Benniston Flashcards
What 3 types of reactions are there at the octahedral site
Ligand substitution
Electron transfer
Activation of ligands
What is ligand stubstitution
Addition or removal of ligands
What do we need to consider when looking at ligand substitution
Does the ligand that is being replaced simply fall off or does the new ligand associate with the complex first? How fast is the ligand replaced and does the surrounding solvent molecule have a part to play in the reaction?
What is electron transfer
Change in oxidation state of the metal centre
What do we need to consider when looking at electron transfer
Most metals have more than one oxidation state and so we can think about electron transfer at the metal site?
What do we mean by activation of ligands
Attack at a ligand
What do we need to consider when looking at activation of ligands
Since a ligand uses a lone pair of electrons to bind to a metal does this donation activate the ligand in any way
What is a mechanism and how do we determine a mechanism
A mechanism describes all the pathways in which a reactant is converted to a product
If a reaction goes by one pathway what would we need to know?
- how the reactant’s ground state get activated
- the structure of a defined intermediate
- how the intermediate goes to the product
When do problems arise for a mechanism
When we get more than one pathway
What conditions are probing of inorganic reactions done under
Pseudo first order conditions
How do we probe mechanisms
One way of probing mechanisms is through the rate law, by running kinetic experiments and determining the rate law
What does the kinetic information from probing mechanisms tell us
- the number and type of each molecule participating in the formation of the transition state
- the rate constant
How could we use the eyring equation to describe reaction mechanisms
The rate of a reaction can be measured over a temperature range and related to parameters in the eyring equation. By rearranging the equation we can plot a graph that should be a straight line. Any deviation from this straight line tells us that there is something odd going on in the reaction
What is KB in the eyring equation
boltzmann’s constant
What is h in the eyring equation
Planks constants
Draw a basic energy reaction pathway for a metal reacting with a ligand
see flash card
How can we relate the free energy change of the reaction
we can relate this to the free energy of activation for the forward and the reverse reaction
∆Gr‡ = ∆Gf‡ + (-∆G)
What is the equation for ∆G
∆G = -RTlnK
What is the equation for ∆Gf
Where f means forward
∆Gf‡ = ∆Hf‡ - T∆Sf
What is the equation for ∆Gr
Where r is the forward reaction
∆Gr‡ = ∆Hr‡ - T∆Sr‡
How can we rewrite the ∆G relating to enthalpy
∆Hf‡ - ∆Hr‡ = ∆Go
What does ‡ mean
activation energy
So ∆Hf‡ means the enthalpy of activation for the forward reaction.
If we have values for ∆H‡ and ∆S‡ what deductions could be made
- Changes in ∆H‡ for similar reactions may indicate that rates of
reaction are due to alterations in bond strengths. - Values of ∆S‡ (positive or negative) may tell us whether reactions
proceed by a DISSOCIATIVE or ASSOCIATIVE mechanism
What is the conjugate base mechanism
In this mechanism the first step is formation of a conjugated base of the metal complex by deprotonating one of its ligands. This allows facilitated dissociation with one of the other ligands. The 5 coordinate intermediate picks up a new ligand.
What is a better parameter than the activation parameter to deduce a mechanism
the volume of activation
What does the volume of activation parameter tell us
tells us the volume change from moving from the starting ground state complex to its activated complex. It can be positive or negative and in some classes close to zero
What is the volume of activation parameter measured by
the parameter is measured by looking at the effect of pressure on the reaction rate
How do we calculate the volume of activation
a plot of ln(k/kp=0) vs P (MPa) will have a slope ‘s’ and so
∆V‡ = RT x S (cm3 mol-1 at 298.2 K)
At another temperature tou would need to calculate R times T
What are the units for ∆V‡
The units of R x T = J K-1 mol-1 x K = J mol-1 but the unit Joule can be broken down further since J = force x distance = N m. If we plot a graph of ln(k/k0) vs P in Pascals the
slope would be 1/Pascals. Since a Pascal is a unit of pressure = force/area = Nm-2. Thus, the unit for R x T x slope = N m mol-1x 1/N m-2 = N m x N-1 m2 mol-1 = m3 mol-1.
Define lability
the ability of a complex to engage in reactions that result in replacing one or more ligands
by others.
What is a labile comples
Complexes which undergo reactions that result in replacing one or more ligands by others very rapidly is known as labile
What are inert complexes
If the reaction of replacing on or more ligands by others proceeds slowly or not at all are considered inert
What is the classification by taube for labile metal ions
Labile metal ions have either a low-energy t2g orbital or at least one electron in a high energy eg orbital
What is the classification by taube for inert metal ions
Inert metal ions will have at least one electron in each t2g orbital and no electrons in eg orbitals
Define dissociative in terms of ligand binding
The ligand departs before the new ligand binds
Define associative in terms of ligand binding
the new ligand binds before any ligands depart
If we start from an octahedral complex and lose a ligand what shape do we produce
Square based pyramid
If we start from an octahedral complex and gain a ligand what shape do we produce
An octahedral wedge
What does CFAE stand for and what is it
Crystal field stabilisation energy (CFSE) calculated for ground state and transition state and
difference defined as the Crystal Field Activation Energy (CFAE)
What is the equation for CFAE
CFAE=CFSE(after) - CFSE(before)
What does a negative CFAE tell us
A negative number tells us that it is favourable to move from an octahedra lgeometry to either a square-based pyramid or octahedral wedge.
Labile complex
What does a positive CFAE tell us
It is favourable to move from a square based pyramid or octahedral wedge to octahedral geometry
Inert complex
