Coordination Chemistry 1 Flashcards
Transition Metal
An element with a partially filled d- (or f-) sub-shell in at least one common oxidation state.
Complex (or Coordination Compound)
Positively charged central ion (or possibly a neutral atom), an acceptor, surrounded in a symmetrical manner by a shell of ions or molecules called ligands.
Acceptor
electrophile or lewis acid
Ligand
nucleophile or lewis base
Monodentate (or Unidentate) Ligands
One donor atom - they can be neutral or anionic ligands
Bidentate Ligands
Two donor atoms
Tridentate Ligands
Three donor atoms
Tetradentate Ligands
Four donor atoms
Hexadentate Ligands
6 donor atoms - [EDTA] 4-
Chelation
Formation of complexes by chelate ligands - simultaneous binding of multiple donor atoms by forming rings around the central atom
Ambidentate Ligands
Ligands that can attach themselves to the other central metal atoms through different atoms
Bridging Ligand
A ligand attached to two or more, usually metallic, central atoms.
Coordination number
Number of ligand atoms directly bonded to the central metal in the complex.
Nuclearity
The number of central metal atoms in a complex
Isomers
Two or more different compournds having the same formula, but different structures.
Stereoisomers
different arrangements of atoms
Structural Isomers
different bonds between atoms
Conformational isomers
interconvertible by bond rotation
Configurational isomers
non-interconvertible by bond rotation
Optical Isomers: enantiomers
- have the same atoms, same sets of bonds but differ in relative orientation of these bonds
- non-superimposable mirror images
- they have identical physical, chemical and spectral properties except for they interact differently with a chiral environment
- they are optically active
Diastereoisomers
not mirror images
Ʌ
left-handed helix for optical isomerism in an octahedral complex with bidentate ligands
Δ
right-handed helix for optical isomerism in an octahedral complex with bidentate ligands
Resolution of optical isomers
A racemic mixture is separated into its two constituent enantiomers by converting the enantiomers into a mixture of diastereoisomers, which differ in physical properties and can therefore be separated (they have different solubilities).
Polarimeter
used to measure the angle of rotation of each enantiomer
- the observed rotation is proportional to the amount of each enantiomer present
Coordination Isomerism
Form of structural isomerism in which the composition of the coordination complex ion varies.
- Made up of both cationic and anionic complex ions
- Neutral species are not allowed
Hydrate (solvent) Isomerism
Possible with water in or out of coordination sphere - the compounds differ by the number of solvent molecules directly bonded to the metal ion.
Ionisation Isomerism
Involves the exchange of ions inside and outside of the coordination sphere.
- Made up of a complex ion and counter ion (counter ion must also be able to be a ligand)
Linkage Isomerism
Coordination compounds with the same composition but differ in their metal atom’s connectivity to a ligand (ambidentate ligands only)
OH 2 ligand
aqua
N 2 ligand
dinitrogen
O 2 ligand
dioxygen
NH 3 ligand
ammine
CO ligand
carbonyl
Cl - ligand
chloro (chlorido)
Br - ligand
bromo (bromido)
[SCN] - ligand
thiocyanato / isothiocyanato
OH - ligand
hydroxo (hydroxido)
CN - ligand
cyano (cyanido)
[NO 2 ] - ligand
nitro / nitrito
Greek derived prefix: 1
mono
Greek derived prefix: 2
di
Greek derived prefix: 3
tri
Greek derived prefix: 4
tetra
Greek derived prefix: 5
penta
Greek derived prefix: 6
hexa
Greek derived prefix: 7
hepta
Greek derived prefix: 8
octa
Multiplicative prefixes: 2
bis
Multiplicative prefixes: 3
tris
Multiplicative prefixes: 4
tetrakis
Multiplicative prefixes: 5
pentakis
Multiplicative prefixes: 6
hexakis
How to achieve large negative values of gibbs free energy?
When K (equilibrium constant) is large - equilibrium lies on RHS.
The stronger the new M-L bond being formed.
What does K 1 , K 2 , etc stand for?
Stepwise Stability Constants
What does β mean for stability constants?
Overall Stability Constant
What information does β give?
Large values of β indicate that the concentration of the complex is much larger than the concentration of its constituents.
What value of β is thermodynamically stable?
β ≈ 10 8
Overall stability constant for stable complexes?
log β > 0 (β > 1) i.e. ΔG° is -ve
Overall stability constant for unstable complexes?
log β < 0 (β < 1) i.e. ΔG° is +ve
How does statistical error impact equilibrium constants?
Successive equilibrium constants will decrease if there are no changes in geometry,
i.e. K 1 > K 2 > K 3 > 4 > K 5 > K 6
How does sterics impact equilibrium constants?
Bulky ligands can hinder the approach of subsequent ligands
How does electrostatic factors impact equilibrium constants?
