Transition elements are those that have an incomplete d-subshell in either their atoms or common ions. Rules for d-block elements:
- Fill 4s before 3d.
- An atom is more stable with either a full or half full 3d subshell.
Even thouh copper doesn't have an incomplete d-subshell in it atoms, it does in one of its common ions - Cu2+.Zinc is not a transition element because it doesn't have a partially filled d-subshell as an atom or an ion.Common properties of tranition elements are:
- Variable oxidation states.
- They and their compounds make good catalysts.
- Form coloured complexes by coordinate bonding.
Transition elemets from Ti to Cu have electrons of similar energy in both the 3d and 4s levels. One particular element can form ions of roughly the same stability by losing different numbers of electrons.This is why ionisation energies of transition elements tend to increase gradually without large jumps in value. Important oxidation states are:
- Chromium - Cr3+ and Cr6+
- Manganese - Mn7+ and Mn2+
- Iron - Fe2+ and Fe3+
- Copper - Cu1+ and Cu2+
Transition Metals as Catalysts
Homogenous catalysts - Catalysts that are in the same state as the reactants/reagents.
Heterogenous catalysts - Catalysts that are in a different state to the reactants/reagents.
Why do transition metals make good catalysts?
- The reactants use empty d-orbitals to form temporary bonds with the catalyst.
- This holds the reactants so the reaction takes place.
- Reactants are 'absorbed' onto the catalysts surface.
Some examples of catalysts and their reactions:
- Iron - Haber Process
- Platinum - Catalytic hydrogenation of oil
- TiCl3 or TiCl4 - Used in the polymerisation of ethene
- V2O5 - Contact Process
Some catalysts work differently with the transition metal changing its oxidation state during the reaction and then back again once complete.
E.G. V2O5 in the Contact process - used to manufacture sulphuric acid.
Formation of Complex Ions
Transition metal ions form bonds with specific numbers of molecules (or ions) that surround the central metal ion.The surrounding molecules (and ions) are lignands.
Metal ions are all positively charged making them electron deficient. The bonds that from between the surrounding ligands and the central metal ions are coordinate bonds. Ligands must have a lone pair of electrons they can donate to the central metal ions.
Ligands - Molecules (or ions) with a lone pair of electrons that can form a co-ordinate bond with a central metal ion. There are three types of ligand:
- Unidentate (or monodentate) - form one coordinate bond.
- Bidentate - form two coordinate bonds.
- Multidentate (or polydentate) - form several coordinate bonds.
Complex Ion Shapes:
Most common coordination number (number of coordinate bonds formed) is 6 - complexes with this number of coordinate bonds are octahedral in shape.The second most common coordination number is 4 - complexes with this number of coordinate bonds are tetrahedral in shape.
SIlver ions (Ag2+) only form linear complexes with a coordinate number of 2.
Representing Complex Ions
All first row transition metal elements form hexaaqua complexes with water ligands. Hexaaqua = 6 water ligands. Hexaaquacopper(II) ion - formed when copper sulphate and copper chloride are dissolved in water.
- [Cu(H2O)6]2+ (Co-ordinate = 6)
- Colour = Blue
- Shape = Octahedral
Some of the water ligands can be substituted/exchanged by adding different reagents. Tetraamminediaquacopper(II) ion - formed when concentrated ammonia solution is added to a solution containing the hexaaquacopper(II) ion.
- [Cu(NH3)4(H2O)2]2+ (Co-ordinate = 6)
- Colour = Deep Blue
- Shape = Octahedral
Tetrachlorocuprate(II) ion - formed when concentrated hydrochloric acid is added to a solution containing the hexaaquacopper(II) ion.
- [CuCl4]2+ (Co-ordinate = 4)
- Colour = Olive-green/yellow-green
- Shape = Tetrahedral
Origins of Colour in Complex Ions
Light is electromagnetic radiation in the visible spectrum and when it falls on pigments they absorb and reflect different wavelengths of its energy.
