Transition Metals

Question 1

What is a d-block element?

Answer 1

An element in which its atoms have their highest-energy electrons in d-orbitals.

Question 2

What is a transition metal?

Answer 2

A transition metal forms at least one stable ion with partially filled d-orbitals.

Question 3

What are some examples of d-block elements which are not transition metals?

Answer 3

Scandium and zinc - when scandium ionises it loses 4s2 and 3d1 electrons to form Sc3+ (it has no electrons in d orbitals). When zinc ionises it loses its 4s2 electrons to form Zn2+ (it has a fully filled d-subshell, so has no partially filled d-orbitals and is therefore not a transition metal).

Question 4

What is the general electron configuration of a d-block atom?

Answer 4

1s2 2s2 2p6 3s2 3p6 3dx 4s2
The value of x will depend on the total number of electrons in the atom

Question 5

What is the electron configuration of a chromium atom?

Answer 5

1s2 2s2 2p6 3s2 3p6 4s1 3d5

Question 6

What is the electron configuration of a copper atom?

Answer 6

1s2 2s2 2p6 3s2 3p6 4s1 3d10

Question 7

In general, when are transition metals the most stable?

Answer 7

Having a half-full, or full set of d-orbitals is the most stable state for a transition metal. Copper and chromium have unusual electron configurations (both have 4s1), to obtain this half-full/full effect. Despite having a higher energy level than the 4s subshell, the energy needed to promote an electron from the 4s to 3d is less than the energetic benefit of having a half-full or full set of d-orbitals. This also impacts how transition metals ionise and which oxidation numbers are the most stable.

Question 8

When transition metals ionise, which electrons are lost first?

Answer 8

In forming ions, transition metals will generally lose their 4s electrons first, followed by 3d electrons.

Question 9

Why are lots of oxidation states available for lots of transition metals?

Answer 9

Successive ionisation energies of transition metals are quite similar (the d-orbital electrons all have similar ionisation energies), so enough energy is present to keep removing electrons until the energy required to ionise the next electron is too great. This means lots of oxidation states are possible, though not necessarily stable.

Question 10

What are the stable oxidation numbers of the transition metals?

Answer 10

All transition metals can form 1+ or 2+ ions by losing their 4s electrons. In general, 2+ ions ads stable, as well as 3+ and 4+ (often found in polyatomic ions). V can be +5, Cr can be +6 and Mn can be +7

Question 11

What is the shorthand for writing electron configurations?

Answer 11

Use noble gases to represent the first part of the electron configuration. For transition metals, this is generally [Ar]3dx 4s2 (x depends on which element it is). For ions, the 4s and some of the 3d may be lost, e.g. Cr 3+ = [Ar] 3d3

Question 12

How are transition metal compounds used in catalysis?

Answer 12

Since transition metals often have multiple stable oxidation states, they can act as catalysts in reactions by being good oxidising and reducing agents. They will be oxidised/reduced to a different stable compound, then reduced/oxidised back to the original compound in the next step. It is a catalyst as it is providing an alternative reaction pathway of lower activation energy, and the original compound is regenerated in a later stage of the reaction.

Question 13

What is a ligand?

Answer 13

A ligand is a molecule which forms coordinate (dative) bonds to a transition metal ion (technically doesn’t have to be a transition metal ion, but does have to be a metal ion), forming a complex ion.

Question 14

What are coordinate bonds?

Answer 14

Coordinate bonds are the dative covalent bonds formed when a ligand donates a lone pair of electrons to a transition metal ion to form a coordination complex/complex ion.

Question 15

What is the coordination number of a complex ion?

Answer 15

The number of coordinate bonds in the complex ion.

Question 16

Where do the electrons from the ligand go when a complex ion forms?

Answer 16

The electrons go into available empty orbitals on the transition metal ion, forming a dative bond between the ion and the ligand.

Question 17

What does a substance need to have in order to be a ligand?

Answer 17

A substance needs a lone pair of electrons available to donate and must have a geometry enabling coordinate bonds to form. Ligands are usually either electrically neutral or negatively charged (as positive ions would repel the metal ions they are bonded to(they can be positively charged, but this is rare)).

Question 18

What are the geometries a complex ion can have?

Answer 18

Linear (angle 180 degrees) - usually forms when silver is the transition metal ion
Tetrahedral (angle 109.5 degrees) - usually forms with Cl- ligands
Square planar (angle 90 degrees and 180 degrees (over the top of the plane)) - usually forms with platinum and nickel complexes or when -CN is the ligand
Octahedral (angle 90 degrees) - most complex ions are octahedral, particularly with water, ammonia, or small neutral molecules as the ligand

Question 19

Why do complex ions with 2 ligands generally form linear complexes?

Answer 19

The linear shape minimises repulsion and maximises separation between bonding and lone pair electrons.

Question 20

Why do complex ions with 2 ligands generally form linear complexes?

Answer 20

The linear shape minimises repulsion and maximises separation between bonding and lone pair electrons.

Question 21

Why do chloride ions generally form tetrahedral shapes in complex ions?

