Transition Metal Chemistry Flashcards
Strength of metal-metal bonds 3d - 5d?
3d << 4d < 5d
Why are 5d metal bonds stronger?
5d elements have more overlapping d orbitals, meaning they also have more metal-metal bonds.
How is [Re2Cl8]2- synthesised?
Perenate [ReO4]- + Zn/HCl --> [Re2Cl8]2-
Explain how the quadruple bond is formed in [Re2Cl8]2-. What happens if electrons are added or removed?
The quadruple bond involves sigma, pi and delta interaction. The sigma bond is formed by the dz^2 orbitals pointing at each other to form dz^2 - p hybrid. The pi bond is formed by the overlap of two d orbitals to form a degenerate pair of pi MO and pi antibonding MO. Lastly the delta interaction is formed by two d orbitals interacting face on. The delta interaction is fairly weak.Addition or loss of electrons causes loss of the delta interaction, and will be triply bonded.
Calculate the number of valence electrons in [Re2Cl8]2-.
Re is in group 7 so has 7 valence electrons. Re(III) has 4 valence electrons. There are two Re atoms so altogether there are 8 electrons.
What was the evidence for a quadruple bond in [Re2Cl8]2-?
- Eclipsed configuration- Very short distance between two Re atoms- Blue colour, caused by the delta --> delta antibonding transition. This requires little energy and can happen in visible region.
How can the Mo analogue to [Re2Cl8]2- be synthesised?
Mo(CO)6 + HOAc/AcO2 --> Mo2(OAc)4Mo2(OAc)4 + HCl --> [Mo2Cl8]4-
How many valence electrons are in [Os2Cl8]2-?
Os is in group 8, has 8 valence electrons. Os is in an oxidation state 2 * x - 8 = -2. x = 3 so Os(III). This gives Os(III) 5 valence electrons. There are two Os atoms so 10 valence electrons total.
Explain why [Os2Cl8]2- is triply bonded and not quadruply bonded? What will the configuration be?
There are 10 valence electrons total, whereas Re had 8 valence electrons. These extra two electrons fill the delta antibonding molecular orbital, thus removing the delta interaction and leaving [Os2Cl8]2- triply bonded. The complex is now free to adopt staggered or eclipsed conformation.
How is Nb6Cl14 and Ta6Cl14 formed? What are they comprised of? What is the structure if another ligand attaches to the complex?
NbCl5 + Cd + high temp --> Nb6Cl14TaCl5 + Cd + high temp --> Ta6Cl14Nb6Cl14 and Ta6Cl14 contain clusters of [Nb6Cl12]2+ and [Ta6Cl12]2+. They are comprised of an octahedron of Na/Ta, bridged by Cl ligands.The [Nb6Cl12]2+ and [Ta6Cl12]2+ are very stable but can accept another ligand to take on the form [Nb6Cl12X6]4- or [Ta6Cl12X6]4-.
What is the structure of the [Mo6Cl8]4+ complex?
[Mo6Cl8}4+ contains an octahedron of Mo, triply bridged by Cl ligands on each face.
Describe the differences of metal carbonyl clusters in 3d - 5d complexes. Give examples.
3d metals are more likely to have a greater number of carbonyl ligands bridging between metals and shorter metal-metal bonds. For example, Ir4(CO)12 has only three terminal carbonyl groups at each Ir. Co4(CO)12 has two terminal carbonyl groups at each Ir, plus two doubly bridging CO between the metals.
Describe the strength of metal-metal bonds 3d - 5d. Give examples.
Strength: 3d < 4d < 5d. 3d metal Fe in Fe(CO)5 requires energy to convert to Fe3(CO)12. 5d metal Os in Os(CO)5 reacts easily to form Os3(CO)12 by the reaction:3Os(CO)5 --> Os3(CO)12 + 3COThe Os-Os bonds are stable and reaction is entropically preferred.
Describe how to get further clusters of Os metals?
By heating Os3(CO)12, you liberate CO and more Os-Os bonds will form. This is a high nuclearity metal carbonyl cluster.
How is a high nuclearity metal carbonyl cluster formed? How are colloids related?
By heating the metal carbonyl complex, you liberate CO until you have a close packed structure of metal atoms with an outer shell of CO ligands.Colloids are similar in that they are fragments of metal lattice surrounded by CO ligands in order to prevent aggregation of the metals.
What is the role of the ligand in metal-metal bonding?
Early transition metals have few electrons, so are stabilised by pi donor ligands such as halides. Later transition metals have lots of electron density so are stabilised by pi acceptor ligands such as CO.
What is the reaction for the formation of a vanadium polyoxoanion? What is the driving force for this reaction? Is it an isopolyanion or heteropolyanion?
