Coordination Complexes Flashcards
What did Alfred Werner identify?
He identified principle features of geometric structures
He observed a violet and a green compound with the same formula CoCl3.4NH3
1 chloride in each compound reacts with Ag+ to form AgCl
He proposed the reactive chloride is not bound to cobalt
CoCl2(NH3)4 Cl-
He proposed the six bonded group are arranged in a symmetric fashion around the cobalt
What are the possible arrangements of the bonded groups around the cobalt - Werner
The arrangements include planar hexagon which has 3 isomers Trigonal prismatic (2 isomers) Octahedral (2 isomers)
Which arrangement did Werner conclude?
He only isolate two compounds (octahedral only has 2 isomers), violet and green
Likely arrangement of 6 ligands is octahedral
This is correct
What is a chiral compound?
This is a complex that is non superimposable on its mirror image and has no internal plane of symmetry
What is geometric isomerism
A complex can adopt two different geometries
Which geometries show isomerism?
Isomerism is possible in square planar complexes of the general formula MX2Y2 and when X and Y are different
Octahedral complexes MX4Y2 complexes exist as cis and trans geometric isomers
Which geometries do not show isomerism
Tetrahedral complexes of the general formula MX2Y2 do not form geometric isomers as there is only one way the four ligands can be arranged
Describe the cis and trans configuration of octahedral with formula MX4Y2
It is cis when the 2 Y atoms are adjacent
It is trans when the 2 Y atoms are adjacent
Cis is adjacent
Trans is opposite
What can octahedral complexes [MX3Y3] exist as?
They can exist can mer and fac
Mer is where 3 of the ligands X/Y in the same plane
Fac is where 3 ligands X/Y are adjacent (triangular face)
What confirmed werners predictions?
Optical isomerism confirmed Werners - octahedral
Optical activity is determining if a compound is chiral (optically active)
What does en mean?
en= ethane-1,2-diamine
The ligand binds using both N donor atoms
A curve can represent the CH2CH2 bridges in the en ligand
What does ^ and triangle mean?
Triangle= right rotation ^= left rotation
What is an enantiomer?
This is the mirror image of a chiral molecule
They are non superimposable
Why is chirality not important for coordination complexes?
This is because metal ligand bonds are labile
This means the ligands can break away and change places so the enantiomer undergoes racemisation
Why can bidentate ligand enantiomers be isolated where monodentate ligands cannot?
Mondentate ligand bonds are labile and so the enantiomers undergo rapid racemisation
Bidentate ligands are less labile so it is possible to isolate these enantiomers
What are bidentate isomers distinguished by and what do they form?
Bidentate ligands form a helix
The enantiomer forming the right handed, clockwise helix is labelled triangle
The left handed anti-clockwise helix is labelled ^
Why are trans complexes not chiral?
This is because they contain a mirror plane
How can you separate diastereoisomers?
They are separated by fractional distillation
How can you separate a racemic mixture of 2 enantiomers?
+ Ba d-tatrate
- BaSO4
And they are separated
What is geometric isomerism?
Geometric isomerism, also known as cis- trans isomerism or EZ isomerism is a form of stereoisomerism
Do enantiomers have the same properties?
Do diastereoisomers have the same properties?
Diastereoisomers have different properties
Enantiomers have the same properties apart from
-reactivity with chiral reagents
- reaction with plane polarised light
What is the most common coordination for d block complexes?
Six coordination is the most common for d block metals
The majority of 6 coordinate complexes are octahedral
What are some important distortions of octahedral complexes?
Stretching or compressing
1) a 4 fold rotation symmetry which results in tetragonal distortion
2) a 3 fold rotation symmetry axis results in trigonal distortion
Why are higher coordination numbers less common for 3d and more common for 4d and 5d?
The central metal atom is large and so can coordinate more than six ligands
What is geometry is most common for 7 coordinate complexes?
Pentagonal bipyramidal?
What is the most common geometry for 8 coordinate?
Square antiprismatic
Cubic geometry is possible but not observed due to greater ligand ligand repulsions
What geometry is common for 2 coordinate complexes?
Linear
It is common for ions with low charge density
What geometry is common for 3 coordinate complexes?
Trigonal planar but is rare
What geometry is common for 4 coordinate complexes?
Tetrahedral or square planar
Tetrachloro complexes can adopt either depending on the nature of the metal ion
When is tetrahedral favoured over square planar?
Tetrahedral is favoured when the central atom is small and the ligands are large
When is the square planar confirmation observed?
Observed for metals with d8 configuration
What isomerism can square planar show and why is this important?
Cis or trans
Cis-Pt(II) are useful in cancer therapy- bind to base of DNA
The trans is not active
What geometry is common for 5 coordinate complexes?
Trigonal bipyramidal and square pyramidal
They are less common
Why do 5 coordinate complexes often exist as intermediate geometries?
The energy difference between square pyramidal and trigonal bypyramidal is small
Ligands are fluxional- often change sites
How do ligands attach to the metal ion?
Via coordinate bonds to the donor atom
What is the denticity of a ligand?
This is the number of donor atoms that are coordinated to the metal ion
What denticity are ligands with 2, 3 ,4,5,6 donor atoms?
2 is bidentate 3 is tridentate 4 is tetradentate 5 is pentadentate 6 is hexadentate
2+ are polydentate
What does a bidentate ligand form when it coordinates to a metal ion?
Forms a chelate ring (5 or 6 memebered)
What types of ligands are there?
Monoatomic anions
Polyatomic anions
Neutral molecules cations
Why are soft metals dangerous?
Soft metal ions can bind to soft complexes in the body which is toxic
What is porphyrin?
This is a macrocyclic ligand
It’s iron complex is present in haemoglobin
What is EDTA?
A hexdentate ligand
It can bond strongly to metals so good in analytical chemistry and medicine
Treats metal poisoning
What are hard metals and what do they bond to?
Hard metals carry a high charge or have a high charge density
Favouring forming bonds to ligands with small electronegative donor atoms (hard ligands)
This bond has a high degree of ionic character
What is a hard ligand?
Ligands that have small electronegative donor atoms
What are soft metals and what do they form bonds to?
Soft metals have a lower charge and tend to be larger and more polarisable
They form bonds to ligands that have larger more polarisable donor atoms (soft ligands)
The bond has a high degree of covalent character
What is a soft ligand?
Ligands that have larger more polarisable donor atoms
What do intermediate metals form bonds to?
They can form strong bonds to both hard and soft Ligands
What do intermediate Ligands form bonds to?
They can form strong bonds to both hard and soft metals
Describe the difference in polarisability in ligand donor atoms or metal ions?
Polarisability (experience a charge shift)
Hard centres are little affected
Soft centres are easily polarised
Describe the bonding between hard/ hard and soft/soft
Affinities are high between centres of the same type
Hard/ hard have highly ionic bonds
Soft/ soft have partly covalent bonds
Using this you can predict the stability of complexes
What are examples of hard and soft electron pair donors?
Hard= fluoride and negatively charged oxygen
Soft= thiolates and sulphides
