Topic 15: Transition metals Flashcards
What is a transition metal?
d-block elements that form one or more stable ions with incompletely-filled d-orbitals
Why do transition metals show a variety of oxidation colour?
What is meant by the term ligand?
molecules which surround the central metal ion
What is a complex metal ion?
A metal ion complex has a metal ion at its centre with a number of other molecules or ions surrounding it.
What causes the colour of aqueous ions, and other complex ions?
This results from the splitting of the energy levels of the d-orbitals by ligands
Electrons jump to uninhabited areas
The light wavelength which is not absorbed is observed
What is the co-ordination number?
The number of dative bonds in a metal ion complex
Dative bonds from lone pairs on the ligands
Why is there a lack of colour in some aqueous ions and other complex ions?
If the metal ion forms a full or empty 3d subshell
What causes colour changes in transition metals?
Variable oxidation numbers/states
Ligand
Co-ordination number
Why do H2O, NH3 and OH- act as monodentate ligands?
They only form one dative bond
Why do complexes with 6 dative covalent bonds have an octahedral shape?
It optimises spatial distribution
- Cannot fit any more ligands around the central metal ion due to electron repulsion / overlap of orbitals
What shape does a complex ion form with the ligand Cl-
Tetrahedral shape
This is because the Cl- ions have a high electron density, meaning they are larger ions, so only 4 can fit around the central metal ion
Give an example of a square planar complex, and how they arise
Cis-platin
There are 4 dative bonds
What is the structure of cis platin?
A platinum ion in the centre
Two Cl ligands and two NH3 ligands attached
How is cis-platin used as an anti-cancer drug?
- First ligand substitution reaction: One of the chloride ligands on cisplatin is substituted for one H2O ligand
- Second ligand substitution reaction. The newly-substituted water molecule is replaced by one of the guanine nitrogen atoms. Dative covalent bonds are formed between the lone pair on one nitrogen atom on a guanine base and the central platinum ion at the centre of cisplatin.
- The process is repeated for the other chloride ligand
- DNA shape is distorted. This prevents DNA replication from taking place.
Why isn’t Trans-platin used in cancer treatments?
It is toxic
Cisplatin has both of its Cl’s on one side, so the two Cl’s can bond to a close together guanine pair. It is bidentate since the two Cl’s are on the same side, so cisplatin can stay attached to the DNA strand and prevent the DNA polymerase from replicating the DNA. This wouldn’t be possible with Trans-platin since the Cl are on the opposite side.
In Trans-platin, the Cl ligands are on opposite sides of the Pt central ion. This means it cannot stay bonded to the DNA strand and prevent DNA replication
What is a bidentate ligand? Give an example.
Have two lone pairs on them, so can form two dative covalent bonds
:NH2CH2CH2H2N:
What is a multidentate ligand? Give an example
Have more than 2 lone pairs, so can form multiple dative bonds
EDTA4- (hexadentate)
What is haemoglobin? (Structure)
an iron(II) complex containing a multidentate ligand
When does ligand exchange happen in haemoglobin?
when an oxygen molecule bound to haemoglobin is replaced by a carbon monoxide molecule
What is the chelate effect?
When bidentate or multidentate ligands replace monodentate ligands in a complex. This results in a more stable ion complex
Why does stability of the complex increase after chelation?
the strength of the ligand bonds broken are very similar to the strength of the new bonds being made. This means the enthalpy changes, ∆H, for many ligand substitution reactions are very small.
The explanation for the increased stability is due mostly to an increase in entropy, ∆S. There is more disorder in the products (increased entropy) as there are more product molecules formed.
What colour is vanadium 5+ (OS), and what compound is it in?
Yellow
VO2 +
What colour is vanadium 4+ (OS), and what compound is it in?
Blue-Grey
VO 2+
What colour is vanadium 3+ (OS), and what compound is it in?
Green
V3+
