General Flashcards
D-electron count =
Group number - ox state
TVEC=
D-electron count + ligand electrons + number of M-M bonds
Why complexes form? The 2 forms of DeltaG?
= deltaH - T × deltaS = -RTlnKf
Why complexes form:
Enthalpic effects?
More ligands means more stable
Number of ligands limited by ligand-ligand repulsion
Why complexes form:
Entropic effects
Chelate rings
Solvation
Which orbital does a pi-acceptor accept electron density into?
LUMO
Which orbital affects sigma donating most?
HOMO
How can sigma donating and pi accepting be described as a team?
Synergic
Which sort of ligand best stabilizes metal complexes with low oxidation states? And why?
Pi acceptors
Relieve negative charge build up at a metal centre
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?
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.
How can you break a strong bond such as N-N or O-O?
Insert more electrons into antibonding orbitals and hence lower bond order.
What affects the strength of the M-L bond for pi-acceptors?
Increased orbital overlap increase strength of bond
What’s special about NO as a ligand?
Can change binding mode
What number of electrons do linear NO and bent NO donate?
Linear is 3
Bent is 1
Whats the strategy for determining whether NO as a ligand is bent or linear in a particular case?
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)
When will Eta2-H-H transform into a dihydride?
When enough electron density is transferred from metal to H2, the H-H sigma bond will break, giving 2 M-H sigma bonds
Alkenes as ligands:
Donate and accept into which orbitals?
What happens when they rotate as ligands?
Donate frrom pi, accept into pi*.
Sigma bonding unaffected by rotation
Pi bonding broken each rotation
What are the interactions involved with a pi-donor ligand?
Sigma donating
And interaction between filled ligand orbital and empty metal orbital
Characteristics of metals that tend to have pi donor ligands?
Low d electron count and/or high ox state
Effects of pi donors on chem of metal complexes in terms of MO Theory?
T2g d orbitals are raised in energy and hence have pi* character, reducing deltaOCT
Effect of pi acceptors on chem of metal complexes in terms of MO theory?
T2g orbitals lower in energy and are pi bonding, hence increasing deltaOCT
Bearing in mind the effect of pi donors on the MO of metal complexes, what spin are pi donor ligands and why?
High spin
Due to decrease in size of deltaOCT
Order of ligand addition is important due to…
Trans-effect and trans-influence
Why are 4d and 5d complexes not labile like certain 3d complexes?
High CFSE and better metal-ligand overlap
What does the dagger notation mean?
“Of activation”
Ligand substitution mechansim for square planar complex:
Via 5 coordinate trigonal bipyramidal intermediate
When, how and why is isomerism achieved in substitution of ligands on a square planar complex?
When transition state is sufficiently lived
Via pseudorotation because trigonal bipyramids and trigonal prismatic geometries having very similar energies
Factors affecting rate of substitution
Entering group
Leaving group
Nature of other ligands in the complex (trans effect)
Whats the trans effect? And how does it come about?
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
What is the free energy of activation on an energy diagram and what is its significance in terms of rates?
Energy difference between ground state and first transition state and a decrease in it will increase ligand substitution rate
Minimising free energy of activation:
Destabilisation of ground state
Stabilisation of transition state
How can ground state be destabilised?
Some ligands weaken the M-L bond trans to them
-repulsion of ligands trans to strong sigma donors
How can the transition state be stabilised?
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.
Order of ligand type’s ability to direct trans-substitution
Pi acceptors > pi donors > sigma donors
Possible mechanisms for ligand substitutions in octahedral complexes
Associative
Dissociative
Interchange (Ia or Id)
Whats the complication in identifying if Ia or Id?
Pseudo 1st order rxn,
Conc dependance on incoming ligand will be masked
Evidence for Id mechanisms
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 👌
Electron transfer reactions
Difference between IS and OS in terms of bond breakages
OS no bonds are broken or formed
IS bonds are broken and formed
Quick overview of OS
Occur between complexes that do not undergo ligand substitution
Quick overview of IS
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
What sorts of complexes are low spin and which are high spin?
LS pi acceptors
HS pi donors and sigma donors
Mechanism of OS transfer
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
Which energies does the total free energy of OS transfer need to take into account?
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
Solvent reorganisation energy. Why are solvents needed and how do they affect rate of electron transfer
Allow 2 positively charged complexes to approach one another
Solvents that interact strongly with complexes (eg hyd bonding) will reduce rate of electron transfer
For OS electron transfer:
Why in octahedral fields is the eg to eg electron transfer slow?
Large change in bond length
Poor overlap of orbitals due to ligand sterics
For OS electron transfer:
Why in octahedral fields is the eg to eg electron transfer slow?
Large change in bond length
Poor overlap of orbitals due to ligand sterics
For OS electron transfer why in octahedral fields are t2g to t2g electron transfers fast?
Small change in bond length
T2g orbitals are between ligands hence better t2g-t2g overlap
Why can phenyl assist OS E.T.?
Has an extended pi system
What is required if a complex has to change electron config to allow electron transfer?
Activation energy
Whats required to happen to an octahedral complex before IS electron transfer?
Dissociation of a ligand so that there is a vacant coordination site to form a bridge.
What determines if a ligand transfers in IS electron transfer?
Relative bond strengths
Factors that affect IS electron transfer rate
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.
How to figure out if OS or IS is gonna take place?
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
What makes NO special as a ligand and what does that then mean it can do?
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
