5.3.1 Transition Elements Flashcards
Also see transition metals colours on Quixote for exhaustive list
What is a transition element?
- a d-block element that forms at least one stable ion with an incomplete d-subshell
What are the exception to the filling of sub shells for transition elements?
Chromium - 3d54s1 (instead of 3d44s2)
Copper - 3d104s1 (instead of 3d94s2)
How do transition metals form ions?
- form positive ions and lose electrons
- lost from the 4s first and then the 3d
- 3d and 4s su shells are very close in energy and once they are occupied, repulsion between electrons causes the 4s to have a higher energy than the 3d
Are scandium and zinc transition metals?
- Sc forms 3+ ion in which the d-sub shell is empty
- Zn forms a 2+ ion in which the d-sub shell is full
- don’t form ions with incomplete d sub shells - not transition metals
What are the physical properties of transition metals?
- similar atomic radius, high densities and high melting points
- metallic bonding: a giant lattice of regularly arranged positive ions in a sea of delocalised electrons with strong electrostatic attractions between them
What are the chemical properties of transition metals?
- form compounds in which the transition metal has variable oxidation states
- form coloured solutions when dissolved in water
- frequently catalyse chemical reactions
- form complex ions
Examples of variable oxidation states
- copper +1 and +2
- iron +2 and +3
Examples of coloured solutions
- copper sulphate is blue
- iron (II) sulphate is pale green
Examples of catalysts
- iron in the Haber process
- manganese oxide in the decomposition of hydrogen peroxide
What is a complex ion?
- a central transition metal bonded to one or more ligands by coordinate bonds
What is a ligand?
- an ion or molecule with a lone pair of electrons that can be donated to a transition metal to form a co-ordinate bond
What is the coordination number?
- the total number of coordinate bonds between a central metal ion and its ligands
What is a monodentate ligand?
- a ligand that donates one pair of electrons to the central metal ion
What is a bidentate ligand?
- a ligand that donates 2 lone pairs of electrons to the central transition metal ion, forming two coordinate bonds
What shape are complex ions with 6 coordination number?
- octahedral
- 90*
What shape are complex ions with 4 coordination number?
- tetrahedral or square planar
- 109.5* or 90*
- nickel, palladium and potassium are square planar
What are stereoisomers?
- same molecular and structural formula and different arrangement of atoms in space
What is the cis isomer?
A-M-A bond angle 90*
What is the trans isomer?
A-M-A bond angle 180*
What are optical isomers?
- non-superimposable mirror images (no plane of symmetry)
What complex ions show optical isomerism?
- octahedral metal with 2 bi and 2 mono ligands
- octahedral metal with 3 bi ligands
What is the use of cisplatin in medicine?
- anti-cancer drug
- binds to DNA and prevents cell division, killing cancer cells
What is ligand substitution?
- a reaction in which one ligand in a complex is replaced by another ligand
What is ligand substitution reaction from [Cu(H2O)6]2+?
[Cu(H2O)6]2+ + little NH3 -> Cu(OH)2
blue solution blue ppt
[Cu(H2O)6]2+ + excess NH3 -> [Cu(NH3)4(H2O)2]2+
blue solution deep blue solution
[Cu(H2O)6]2+ + Cl- -> [CuCl4]2-
blue solution yellow solution
Why might the solution appear green?
- contains a mixture of complex ions in equilibrium
- position and colour can be changed by adding water or chlorine
What is the ligand substitution reaction from [Cr(H2O)6]3+?
[Cr(H2O)6] + little NH3 -> Cr(OH)3
violet solution grey/green ppt
[Cr(H2O)6] + excess NH3 -> [Cr(NH3)6]2+
violet solution purple solution
What is the biochemical importance of iron?
- haemoglobin contains haem groups which contain iron ion
- oxygen acts as a ligand and binds reversibly to Fe2+
- oxygen and CO both use same binding site
- CO ligands bind more strongly and irreversibly
- reduces ability of blood to transport oxygen around the body
Copper colours
- Cu2+(aq) blue solution
- Cu(OH)2 blue precipitate
Iron colours
- Fe2+(aq) pale green solution
- Fe(OH)2 dark green precipitate (standing red-brown)
- Fe3+(aq) yellow solution
- Fe(OH)3 orange-brown ppt
Manganese colours
- Mn2+ (aq) pale pink (dilute colourless)
- Mn(OH)2 light brown ppt
Chromium colours
- Cr3+(aq) violet solution
- Cr(OH)3 grey-green ppt
- with excess NaOH, ppt will dissolve into green solution
How is oxygen transported around the body?
- oxygen has a lone pair of electrons and so can act as a ligand, forming a coordinate bond to Fe2+ ions in haemoglobin
- this is reversible so oxygen can be carried round the body to where it is needed
