2+ in the oxidation of ethanedioate ions.

Both are slow without a catalyst as both reactant ions are negative, so repel

What’s autocstalysis

the formation of a product that increases the rate of reaction.

E.g. in the oxidation of ethanedioate ions, the Mn 2+ produced acts as a catalyst increasing the rate of reaction as the titration progresses as more product is produced.