Coordination Chem Flashcards
How do you classify ligands?
- State if acylic or cyclic
- Number and type of donor atoms
- Saturated or unsaturated
What are the types fo bidentate ligands?
1,1 when both donor groups bonded to same C
1,2 when one bonded to carbon and other bonded to adjacent C
etc
What are the types of isomerism in TM complexes?
Structural - same atoms, different order in complex
Stereoisomers - different arrangement in 3D space
What are the forms of structural isomerism?
Hydration - ligand replaced by water of hydration
Ionisation isomerism - interchange of anionic ligands
Linkage isomerism - when ligand has >1 lewis base (i.e. -NO2)
What are the monodentate geometric isomerism in square planar & octahedral geometries?
What are the octahedral geometric isomerism when there are tridentate ligands?
What type of ligands prefers fac or mer isomers?
Fac - ligands not flat (e.g. bound to cyclohexane ligand)
Mer - requires flat (e.g. aromatic)
What is the requirement for a chiral complex?
No Sn axis
Test as S1 = σ and S2 = i
What is the equilibrium association constant (Ka) for complex formation?
Ka = Kon/Koff
A + B ⇔ AB
Where kon is forward reaction and koff is backwards
What is the stability of complexes in terms of kinetics and thermodynamics?
Thermo - free energy changes when reversible, described by stable/unstable
Kinetic - rate of exchange proccess, descirbed by inert/labile
How do you determine Ka?
UV-vis spec
Electrochemical methods
NMR
What is the stepwise stability constant?
βn = K1 * K2 * K3*…Kn
βn = [MLn] / ([M][L]n]
Different units to Kn
What is the trend in Ka for successive binding?
Usually a general decrease as:
Stat less likely
Steric hindrance increasing (if larger than former ligand)
Coulombic factors if ligand more charged than former so more repulsion
What occurs ot the Ka of Cu?
Decreases to n = 4 generally and then sig decreases to n=5/6
Cu2+ is d=9 so JT distortion so compresses in xy plane and elongates in z-axis
Stronger bonds to ligand in xy plane (1st 4 ligands bind here)
What are some common deviations from standard stepwise stability constant?
Stereochemical change - i.e. changing from octahedral to tetrahedral
Spin state change - high to low requires large increase in Δ by overcoming pairing e-
What is the Irving-Williams series?
Log K (y-axis) vs M2+ complex (x-axis)
Of first row TM and can be with different ligands
What are the explanations of the trends in the Irving-Williams series?
COMMON Q
General increase: Zeff increases across series - r+ decreases, lowers 3d of TM so better energy match
Unusual increase after Mn: LFSE - increases from 0 at Mn2+ as e- fill the t2g to Ni2+ then decrease. Increases stability of complex
Cu2+ is high (when n=1-4) as JT effect - strong binding of ligand in xy plane
How does LFSE change across M2+ across from V2+ to Zn2+?
Decreases from V2+ to 0 at Mn2+
Increases from Mn2+ to Ni2+
Decreases to Cu2+ but still higher K due to JT effect
No LFSE for Zn2+
How does the irving-williams series change for K1 to K3 dissociation constants?
K2 is same as K1 but all values are lower
K3 is lower in general, exception is dip at Cu2+ (instead of peak)
Why is there a lower than expected K3 value for octahedral Cu2+ compounds?
(when en complex)
Ligand too rigid for JT distortion, so the compound must lose the JT distortion
What is Hard soft acid base theory?
Hard metal ions pair with hard ligands
Soft metal ions pair with soft ligands
What are examples of hard metal ions?
Mn2+, Sc3+, Cr3+, Fe3+, Ti3+
Group 1 & 2, Ln3+
What are examples of hard ligands?
F-, R2O, ROH, OH-, NR3, Cl-
What are examples of soft metal ions?
Hg+, Hg2+, Cu+, Ag+, Au+, Pd2+
Generally 2nd or lower row TM
What are examples of soft ligands?
R2S, R3P, CO, CN-
What are hard-hard interactions defined by?
Ionic character
Large HOMO-LUMO gap
Dominated by desolvation of acid/base - ΔS
How are soft-soft interactions defined?
Covalent character
Small HOMO-LUMO gap
Dominated by orbital overlap - ΔH
How does geometric complementarity effect binding affinity?
