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Metal-Ligand Bonding > General > Flashcards

General Flashcards

1
Q

D-electron count =

A

Group number - ox state

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2
Q

TVEC=

A

D-electron count + ligand electrons + number of M-M bonds

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3
Q

Why complexes form? The 2 forms of DeltaG?

A

= deltaH - T × deltaS = -RTlnKf

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4
Q

Why complexes form:

Enthalpic effects?

A

More ligands means more stable

Number of ligands limited by ligand-ligand repulsion

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5
Q

Why complexes form:

Entropic effects

A

Chelate rings

Solvation

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6
Q

Which orbital does a pi-acceptor accept electron density into?

A

LUMO

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7
Q

Which orbital affects sigma donating most?

A

HOMO

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8
Q

How can sigma donating and pi accepting be described as a team?

A

Synergic

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9
Q

Which sort of ligand best stabilizes metal complexes with low oxidation states? And why?

A

Pi acceptors

Relieve negative charge build up at a metal centre

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10
Q

How does the IR frequency of a pi accepting ligand bond (i.e. C-O bond) reflect the nature of the metal and its interaction with the ligand?

A

The more electron dense the metal, the higher the IR frequency is and also the more pi accepting the nature of the M-L interaction is.

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11
Q

How can you break a strong bond such as N-N or O-O?

A

Insert more electrons into antibonding orbitals and hence lower bond order.

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12
Q

What affects the strength of the M-L bond for pi-acceptors?

A

Increased orbital overlap increase strength of bond

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13
Q

What’s special about NO as a ligand?

A

Can change binding mode

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14
Q

What number of electrons do linear NO and bent NO donate?

A

Linear is 3

Bent is 1

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15
Q

Whats the strategy for determining whether NO as a ligand is bent or linear in a particular case?

A

1- ignore all NO’s and do a TVEC
2- add either 1 or 3 electrons per NO until you reach 18 and hence can determine if bent or linear
3- determine oxidation state by first ignoring NO and then adding NO+ (linear) and NO- (bent)

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16
Q

When will Eta2-H-H transform into a dihydride?

A

When enough electron density is transferred from metal to H2, the H-H sigma bond will break, giving 2 M-H sigma bonds

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17
Q

Alkenes as ligands:
Donate and accept into which orbitals?
What happens when they rotate as ligands?

A

Donate frrom pi, accept into pi*.
Sigma bonding unaffected by rotation
Pi bonding broken each rotation

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18
Q

What are the interactions involved with a pi-donor ligand?

A

Sigma donating

And interaction between filled ligand orbital and empty metal orbital

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19
Q

Characteristics of metals that tend to have pi donor ligands?

A

Low d electron count and/or high ox state

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20
Q

Effects of pi donors on chem of metal complexes in terms of MO Theory?

A

T2g d orbitals are raised in energy and hence have pi* character, reducing deltaOCT

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21
Q

Effect of pi acceptors on chem of metal complexes in terms of MO theory?

A

T2g orbitals lower in energy and are pi bonding, hence increasing deltaOCT

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22
Q

Bearing in mind the effect of pi donors on the MO of metal complexes, what spin are pi donor ligands and why?

A

High spin

Due to decrease in size of deltaOCT

23
Q

Order of ligand addition is important due to…

A

Trans-effect and trans-influence

24
Q

Why are 4d and 5d complexes not labile like certain 3d complexes?

A

High CFSE and better metal-ligand overlap

25
Q

What does the dagger notation mean?

A

“Of activation”

26
Q

Ligand substitution mechansim for square planar complex:

A

Via 5 coordinate trigonal bipyramidal intermediate

27
Q

When, how and why is isomerism achieved in substitution of ligands on a square planar complex?

A

When transition state is sufficiently lived

Via pseudorotation because trigonal bipyramids and trigonal prismatic geometries having very similar energies

28
Q

Factors affecting rate of substitution

A

Entering group
Leaving group
Nature of other ligands in the complex (trans effect)

29
Q

Whats the trans effect? And how does it come about?

A

The ability of a ligand to direct the replacement of the ligand trans to it
Essentially arises from the competition of trans ligands for the same orbitals

30
Q

What is the free energy of activation on an energy diagram and what is its significance in terms of rates?

