Topic 15 - Transition Metals
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What are transition metals?
d-block elements which can form one or more stable ions with incompletely-filled d-orbitals -aka every d-block element except zinc and scandium
Why are zinc and scandium not transition metals?
• zinc’s ion Zn2+ has a full d-subshell - scandium’s ion Sc3+ has an empty d-subshell
Why do transition metals show variable oxidation number?
they can form ions by loosing electrons from their 4s or 3d subshells
What is a ligand?
a molecule/negative ion with a lone pair of electrons which can form a dative bond with the empty, available d-orbitals of a transition metal
What is a complex ion?
a central transition metal ion surrounded by ligands
What is the coordination number (of a complex ion)?
the number of molecules/atoms a transition metal can datively bond to -aka number of ligands
How do you name complex ions?
• number of ligands (di, tri, tetra, penta, hexa etc) • ligand (aqua, ammine, hydroxo, chloro, fluoro, cyano, etc) • name of transition metal (or latin for anionic complexes: chromate, ferate, cobaltate, aluminate, vandate, cuprate, etc)
What is a cationic complex?
a complex ion with a positive charge
What is an anionic complex?
a complex ion with a negative charge
What is a monodentate ligand?
a ligand with one long pair of electrons/that can form one dative bond eg. Cl-, H20, CN-, NH3
What is a bidentate ligand?
a ligand with two long pairs of electrons/that can form two dative bonds eg. NH2CH2CH2NH2 (1,2 diaminoethane)
What is a polydentate ligand?
a ligand with multiple long pairs of electrons/that can form multiple dative bonds eg. EDTA2- (hexadentate ligand)
Where does the colour of transition metal ions and complex ions come from?
splitting of energy levels of the d-orbitals by ligands • repulsion between the 3d electrons and the ligand’s lone pair of electrons increases the energy of the transition metal’s d-orbitals • some d-orbitals rise in energy more than others, which creates a gap between the d-orbitals • a particular wavelength of light is absorbed, which promotes electrons within the d-orbitals • the complementary colour is reflected
Why do some aqueous transition metal ions and complex ions have a lack of colour?
no d-orbital electrons or d subshell is full so no electrons jump so no energy is absorbed
What could cause a change of colour in transition metal ions?
• change in oxidation number • change in ligand • change in coordination number
How is haemoglobin an example of a complex ion?
it is an iron (II) complex containing a multidentate ligand -ligand exchange rxn occurs when an oxygen molecule bond to haemoglobin it replaced with a carbon monoxide molecule
What is the colour of the vanadium ion with an oxidation number of +5 aka VO2 +?
yellow
What is the colour of the vanadium ion with an oxidation number of +4 aka VO 2+?
blue
What is the colour of the vanadium ion with an oxidation number of +3 aka V3+?
green
What is the colour of the vanadium ion with an oxidation number of +2 aka V2+?
violet
How can Cr2O7 2- be reduced to Cr3+ and then reduced further to Cr2+?
using zinc (as reducing agent) in dilute acid
How can Cr3+ be oxidised to Cr2O7 2-?
using hydrogen peroxide in alkaline conditions followed by acidification
2CrO4 2− + 2H+ ⇌
Cr2O7 2− + H2O
What is observed when a little sodium hydroxide is added to chromium ions?
green solution changes to grey-green ppt