Each successive replacement of a water ligand with an anionic ligand reduces the positive charge on the complex.
How does geometry impact equilibrium constants?
Large differences in values occur when there is a change in geometry
Chelation
formation of complexes by chelate ligands, simultaneous binding of multiple donor atoms by forming rings around the central atom.
Chelate Effect
Chelate complexes are more stable than complexes with similar monodentate ligands due to higher stability constants.
Why does chelation decrease gibbs free energy ?
- Because similar ligands are being replaced, there is no effect on ΔH
- Difference is ΔS as it creates more disorder
Macrocyclic ligand
3 or more potential donors in a ring of at least 9 atoms.
Macrocyclic effect
Stability constants for complexes of macrocyclic ligands are higher than those for their acyclic counterparts thus making more stable complexes.
How does the macrocyclic effect decrease gibbs free energy?
It can be due to ΔH or ΔS
Hard acids (acceptors)
- non-polarisable cations
- small radius
- high effective nuclear charge
(higher charge-to-radius ratio) - high energy LUMO
Hard acids examples
- most metals in normal oxidation states
- H +
- all electropositive metals (groups I, II, etc)
Soft acids (acceptors)
- more polarisable cations
- larger radius
- lower charge-to-radius ratio
- lower energy LUMO than hard acids
Soft acids examples
- Cu (I), Rh (I), Ag (I), Au (I)
- Pd (II), Pt (II), Cd (II), Hg (III), Au (III)
- all d-block metals in M(0)
Examples of borderline acids (acceptors)
- Fe (II), Co (II), Ni (II), Cu (II)
- Tl, Pb, Bi
Hard bases (donors)
- small
- non-polarisable
- electronegative
- difficult to oxidise
- low energy HOMO
Soft bases (donors)
- big
- more polarisable
- easier to oxidise
- higher energy HOMO
How are most stable complexes formed? (acids and bases)
- complex of a hard acid with a hard base
- complex of a soft acid with a soft base
LIKE GOES WITH LIKE
What is crystal field theory (CFT)?
it describes the breaking of orbital degeneracy in transition metal complexes due to the presence of ligands
- it describes the strength of the metal-ligand bonds
Assumptions made in crystal field theory
- central metal ion is the point positive charge
- ligands are regarded as dipoles or anions
- ligands are the point negative charges
- ionic bonding from electrostatic forces between positive and negative charges
NO ORBITAL OVERLAP OR ELECTRON SHARING
What orbitals are filled first for d-block complexes?
d orbitals are filled before s
How do you calculate d n configuration?
d n configuration = Group no. - Oxidation State
Hund’s Rule
if there is more than one degenerate orbital, electrons first occupy separate orbitals with parallel spins
Pauli Exclusion Principle
no more than two electrons can occupy a single orbital
if two electrons occupy a single orbital the spins must be paired
Pairing Energy, P
the energy penalty for pairing two electrons in a single orbital
(pairing energies of 4d and 5d metals tend to be lower than for 3d)
When is an octahedral complex high spin?
Δo<P
if Δo is small, there isn’t much different between t 2g and e g
When is an octahedral complex low spin?
Δo>P
when there is a large energy difference
what sort of spin are 4d and 5d metal complexes?
low spin complexes
Generally for tetrahedral complexes, what sort of spin do they have?
Pairing is more unfavourable than filling t 2 orbitals so tetrahedral complexes are usually high spin
Factors affecting the magnitude of Δo
- metal oxidation state
- position in periodic table
- the spectrochemical series of ligands
How does metal oxidation state affect Δo?
- as charge on metal increased, Δo gets larger
- smaller sized, highly charged ions
thus smaller M-L bonds and stronger interactions and so stronger splitting
(ELECTROSTATIC EFFECT)
How does the position on the periodic table affect Δo?
larger d orbitals have stronger interactions
(5d>4d>3d)
Δo (1st row) < Δo (2nd row) < Δo (3rd row)
(SIZE EFFECT)
How can you distinguish between high spin and low spin complexes?
by their magnetic properties
look at the no. of unpaired electrons
diamagnetic: no unpaired electrons
paramagnetic: has unpaired electrons
What causes distortions of structures in complexes?
asymmetric filling of the d-orbitals
What structure do complexes with bulky ligands favour?
Tetrahedral structure due to the geometry having bigger bond angles
What d n electron count shows both high and low spin?
d 6
How can isomers be distinguished?
Using IR spectroscopy and X-ray crystallography
How can isomers be distinguished by properties of the ligands?
1H NMR spectroscopy
What do you use in the resolution of optical isomers?
Potassium sodium (+) tartrate
Stepwise vs Overall stability constants
Stepwise stability constants are equilibrium constants given for each step of the process of ligand substitution.
The overall stability constant is the equilibrium constant of the overall reaction.