VO4 3- + H+ --> [V10O28]6-Depending on the pH, [V10O27(OH)]5-, [V10O26(OH)2]4- etc. can be formed. Excess acid will form VO2+.The driving force for the formation of the polyoxoanion is the metal-oxygen bonds.Isopolyanion.
What is the structure of [V10O28]6-?
The vanadium atoms are octahedral and joined together through oxygens. The oxygens are 2 bridging at the edge, 3 bridging at the face and 6 bridging in the centre. There are terminal oxygens at the vertices.
Describe the formation of molybdenum and tungsten polyoxoanions. What is their structure? Isopolyanion or heteropolyanion?
Protonation of aqueous molybdenum and tungsten solutions will form the polyoxoanions.The oxygens will be terminal at the vertices, doubly bridging along edges of the octahedra, six bridging in the centre and may be quadruply bridging.Isopolyanion.
What is a common heteropolyanion and how is it synthesised?
Phosphomolybdate [PMo12O40]3- and phosphotungstate [PW12O40]3- are very stable heteropolyanions.PO43- + MoO42- + H+ --> [PMo12O40]3-PO43- + WO42- + H+ --> [PW12O40]3-
What is the structure of [PMo12O40]3- and [PW12O40]3-? What is the name for the base structure?
It consists of a tetrahedron of PO4 surrounded by MoO6 octahedra. The oxygens can be terminal, doubly bridging between octahedra and quadruply bridging between PO4 tetrahedron and MoO6 octahedron.Keggin structure.
What is another polyoxoanion besides [PMo12O40]3- and [PW12O40]3- that has the Keggin structure?
AlAl12O40. Used as a flocculating agent.
What are some applications of [PMo12O40]3-?
Determination of Ge and AsDetermination of P and Si in metalDetermination of P in seawater - add a reducing agent to a sample + phosphomolybdate and it will turn blue, intensity of colour corresponds to P content when measured against a series of standard solutions.
Describe the Dawson structure, Anderson structure and the Big Wheel.
Dawson structure: three Mo centres removed from two Keggin structures and joined together.Anderson structure: The centre octahedron is XO6 surrounded by 6 octahedra.Big Wheel: self assembly of Mo clusters in solution.
What is the trans effect, some examples and how they make substitution quicker? How are they related to cisplatin and transplatin?
Trans effect is a kinetic phenomenon, makes rate of substitution of ligand trans to it faster. PEt3 and H are strong trans effect ligands. NH3 < Cl. Strong trans effect ligands will be strong sigma donors or strong pi acceptors. The pi acceptor will remove electron density from the metal and facilitate nucleophilic attack.Used to form cisplatin and transplatin.
What is the trans influence, a strong trans influence ligand, and ways to measure the effect of trans influence?
Trans influence is a thermodynamic effect of lengthening and weakening the bond trans to it. Me is a strong trans influence ligand.X-ray crystallography: directly measure bond lengths.NMR: shorter bond lengths will have larger coupling constant.IR: shorter bonds will have a higher stretching frequency.
What is oxidative addition?
The addition of a small molecule to a complex, forming two bonds and an increase in oxidation state by 2.
What is reductive elimination? Draw an example. What is one condition of reductive elimination?
The removal of a small molecule from a complex, decreasing oxidation state by 2.Must be cis.
Give an example of a complex that can undergo oxidative addition. Draw reaction and mechanism.
Vaska's complex trans-IrCl(CO)(PPh3)2 can undergo oxidative addition.
Wilkonson's complex can be formed by reductive elimination. Draw the equation.
RhCl3.xH2O + 3PPh3 + ethanol --> RhCl(PPh3)3 + CH3CH2O + 2HClThe ethanol reduces Rh(III) --> Rh(I). Done under reflux.
How is Vaska's complex synthesised?
IrCl3.xH2O + excess PPh3 + aniline --> trans-IrCl(CO)(PPh3)2The CO comes from decomposition of DMF solvent at boiling point.Done under reflux. Aniline accelerates the reaction.
Wilkonson's and Vaska's complexes are 16e but can become 18e. How so?
Can bond O2 which donates 2 electrons. O2 bonds sideways. Vaska's complex can bond with C60, which also acts as a donor ligand.
Why should there be care in the strength of adsorption to a catalyst?
Too weak and it won't catalyse, too strong and the catalyst will be poisoned.
Which elements are the best catalysts?
The platinum group, they require the lowest temperature. Early transition metals form stable formates and may not catalyse reaction, coinage metals require high temperature and are too weakly bonding.
What are the steps in a rhodium catalysed reaction?
1: Coordination of alkene2: Insertion of molecule into R-H bond3: Oxidative addition of H24: Reductive elimination of H + R to give catalyst + product
What are some facts about platinum as anticancer drugs?
Cisplatin better than transplatinPt(II) better than Pt(VI), some Pt(VI) used as the body reduces it to Pt(II)