Higham L1-3 Flashcards
What does the magnitude of delt o depend on
1) metal oxidation state
2) 1st, 2nd, 3rd row metal
3) the nature of the ligand
How does the metal oxidation state affect delta O
the higher the oxidation state the larger delta o, as higher oxidation states bring ligands closer and increase electron repulsion
How do the position in the group affect delta o
delta O gets bigger descending a group and the big orbitals interact more with the ligand, metals at the top of the group are more likely to be paramagnetic and high spin
what is the problem with CFSE
CFSE considers ligands as point negative charge (electrostatic model) but if delta o does change when the ligand changes the model needs to be amended to consider L
What is the spectrochemical series
Some ligands are better at causing a larger delta o splitting than others, this leads to a league table of ligands being drawn up (spectrochemical series)
How can you follow transition energies
using UV-Vis spectroscopy
Which ligands cause delta o to be small
Weak field (high spin) ligands
I-, Br-, F-, OH-
What groups are in the middle of the spectrochemical series
H2O and NH3
Which ligands cause delta o to be big
strong field (low spin) ligands
PR3, CN-, CO
What sort of character do transition metal complexes have
transition metal complexes have considerable covalent character
What does the covalent bonding model involve
covalent bonding model involves molecular orbital treatment which must account for splitting of metal d orbitals and incorporate ligand sigma and pi orbitals.
Treat ligands as a group
What does the 4S orbitals give rise to
most symmetric combination (ligands group orbitals) interacts with metal 4S orbitals. This gives 1 bonding a1g and 1 anti bonding a1g* MOs
what does the metal 4p orbitals give rise to
To generate the interactions with the 4p orbitals we need ligand group orbitals that have t1u symmetry. This gives 3 triply degenerate bonding t1u and 3 anti bonding t1u* MOs
Define gerade
The term gerade is given to orbitals that are even or symmetric with respect to an inversion centre so the a1g is gerade while t1u is ungerade
where does the eg and eg* MO come from
of the metal 3D orbitals only the dz^2 and the dx^2-y^2 point at the ligands, this gives 2 degenerate bonding eg MOs and 2 degenerate anti bonding eg* MOs
Where does the t2g energy level come from
the remaining metal d orbitals t2g do not interact with ligand sigma orbitals because they point in between the ligands and therefore are non-bonding
Describe the MO energy level diagram (sigma bonding only)
6 LGOs interact with 6 metal orbitals of corresponding symmetry to give 6 bonding and 6 anti bonding MOs left with 3 non bonding t2g orbitals
Where do the electrons from the lone pair on the ligands fill up
the eg, t1u and a1g are perfectly placed to accommodate the 12 electrons from the 6 ligands lone pairs, these will become filled
Describe the eg* orbital
eg* orbitals mainly metal based but have some ligand character
Which ligands actually undergo only sigma bonding
ligands in the middle of the spectrochemical series
E.g NH3, H2O and NR3 R=alkyl
Describe how M-L pi bonding occurs
ligand must have suitable orbitals with pi symmetry, these orbitals do interact with the metal t2g orbitals
What does the nature of the pi interactions depend on
depends on the relative energies of the metal t2g and ligand pi orbitals and which orbitals are occupied
How do the Px,Py and Pz pi orbital from the ligand work in concert
they work in concert (ligand group orbitals) to form 3 host orbitals of suitable symmetry overlap for the 3x t2g orbitals
What happens if the orbitals on the ligands are full
Ligand is acting as a pi base/ pi donor
What happens if the orbitals on the ligands are empty
Ligand is acting as a pi acid/ pi acceptor
Describe the bonding of pi donor ligands and metals
need filled pi orbitals on the ligand and empty metal t2g orbitals, generally the donor atom has two or more lone pairs that are not involved in sigma bonding.
Describe the effect of pi donors on sigma only MO diagrams
Energy of the filled ligand pi orbital are lower then the metal d orbitals resulting t2g MOs are ligand-based and full. The non bonding t2g becomes metal based and antibonding. Eg and eg* orbitals remain unaffected
Give examples of pi donor ligands
Cl-, OH-, O2-
Which sort if metals have empty t2g orbitals
metals which have a high oxidation state
Describe the bonding of pi acceptor ligands and metals
need empty pi orbitals on the ligands and filled metal t2g orbitals for metal to ligand bonding (metals in low oxidation states)
What is metal to ligand pi bonding also called
back bonding
When do we see back bonding
we see back bonding for CO,
alkenes and phosphine
Pi acceptor ligands!