Answer 21

Chloride ions are quite large ligands (especially compared to small molecules like water or ammonia), so not as many can fit around a transition metal ion. Chloride ions are also charged, so repel each other more than neutral ligands would.

Question 22

A complex ion with 4 ligands will be tetrahedral or square planar - what decides which it is?

Answer 22

The lone pairs on metal ions influence the shape (Pt and Ni will often form square planar structures). The type of ligand used also influences shape (-CN ions are common ligands to adopt this geometry due to the fact that they are strong field ligands (cause significant electron pairing in the central metal ion, leading to dsp2 hybridisation and therefore square planar structures).

Question 23

What is the denticity of a ligand?

Answer 23

The number of coordinate bonds they can form to a single metal ion (this is sometimes the number of available lone pairs, but not always as sometimes the geometry means that the bond angle between two available lone pairs is too tight to bond to the same metal ion).

Question 24

What is a monodentate ligand?

Answer 24

A ligand which forms one coordinate bond to a metal ions (e.g. H2O, NH3, Cl-, -CN)

Question 25

What are bidentate, tridentate and multidentate ligands?

Answer 25

Bidentate ligands form two coordinate bonds (e.g. ethanedioate ions, 1,2-diaminoethane). Tridentate ligands form three coordinate bonds (e.g. diethylenetriamine, iminodiacetate anion). Multidentate ligands are ligands which generally form more than 3 coordinate bonds (but there are specific names for these, such as hexadentate=6 coordinate bonds (e.g. ethylenediamine tetraacetic acid (EDTA)). Note: EDTA has an overall -4 charge. You cannot be more than hexadentate as one transition metal ion can accept a maximum of 6 coordinate bonds.

Question 26

What is the geometry of the Fe2+ in haemoglobin?

Answer 26

There are 4 coordinate bonds between Fe2+ and nitrogen atoms in the haem group. There is another coordinate bond between the Fe2+ and the globin protein, so the initial structure is square pyramidal. The Fe2+ can accept a 6th coordinate bond to the O2, forming Fe3+ and changing the structure to octahedral. This changes the geometry of the other haem groups and makes it easier for other oxygens to bind.

Question 27

Why is carbon monoxide poisoning so dangerous (in terms of ligands)?

Answer 27

O2 is a terrible ligand, which means that it can dissociate from haemoglobin to be taken up and used by cells. However, this means that is CO gets into the blood, it will datively bind to the iron and never leave, as CO is a much better ligand than O2. This reduces your capacity for oxygen and can ultimately cause death.

Question 28

What is cis-trans isomerism in complex ions?

Answer 28

In square planar geometries, if the same ligand is adjacent, this is the cis isomer, if the same ligand is opposite (i.e. bonded across the metal ion), this is the trans isomer. In octahedral geometries there must be 2 of one ligand and 4 of the other in order to show cis-trans isomerism. If two of the same ligand are at any position 90 degrees to each other, this is the cis isomer. If the two same ligands are at any position 180 degrees to each other, this is the trans isomer.

Question 29

What is the structure of cisplatin?

Answer 29

Cisplatin is a platinum ion with two coordinate bonds to chloride ions (which are cis to each other) and to two ammonia molecules.

Question 30

How does cisplatin work?

Answer 30

During DNA replication, the two strands of DNA separate and the bases are exposed. Cisplatin can undergo a ligand substitution and swap its Cl- ligands for the nitrogen on guanine which has an available lone pair. If there are two guanines either next to each other or one base apart, the geometry of cisplatin enables two coordinate bonds to form between the platinum ion and the DNA strand. DNA polymerase could break one coordinate bond to the platinum ion, but not two, so the action of DNA polymerase would stop, leading to cell cycle arrest and cell death. This affects all cells in the body but disproportionately affects cancer cells as they are replicating at a faster rate than normal body cells.

Question 31

What causes colour to be seen in solution of transition metal complexes?

Answer 31

When ligands bond to a central metal ion, they cause the energy levels of d orbitals to split (due to changes in the shapes of the d orbitals which may increase or decrease repulsion between electrons, affecting energy levels). The difference between energy levels is the energy gap and this determines which wavelengths of light will be absorbed by the solution. Energy from light which has a wavelength corresponding to the energy gap between d orbitals will be absorbed to promote electrons to the higher energy orbitals from the lower energy orbitals. The remaining light is transmitted through the solution, which is the colour you see (so the colour seen is the complementary colour of the colour absorbed). The electrons will relax back down, but the energy released is given out as heat, not light (this is not the cause of the colour of the solution, except in the cause of fluorescence or phosphorescence).

Question 32

What affects the colour of a complex ion?

Answer 32

The colour depends only on the energy gap between the d orbitals (assuming white light is used). The energy gap depends on the transition metal ion present, the ligands, the coordination number of the complex and the charge on the transition metal ions, as these affect the electron configuration (which impacts the promotion of electrons to higher energy levels) and the geometry of the complex, as this impacts how the d orbitals split in energy and therefore the energy gap.

Question 33

Metals such as magnesium can form complex ions in water, but these are not coloured - why?

