Higham L4-6 Flashcards
Rank the following π acceptor ligands in terms of their π acceptor strengths
CO, CN-, N2, NO+
Increasing π acceptor strength
CN- < N2 < CO < NO+
So CN- is the least likely to accept electron donation from the metal
What does isoelectronic mean
Have the same number of electrons
Describe the σ bonding/donation for CN-
σ donation through lone pair on C therefore a strong σ donor. Negative change of electron rich CN- increases σ donor strength to electropositive metal, high electrostatic attraction. With metals in normal oxidation states it is not necessary to invoke pi back bondinf
Describe the π bonding CN- complexes
π accepting tendencies of CN- diminished by the negative charge. Higher energy of π* orbitals of CN- compared to CO - poorer overlap with metal t2g. Therefore a weaker π acceptor than CO. π bondinf does occur with lower oxidation states
Describe the toxicity of cyanide and the impact it has on the body
Cyanide is highly toxic it inhibits mitochondrial respiration, can treat cyanide poisoning with hydroxocobalamin which is a form of vitamin B12
Contrast dinitrogen with carbon monoxide
N2 is a weaker sigma donor and weaker pi acceptor than CO, N2 is a homonuclear diatomic molecule whereas CO is a Heteronuclear diatomic molecule. As both ends of N2 are the same there is no dipole so there is no distortion of the electron cloud
Describe how dinitrogen can bind
N2 can bind end on or be bridged between two metal centres
What are the two ways in which NO can bind
NO can bond as either a 3 electron donor which forms a linear M-NO or a 1 electron donor which forms a bent M-NO
When do bent M-NO linkages form
Bent M-NO linkages form when back bonding is not an important consideration
What technique can be used to distinguish between bent and linear M-NO
Linear and bent NO ligands can be distinguished using IR: linear groups absorb at 1650-1900 bent nitrosyls absorb at 1525-1690
What is a phosphine
Phosphines are PR3
R-alkyl or aryl
What is a phosphite
Phosphites are P(OR)3
R- alkyl or aryl
Which of the following are more suitable for pi backbonding PR3 or NR3
PR3 ligands have suitable empty orbitals for pi back bonding unlike alkyl amines NR3 which have no empty p or d orbitals of low enough energy
Why are phosphines and phosphites a very important group of ligands
As we can change their steric and electronic properties
Describe sigma bonding from PR3 to the metal
PR3 lone pair sigma bonding from by donating two electrons into empty metal sigma orbital (d orbital)
Describe pi bonding from PR3 to the metal
PR3 sigma bond accepts electron density from filled metal pi orbital
Describe how we can we can modify phosphorus ligands
We can alter there sterics by making them very big or very small by varying the R groups. We can also alter electronics by varying elements of different electronegativities.
Describe the structures of phosphorus ligands
Phosphorus ligand can be monodentate, bidentate or polydentate
What is the benefit of synthesising large phosphines
Ligand dissociation is favoured by bulkier groups. The large groups can be used as protecting groups, can stop the metal undergoing decomposition
How can we measure the different steric effects of different phosphines
The steric effects is measured in therms of the tolman cone angle
What is the method of determining the tolman come angle
Hold the metal and the phosphorus at a distance of 2.28 angstroms, the three R groups are then sat in a cone like shape, you then measure the cone angle. The cone just encloses van der waals surface of all ligand atoms
What does an increased cone angle favour
Lower coordination numbers
The formation of isomers with minimum sterics interactions
Increases rates and equilibrium constants in dissociative reactions
Describe how we can vary the pi acceptor ability for phosphines and phosphites
If we contain a complex with both CO and PX3 we can increase the pi acceptor ability by increasing the electronegativity, this causes more back bonding to PX3 therefore less electron density goes into CO pi* antibonding orbital
How does ether bond to a transition metal
There is a pi bond and a sigma bond between the two carbons in ethene. If the two p orbitals are in the same phase, this pi cloud is then able to sigma bond with an empty metal d orbital accepting electron density. The lumo is formed when these p orbitals are out of phase and overlap with the transition metal and accept electrons
What does there need for sigma and pi bonds to form metal to ethene
Plane of alkene must be perpendicular to metal alkene bond
Explain how the C=C bond weakens when it binds to the transition metal
Sigma donation to M and pi back bonding from M causes C=C to lengthen and weaken: more like single bonds. These carbons tends to have more sp3 character/ be more pyramidal in geometry
What is η2
2 carbons bonded to metal
What is η3
3 carbons bonded to metal
C3H5 allyl radical or C3H5- anion
What is the two ways in which allyl ligand can bond
The allyl ligand can bond in a sigma fashion to a which which will also need to donate an electron or pi orbitals can also be involved to give η3 ie bonded through all 3 carbons
What is η4
4 carbons bonded to the metal
1,3 butadiene
What is η5
5 carbons bonded to the metal
The cylcopentadienyl anion (cyclopentadienide) ligand cp
How do we produce the cyclopentadienyl anion from cyclopentadiene
Cyclopentadiene C5H6 is acidic, deprotonation with NaH gives aromatic anion [C5H5]- as Na[C5H5] and H2
Describe M-Cp complexes
Cp is very stable as it obeys huckles rule and is considered to be aromatic. The resultant complexes when this binds to a metal are very stable and the M-Cp bonds do not easily dissociate and Cl ligand don’t usually get involved in chemical reactions
What is the effective atomic number
It is the sum of valence electrons on the metal and the electrons donated by the ligands
Describe how we use the neutral atom method for electron counting
1) count the metal as being in the zero oxidation state: the group its in gives the number of electrons it gives
2) add up the number of electrons that the ligand contributes
3) factor in for the overall charge on the complex
Give examples of neutral L type ligands
PR3, NH3, H2O, ethene etc
give examples of L type ligand
Cl, H, OH
What is an L type ligand
If can you get them in a bottle or on there own without a metal then they are an L type
What is an x type ligand
if it is a ligand which is normally bound to something else and not found on its own
How many electrons does each Cp contribute
5 electrons
How many electrons does η4 contribute
4
How many electrons does η3 contribute
3
For the neutral atom method list the ligands which contribute one electron
H, Cl, OH, OR, CN, CH3, OH, NO bent
For the neutral atom method list the ligands which contribute 2 electrons
PR3, NR3, NH3, H2O, CO, alkenes, RCN, imido bent M=NR and M=O
For the neutral atom method how many electrons does linear NO donate
3
How many electrons does imido linear M=NR and M=O contribute
4
How many electrons does linear alkoxide donate
5
How many electrons does benzene donate
6
Explain what happens when chlorine is used as a bridging ligand between two metals
To form on M-Cl bond the metal has to give up one of its electrons to the chlorine. Because this chlorides is interacting with the second metal, it has a lone pair which is perfectly inline to act as an L electron donor with the second metal