A

Energy difference between ground state and first transition state and a decrease in it will increase ligand substitution rate

31
Q

Minimising free energy of activation:

A

Destabilisation of ground state

Stabilisation of transition state

32
Q

How can ground state be destabilised?

A

Some ligands weaken the M-L bond trans to them

-repulsion of ligands trans to strong sigma donors

33
Q

How can the transition state be stabilised?

A

Strong pi acceptors stabilise the transition state by accepting electron density from the incoming nucleophile and hence relieving charge build up at the metal centre.

34
Q

Order of ligand type’s ability to direct trans-substitution

A

Pi acceptors > pi donors > sigma donors

35
Q

Possible mechanisms for ligand substitutions in octahedral complexes

A

Associative
Dissociative
Interchange (Ia or Id)

36
Q

Whats the complication in identifying if Ia or Id?

A

Pseudo 1st order rxn,

Conc dependance on incoming ligand will be masked

37
Q

Evidence for Id mechanisms

A

Entering group effect
- not huge difference in rate constants when varying incoming group therefore suggesting no Ia
Plotting logK vs logk
- as thermodynamic driving force for the reaction increases, the reaction rate also increases (supports Id)
Steric effects
-bulky leaving group increases rate for Id and bulking entering group would decrease rate for Ia
Electron count of intermediate
- Ia would be 20e (🚫)
-Id would be 16e 👌

38
Q

Electron transfer reactions

Difference between IS and OS in terms of bond breakages

A

OS no bonds are broken or formed

IS bonds are broken and formed

39
Q

Quick overview of OS

A

Occur between complexes that do not undergo ligand substitution

40
Q

Quick overview of IS

A

Occurs between complexes via a bridging ligand.

At least one of the complexes must be labile or have a vacant coordination site to allow the bridge to form

41
Q

What sorts of complexes are low spin and which are high spin?

A

LS pi acceptors

HS pi donors and sigma donors

42
Q

Mechanism of OS transfer

A

The ions come into close proximity forming a precursor complex.
As electron is being transferred, this is the highest energy point of the complex as the electron has equal probability of being on either metal.
Complex relaxes to the successor complex

43
Q

Which energies does the total free energy of OS transfer need to take into account?

A

Energy to bring the reactants together
Solvent reorganisation energy
Energy required for reorganisation of bond lengths to make the interacting orbitals of the same energy

44
Q

Solvent reorganisation energy. Why are solvents needed and how do they affect rate of electron transfer

A

Allow 2 positively charged complexes to approach one another

Solvents that interact strongly with complexes (eg hyd bonding) will reduce rate of electron transfer

45
Q

For OS electron transfer:

Why in octahedral fields is the eg to eg electron transfer slow?

A

Large change in bond length

Poor overlap of orbitals due to ligand sterics

46
Q

For OS electron transfer:

Why in octahedral fields is the eg to eg electron transfer slow?

A

Large change in bond length

Poor overlap of orbitals due to ligand sterics

47
Q

For OS electron transfer why in octahedral fields are t2g to t2g electron transfers fast?

A

Small change in bond length

T2g orbitals are between ligands hence better t2g-t2g overlap

48
Q

Why can phenyl assist OS E.T.?

A

Has an extended pi system

49
Q

What is required if a complex has to change electron config to allow electron transfer?

A

Activation energy

50
Q

Whats required to happen to an octahedral complex before IS electron transfer?

A

Dissociation of a ligand so that there is a vacant coordination site to form a bridge.

51
Q

What determines if a ligand transfers in IS electron transfer?

A

Relative bond strengths

52
Q

Factors that affect IS electron transfer rate

A

Depending on inertness or lability, formation of the bridging complex can be the RDS
Electron config
- sigma* orbitals interact strongly with bridging ligand
Bridging ligand
- bulk etc
- M to L to M (2 step process) gets around the need for the reorganisation energy to account for both complexes at the same time.

53
Q

How to figure out if OS or IS is gonna take place?

A

If its possible to go pi/pi* to pi/pi* then OS will take place
If rearrangement of orbital energies is required for OS then IS will take place instead

54
Q

What makes NO special as a ligand and what does that then mean it can do?

A

Exists as either 1 or 3 electron donor. Can change from 3 electron to 1 electron donor to allow for an associative mechanism of ligand substitution

Metal-Ligand Bonding (1 decks)

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