Improved size match gives larger binding affinity
In cryptands - larger the group favours larger cations, as determined by hard interactions so depends on release of solvation molecules from aqua-complex (ΔS favourable)
What is the definintion of the chelate effect?
Enhanced stability of complexes containing chelate rings, as compared to stability of a system with fewer chelate rings
Why does the chealate effect occur?
Entropically favourable - more molecules in products than reactants
Enthalpically favourable - polar donor groups covalently linked to as some lp-lp repulsion in ligand synthesis, and alkyl linking donor atoms tends to increase basicity due to inductive effect
Why does the binding of one of donor atoms bind to a ligand?
After first has bonded, the second is tethered to the first so effective concentration much higher than when free in solution
This changes the equilibrium
How does the chealate effect change as ring size increases and why?
Decreases generally
Repulsion between donor atoms overcome in ligand synthesis becomes less significant as not as close
Effective concentration lower as the chain increases
What is the special case of 5/6 membered chelate rings with N donors?
Small ions prefer 6-membered as donor atoms closer together due to cycohexane chair
Large ions prefer 5-membered as donor atoms further apart
What is the definition of macrocyclic ligands and their properties?
Polydentate ligand with at least 3 donor atoms, with a cyclic structure
Thermo stable - high binding constant
Kinetically inert - rate of removal of M is slow
What is the macrocyclic effect?
Macrocylclic ligand more stable than acyclic analogues
Have to look at data to determine if enthalpically or entropically driven
What is the effect for enthalpic preference of macrocylic?
Complentarity - size match leads to better M-L bonds
Solvation of ligand - macrocylic ligands less strongly solvated as less exposed to solvent, less ligand-solvent bonds to break
Ligand pre-organisation - synthesis overcomes repulsive forces between polar donor groups
What is the cause of entropic effect of macrocyclic effect?
Conformational entropy - the more organised the more favourable ΔS, less conformatioally flexible so lose fewer degrees of freedom upon complexation
What is the kinetic contribution to macrocyclic effect?
kf is formation, kd is dissociation, K = [MLn+]/[Mn+][L]
K doesn’t say anything other than absolute values of kf and kd
kf / kd is sig larger for cyclic
How does kf and kd compare in cyclic and acylic?
kf - cyclic 102-103 slower than acyclic, could be any step from:
Formation of outer-sphere complex, exchange ooen solvent for a new M-L bond, repeat to form new M-L bonds
kd - cyclic 105-107 times slower than acylclic
What is the effect of preorganisation on stability?
More organised for binding & low solvation before ocmpelxation means a more stable complex
What is the preorganisation of different macrocylic complexs?
From most organised & stable to least:
spherand > cryptand > crown ether > acyclic polyether
What is the template effect?
Metal ions promote cyclisation reactions which are otherwise not possible
M-L interactions used to preorganise compnents into desired geometry for reaction
Favoured when conformation of reaction not thermo or kinetically favoured
What are some examples of macrocylic synthesis?
Curtis synthesis of Schiff base macrocycle
Schiff-Base condensation - depending on size of M can have 1+1 of separate starting materials or 2+2
S3 macrocycle - fac NC-Me (weakly coordinating) replaced with 3 S contianing ligand
What is the thermodynamic and kinetic template effect?
Thermo - metal ion complexation stabilises one compnent of eqm and shifts it
Kinetic - coordination of macrocylic precursor holds reactive group in correct geometry, preventing polymerisation
What is the de-metallation problem and solution with the template effect?
Can be difficult to remove the metal template
Solutions: add competing ligand (high conc), reduce/protonate donor groups so less affinity to metal, redox of metal
What is the identity problem and solution with the template effect?
Hard to identify correct Mn+ for a process
Solutions: trial/error, metal exchange reactions, other synthetic routes like high dilution synthesis
What is the high dilution synthesis method?
Synthesis of macrocycles - lower polymerisation via intermolecular reactions by working at low conc, makes intramolecular more likely
What are the two common mechanically locked molecules?
Rotaxane - ring surrounding an axle with stoppers to prevent dethreading
Catenane - formed of interlinked rings held mechanically
Both require metal templation for any decent yield
What metals are used in template synthesis of catenanes and rotaxanes?
Metal depends on coordination needed
Td - Cu(I)
Square planar D4h - Pd(II), Pt(II)
Oh - first row TMs
Linear - Ag(I), Au(I)
Define a molecular machine
Molecules which carry out some type of controlled mechanical motion/work in response to external stimuli