Red end of the spectrum:
- Low energy/frequency
- Longer wavelength
Purple end of the spectrum:
- High energy/frequency
- Shorter wavelength
If an electron absorbs wavelengths within a certain range - 400nm to 700nm - it will reflect a colour that we can see that is made up of all the other visible wavelengths of light, minus the wavelengths it absorbs. Only electrons in d-orbitals do this. If an ion or compound has atoms with no d-orbitals or if they have a full d10 the ion will absorb outside the visible region and appear colourless or white.In transition metal atoms all 5 d-orbitals have the same energy. However when they are joined to ligands in a complex ion the 5 d-orbitals become split into those that are higher (2) and those that are lower (3) in terms of energy.An electron in a lower energy d-orbital may absorb energy from wavelengths of light and will make a transition to a higher energy d-orbital. This is a d-d transition. deltaE has been absorbed (deltaE = hf) and the other wwavelengths of light are reflected and seen as a colour.
Precipitation Reactions of hexaaqua ions with OH-
Solution: Colour: Initial addition of OH- Addition in excess of OH-
[Cu(H2O)6]2+ Light Blue dark jelly layer darker precipitate
[Fe(H2O)6]2+ Almost clear dark jelly layer darker precipitate
[Fe(H2O)6]3+ Straw yellow jelly layer (same colour) darker precipitate
[Cr(H2O)6]3+ Light green darker jelly layer darker green precipitate
On initial addition of OH- a precipitate of the metal hydroxide forms that has no charge and comes out of solution.
[M(H2O)6]2+ + 2OH- ----> [M(H20)4(OH)2] + 2H2O
[M(H2O)6]3+ + 3OH- ----> [M(H2O)3(OH)3] + 3H2O
E.G Fe(H2O)6]2+ + 2OH- ----> [Fe(H20)4(OH)2)] + 2H2O
In these reactions we can consider the OH- ions to be acting as Br
onstead - Lowry bases by accepting H+ from water ligands, which are acting as Br onstead - Lowry acids. In some case (M3+ ions) adding more OH- ions leads to more H+ ions being removed from remaining H2O ligands. This causes negatively charged complex ions to form. Because the complex ion is charged the precipitate re-dissolves.
Importance of Transition Elements - Trace Elements
Vanadium - Found in some marine creatures.
Manganese - Found in bones, tissues and some organs / Present in many enzye=me systems.
Iron - Present in the blood in haemoglobin / Present in all tissues and organs - a necessary part of a healthy diet as iron deficiency leads to anaemia.
Cobalt - Component of vitamin B12 for formulation of blood and brain and nervous system function.
Chromium - Regulates blood glucose levels because it is a cofactor in the insulin hormone system.
Copper - Important in the formation of haemoglobin.
Zinc - Important in them metabolism of glucose and found in many enzyme systems.
Economic Importance of Transition Elements
Scandium - Alloyed with aluminium to form metal baseball bats.
Titanium - Used in high tensile steels and helicopter rotors / titanium oxide is an important pigment / used for glasses frames / used for surgical plates becuase it is unreactive towards body fluids.
Vanadium - Vanadium steels are rust resistant and useful in machine tools / vanadium oxide is used in ceramics and is the ctalyst in the Contact Process for production of sulphuric acid.
Chromium - Used in the manufacture of stainles steel / heat resistant chromium alloys are used for high temperature chemical apparatus / chromium plating mmakes base metals attractive and corrosion resistant.
Manganese - Manganese steels are tough and important to improving roiling and forging properties / manganese dioxide is used in dry batteries.
Iron - Iron and steel manufacture / Iron is the catlyst in the Haber Process for ammonia production.
Economic Importance of Transition Elements (2)
Cobalt - Alloying element in high temperature super alloys used for casting airfoils and structural parts of jet turbine engines / improves properties in rechargeable batteries / essential in making permanent magnets.
Nickel - Chemical catalyst in processes like the hydrogenation of vegetable oils and reforming hydrocarbons / Nickel electroplating protects base metals / component of stainless steel / alloys are using in glasses frames.
Copper - Essential for all electrical apparatus and computer manufacture and operations.