Answer 33

The process of the water ligands binding to the metal ion will split the d orbitals in the same way as a transition metal ion, but there are no electrons in the d orbitals, so the absorption of certain wavelengths of light will not occur, and so all of the white light will be transmitted and therefore no colour seen. Any metal ion which has either empty or full d orbitals will not produce coloured complexes as the electrons can’t move between energy levels.

Question 34

How can calorimetry be used to find the concentration of transition metal ions in solution?

Answer 34

The more concentrated a coloured solution, the more light it will absorb. A colorimeter can be used to measure the absorbance of a solution, which can be compared against solutions of known concentration to make a calibration curve.

Question 35

What are the colours of compounds of vanadium in different oxidation states in aqueous solution?

Answer 35

VO2 + (+5 state) - yellow VO 2+ (+4 state) - blue V3+ - green V2+ - violet You Better Get Vanadium

Question 36

What colour is chromium in the +6 oxidation state in aqueous solution?

Answer 36

Orange (dichromate ions Cr2O7 2-)

Question 37

What colour is chromium in the +3 oxidation state in aqueous solution?

Answer 37

Green (Cr 3+)

Question 38

What colour is iron in the +3 oxidation state in aqueous solution?

Answer 38

Yellow (Fe 3+)

Question 39

What colour is iron in the +2 oxidation state in aqueous solution?

Answer 39

Pale green (Fe 2+)

Question 40

What colour is cobalt in the +2 oxidation state in aqueous solution?

Answer 40

Pink (Co 2+)

Question 41

What colour is copper in the +2 oxidation state in aqueous solution?

Answer 41

Pale blue (Cu 2+)

Question 42

What colour is the chromate (CrO4 -) ion in aqueous solution?

Answer 42

Yellow

Question 43

What colour is Fe2+ in OH-?

Answer 43

Dark green precipitate which does not dissolve in excess alkali (turns brown on standing due to oxidation from air)

Question 44

What colour is Fe3+ in OH-?

Answer 44

Brown precipitate

Question 45

What colour is Cu2+ in OH-?

Answer 45

Dark blue precipitate

Question 46

What colour is Co2+ in OH-?

Answer 46

Dark blue solution —> pink precipitate on standing

Question 47

What colour is Cr3+ in OH-?

Answer 47

Green precipitate which does dissolve in excess OH- to form a green solution.

Question 48

What colour is Cu2+ with excess NH3?

Answer 48

dark blue solution

Question 49

What colour is Co2+ with excess NH3?

Answer 49

brown solution

Question 50

What colour is Cr3+ with excess NH3?

Answer 50

Violet solution

Question 51

What colour is Cu2+ with Cl-?

Answer 51

Yellow solution

Question 52

What colour is Co2+ with Cl-?

Answer 52

Blue solution

Question 53

What is a catalyst?

Answer 53

A catalyst speeds up the rate of a reaction by providing an alternate reaction pathway of lower activation energy. They are not used up in the reaction.

Question 54

What is the difference between a heterogeneous catalyst and a homogeneous catalyst?

Answer 54

Heterogenous catalysts are in a different phase to the reactants, homogeneous catalysts are in the same phase as the reactants.

Question 55

How do heterogenous catalysts work?

Answer 55

Usually a solid catalyst with gaseous reagents. If this is the case, the reactants adsorb onto the surface of the catalyst at active sites. Bonds in the reactants are weakened by this interaction and reactant molecules are more likely to collide and their orientation is favourable. The reaction occurs on the catalyst surface and the product molecules desorb from the surface.

Question 56

What makes a good heterogenous catalyst?

Answer 56

The strength of adsorption to the surface must be strong enough to get the reactants to stick for long enough to react, but the product molecules must also be able to desorb, so the attraction can’t be too strong. In general, metals which make good catalysts include Fe, Co, Ni, Ru, Rh, Pd, Pt.

Question 57

How can heterogenous catalysts become more effective?

Answer 57

They become more effective if the surface area is increased, which provides more active sites on which reactions can occur. This is achieved by having a mesh/honeycomb structure.

Question 58

What is catalyst poisoning?

Answer 58

Some substances will adsorb to the surface of a catalyst and never desorb. This blocks active sites and makes the catalyst less effective. The only way to save a poisoned catalyst is by sanding off the poisoned layer and revealing a new layer of catalyst underneath.

Question 59

How do homogenous catalysts work?

Answer 59

Usually reactants and catalyst are in the aqueous phase. An example is acid catalysts (for example in esterification), which involve the acid reacting with one reactant to form an intermediate which will then react with the other and the original H+ is regenerated. Transition metal ions can also act as homogenous catalysts, for example in the reaction between I- and S2O8 2-, Fe2+ reacts with S2O8 2- to form SO4 2- and Fe3+. The Fe3+ reacts with the iodide ions to make iodine and Fe2+, which can cycle back into the first reaction. This is very slow without the catalyst as the negative ions repel each other.

Question 60

What is autocatalysis?

Answer 60

When a reaction is catalysed by one of its products, so it starts slowly then increases as more catalyst is made.