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Year 2 - Chem > Coordination Chem > Flashcards

Coordination Chem Flashcards

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1
Q

How do you classify ligands?

A
  1. State if acylic or cyclic
  2. Number and type of donor atoms
  3. Saturated or unsaturated
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2
Q

What are the types fo bidentate ligands?

A

1,1 when both donor groups bonded to same C

1,2 when one bonded to carbon and other bonded to adjacent C

etc

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3
Q

What are the types of isomerism in TM complexes?

A

Structural - same atoms, different order in complex

Stereoisomers - different arrangement in 3D space

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4
Q

What are the forms of structural isomerism?

A

Hydration - ligand replaced by water of hydration

Ionisation isomerism - interchange of anionic ligands

Linkage isomerism - when ligand has >1 lewis base (i.e. -NO2)

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5
Q

What are the monodentate geometric isomerism in square planar & octahedral geometries?

A
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6
Q

What are the octahedral geometric isomerism when there are tridentate ligands?

A
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7
Q

What type of ligands prefers fac or mer isomers?

A

Fac - ligands not flat (e.g. bound to cyclohexane ligand)

Mer - requires flat (e.g. aromatic)

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8
Q

What is the requirement for a chiral complex?

A

No Sn axis

Test as S1 = σ and S2 = i

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9
Q

What is the equilibrium association constant (Ka) for complex formation?

A

Ka = Kon/Koff

A + B ⇔ AB

Where kon is forward reaction and koff is backwards

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10
Q

What is the stability of complexes in terms of kinetics and thermodynamics?

A

Thermo - free energy changes when reversible, described by stable/unstable

Kinetic - rate of exchange proccess, descirbed by inert/labile

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11
Q

How do you determine Ka?

A

UV-vis spec

Electrochemical methods

NMR

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12
Q

What is the stepwise stability constant?

A

βn = K1 * K2 * K3*…Kn

βn = [MLn] / ([M][L]n]

Different units to Kn

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13
Q

What is the trend in Ka for successive binding?

A

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

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14
Q

What occurs ot the Ka of Cu?

A

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)

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15
Q

What are some common deviations from standard stepwise stability constant?

A

Stereochemical change - i.e. changing from octahedral to tetrahedral

Spin state change - high to low requires large increase in Δ by overcoming pairing e-

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16
Q

What is the Irving-Williams series?

A

Log K (y-axis) vs M2+ complex (x-axis)

Of first row TM and can be with different ligands

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17
Q

What are the explanations of the trends in the Irving-Williams series?

COMMON Q

A

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

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18
Q

How does LFSE change across M2+ across from V2+ to Zn2+?

A

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+

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19
Q

How does the irving-williams series change for K1 to K3 dissociation constants?

A

K2 is same as K1 but all values are lower

K3 is lower in general, exception is dip at Cu2+ (instead of peak)

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20
Q

Why is there a lower than expected K3 value for octahedral Cu2+ compounds?

(when en complex)

A

Ligand too rigid for JT distortion, so the compound must lose the JT distortion

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21
Q

What is Hard soft acid base theory?

A

Hard metal ions pair with hard ligands

Soft metal ions pair with soft ligands

22
Q

What are examples of hard metal ions?

A

Mn2+, Sc3+, Cr3+, Fe3+, Ti3+

Group 1 & 2, Ln3+

23
Q

What are examples of hard ligands?

A

F-, R2O, ROH, OH-, NR3, Cl-

24
Q

What are examples of soft metal ions?

A

Hg+, Hg2+, Cu+, Ag+, Au+, Pd2+

Generally 2nd or lower row TM

25
Q

What are examples of soft ligands?

A

R2S, R3P, CO, CN-

26
Q

What are hard-hard interactions defined by?

A

Ionic character

Large HOMO-LUMO gap

Dominated by desolvation of acid/base - ΔS

27
Q

How are soft-soft interactions defined?

A

Covalent character

Small HOMO-LUMO gap

Dominated by orbital overlap - ΔH

28
Q

How does geometric complementarity effect binding affinity?

A

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)

29
Q

What is the definintion of the chelate effect?

A

Enhanced stability of complexes containing chelate rings, as compared to stability of a system with fewer chelate rings

30
Q

Why does the chealate effect occur?

A

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

31
Q

Why does the binding of one of donor atoms bind to a ligand?

A

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

32
Q

How does the chealate effect change as ring size increases and why?

A

Decreases generally

Repulsion between donor atoms overcome in ligand synthesis becomes less significant as not as close

Effective concentration lower as the chain increases

33
Q

What is the special case of 5/6 membered chelate rings with N donors?

A

Small ions prefer 6-membered as donor atoms closer together due to cycohexane chair

Large ions prefer 5-membered as donor atoms further apart

34
Q

What is the definition of macrocyclic ligands and their properties?

A

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

35
Q

What is the macrocyclic effect?

A

Macrocylclic ligand more stable than acyclic analogues

Have to look at data to determine if enthalpically or entropically driven

36
Q

What is the effect for enthalpic preference of macrocylic?

A

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

37
Q

What is the cause of entropic effect of macrocyclic effect?

A

Conformational entropy - the more organised the more favourable ΔS, less conformatioally flexible so lose fewer degrees of freedom upon complexation

38
Q

What is the kinetic contribution to macrocyclic effect?

A

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

39
Q

How does kf and kd compare in cyclic and acylic?

A

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

40
Q

What is the effect of preorganisation on stability?

A

More organised for binding & low solvation before ocmpelxation means a more stable complex

41
Q

What is the preorganisation of different macrocylic complexs?

A

From most organised & stable to least:

spherand > cryptand > crown ether > acyclic polyether

42
Q

What is the template effect?

A

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

43
Q

What are some examples of macrocylic synthesis?

A

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

44
Q

What is the thermodynamic and kinetic template effect?

A

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

45
Q

What is the de-metallation problem and solution with the template effect?

A

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

46
Q

What is the identity problem and solution with the template effect?

A

Hard to identify correct Mn+ for a process

Solutions: trial/error, metal exchange reactions, other synthetic routes like high dilution synthesis

47
Q

What is the high dilution synthesis method?

A

Synthesis of macrocycles - lower polymerisation via intermolecular reactions by working at low conc, makes intramolecular more likely

48
Q

What are the two common mechanically locked molecules?

A

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

49
Q

What metals are used in template synthesis of catenanes and rotaxanes?

A

Metal depends on coordination needed

Td - Cu(I)

Square planar D4h - Pd(II), Pt(II)

Oh - first row TMs

Linear - Ag(I), Au(I)

50
Q

Define a molecular machine

A

Molecules which carry out some type of controlled mechanical motion/work in response to external stimuli

51
Q
A

Year 2 - Chem (23 decks)

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