Higham L4-6 Flashcards
(50 cards)
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