Describe the bonding within the carbonyl ligand
total of 10 electrons. 6 will form the sigma and 2 pi bonds (the triple bond in CO). The remaining 4 electrons will be present as 2 lone pairs, one on the carbon and one on the oxygen
Why do metals bind to the carbon lone pairs and not the oxygen one
dative bond from O to C leaves O partially positive. Other bonds almost cancel this out as O pulls electrons towards itself due to it being more electronegative the lone pair on O is more stabilised , therefore the lone pair on carbon is therefore higher in energy and destabilised this is the HOMO. The 2x filled pi orbitals will have more oxygen character as a result of its greater electronegativity, therefore the empty pi* orbitals will have more carbon character so the metal bonds to the carbon via both sigma and pi components
Give an example of pi acceptor ligands
CO or phosphines
What are high oxidation states stabilised by and why
High oxidation states are stabilised by pi donor ligands. Electron donation to metal from ligands via sigma and pi for metals which leach electrons in high oxidation states
What are low oxidation states stabilised by and why
Low oxidation states are stabilised by pi acceptor ligands. High electron density at metal requires mechanism for removal - back bonding via pi system
Describe the effect that a pi base/donor has on delta 0
Delta 0 decreases
Describe the effect that a pi acid/acceptor has on delta 0
Delta 0 increases
Where does the C-O sigma bond arise from
The carbon 2s and one of the 2p will for 2 hybrid SP molecular orbitals. Oxygen 2s and one of the 2p will for 2 hybrid sp molecular orbitals. When the SP hybrid orbital from carbon overlaps with the SP hybrid orbital from the carbon we form the C-O sigma bond
How do the pi bonds for in C-O
Hybridisation for carbon results in 2SP molecular orbitals and 2 p orbitals remain, this is the same for oxygen. When a p orbital from carbon overlaps with oxygen we form one pi bond but since we have two p orbitals for each element we for 2 pi bonds
Describe how all ten electrons in C-O are used
4 electrons from carbon, 6 electrons from oxygen.
One sigma and two pi bonds take into account 6 electrons
The second SP hybridised MO for carbon houses a lone pair and the second SP hybridised MO for oxygen houses a lone pair
What is found in the 1σ orbital for C-O
Lone pair of electrons on the oxygen
What is found in the 2σ orbital for C-O
Sp hybrid, this is the carbon oxygen single bond, this is populates with two electrons
What is found in the 3 σ orbital for C-O
Homo of the molecule where the lone pair of the carbon sits
What is found in the 1 π orbital for C-O
Two degenerate orbitals of π symmetry created by the overlap of the carbon-oxygen py and Pz orbitals, this will have higher oxygen character
What is found in the 2 π orbital for C-O
The 2 anti bonding orbitals for the pi overlap of py and Pz these as a result have more carbon character
How does the carbonyl ligand bond to a metal
The 3σ HOMO has the correct symmetry and energy will have good overlap with an empty metal orbital of the same symmetry and we can form a metal-carbon σ bond. If the metal orbital contains electrons it can back donate into the C-O 2π* orbital
What would u expect to happen to the length of the bond when you have both sigma and pi bonding to a metal
If the bonding from metal to ligand has both a sigma and a pi component this should strengthen the bond, increase the bond order and reduce the bond length. A shorter metal-ligand bond is evidence for pi back bonding
What can we use to measure bond lengths
X ray crystallography
How can measuring bind strengths be evidence for back bonding from metal to C in a carbonyl
Back bonding will reduce the C-O bond order to give lower CO stretching frequency than in free CO. The metal carbon bond becomes stronger as the CO bond becomes weaker
What do we use to measure bond strengths
IR spectroscopy
What affects the IR spectra of a metal carbonyl complex
1 bonding mode of CO
2 charge on the complex (metal oxidation state:
3 other ligands e.g phosphines
Describe the change in the IR spectrum for carbonyls when we go from 1 to 2 to 3 metals attached to the C on the carbonyl
When we go from 1 metal to 3 metals we have more pi electron density pumped into the CO antibonding orbital making it weaker. Therefore if we have 3 metals to CO there will be a smaller number observed in its vCO
Describe how the charge in the complex/ metal oxidation state affects the IR
If the metal is an anionic it will pump more electron density into the CO antibonding orbital making it weaker therefore lower number in vCO
Describe the three ways we can prepare metal carbonyls
1) direction reaction between M and CO
2) reductive carbonylation
3) photolysis or thermolysis
Describe the direct reaction between M and CO
Roast the metal in the presence of carbon monoxide, for Ni and Fe
Describe reductive carbonylation
Reduction of a commonly available metal salt in the presence of CO, the reducing agent may be hydrogen, a metal (Na, Mg, Al) or carbon monoxide itself
What is the problem with thermolysis
Thermal reactions can lead to a mixture of products
What are the benefits of using photochemical reactions to make metal carbonyls over thermal reactions
Photochemical reaction conditions can give much cleaner reactions as there js a greater degree of control over substitution
