C3211 Final Flashcards
D-Block Elements
Arising from filling of 3d, 4d, and 5d subshells
Transition Elements
Has an incomplete d subshell. Group 12 elements (Zn,Cd,Hg) are d-block elements but not transition elements
Characteristic properties of lanthanides and actinides (3):
- Lustrous, malleable, high electrical and thermal conductivities. High melting/boiling points. Hard and strong.
- Multiple oxidation states
- Lewis acid properties; strong tendency for forming coordination cmpds with Lewis bases
Why are they called ‘complex’ compounds?
Bc of difficulties they caused for chemists studying them
What type of interaction is present in a coordinate bond?
Its a combination of an interaction based on electrostatics and covalent interaction
Denticity
Describes number of donor atoms contained by a ligand
Chelating ligands
Form a ring with the central metal; chelate means crab’s claw.
What denticity of ligands are important as bioinorganic ligands?
Tridentate and multidentate ligands; porphyrins, corrins, etc.
Common ligands
See flash cards
Alfred Werner
Identified geometries of d-metal complexes. He studied Co3+. Noticed different colors.
How did Werner characterize and separate very similar complexes (CoCl3 with NH3)?
He used silver nitrate to precipitate out AgCl, which indicates the strength of Cl- association.
What did Werner suggest from his studies?
Complexes possessing six ligands attached to each Co3+ ions. Differentiated primary and secondary valence to distinguish strong and weak bonds.
Rules for Naming Complexes
Review
Coordination number
Number of ligand metal interactions
Inner sphere complexes
Ligands directly attach to central metal
Outer sphere complexes
Ion pairs and a secondary coordination sphere
What 3 factors determine coordination number?
- Size of central atom/ion
- Steric interaction between ligands
- Electronic interactions
CN 1 or 2 (Description)
Very rare. Requires very bulky ligands to protect the coordination sphere of the metal. Later transition metals.
CN 1 or 2 (example)
CN=1: [2,3,5-Ph3C6H2Cu]
CN=2: Hg(Ch3)2
CN of 3 (description)
Preferred by later TMs. CN may not be apparent from written formula as bridging may occur.
CN of 3 (example)
K[Cu(CN)2]
CN of 4 (description)
Very common; favored for early TMs, especially 3d, and large ligands.
Geometries of CN 4
Tetrahedral
Square Planar
What metal configuration typically form square planar complexes?
d8
Ex of CN 4
Pt(Cl2(NH3)2)
CN of 5 (description)
Less common, but important for reactivity and rxn mechanisms.
Geometries of 5 CN complexes
Square pyramidal
Trigonal bipyramidal
What influences geometry of CN 5?
Ligands strongly influence which geometry is observed. They also may interconvert
Berry pseudorotation
Common mechanism of interconversion between geometries of CN5 complexes. Can be studied with NMR
CN 6 (description)
Most common. Almost all octahedral (trigonal prismatic sometimes). Distortions are possible. Possess cis/trans and fac/mer isomerism.
Higher coordination numbers than 6
CN7 or CN8 is possible for 4d and 5d and lanthanides. Less common though.
Formal oxidation state of metal in complexes
- Identify charges on the ligands
- Look at total charge of molecule
- Charge L + MOS = total charge
Electron Configurations in Complexes
nd orbitals are lower in E than (n+1)s orbitals, so 3d becomes filled before 4s.
Electron counting rules for number of d electrons
- Count total number VE
- Deduce OS
- Subtract
Types of isomerism:
- Solvent isomers
- Ionization isomers
- Coordination isomers
- Linkage isomers
- Stereoisomers
Chirality
cannot be superimposed on their own mirror image and have the ability to polarize light
Lambda isomers
Has a left-handed (counterclockwise) screw axis
Delta isomers
Have a right-handed (clockwise) screw axis
Is there a correlation between delta/lambda and optical rotation?
Each isomer may cause + or - rotation; different for each compound
Enantiomers
Have the same physical properties of solubility, melting point, etc. There are chiral separation columns. Some synthesis methods involve preceding to make one stereoisomer more soluble than the other.
What chelation rings are preferred?
5 or 6 membered rings
Ionization Isomerism
Compounds that have the same formula but differ in their ion arrangement in solution (i.e. differ in inner/outer spheres)
Coordination Isomerism
Overall ratio of ligands to metals remains constant, but arrangement of coordination sphere around each metal varies.
Linkage isomerism
Some ligands possess different donor atoms. Many factors impact the bonding, such as solvent and HSAB.
What must any theory of bonding seek to explain?
- Physically observed behaviour of complexes
What specific questions do we ask about bonding in complexes?
- Why do certain metals and ligands favour particular oxidation states and geometries
- Can we account for colours of complexes
- Can we account for magnetic properties of complexes
- Why are some compounds more reactive than others?
Crystal Field Theory
Simple model that is based on the assumption that the metal-ligand interaction is primarily electrostatic
Details of CFT
-Simplify ligands to be point charges
-Positive charge of nucleus attracts the ligands
-Transition M has partially filled d orbitals which have repulsive interactions with the ligand, the magnitude of which depends on the angle of approach.
-D orbitals split.
Describe general CFT splitting in octahedral geometry
Interaxial orbitals are stabilized by 2/5 do, axial orbitals are destabilized by 3/5 do
Draw CFT for octahedral
See notes
What does CFT fail to describe?
Covalent interactions or possible multiple metal-ligand bonds
Barycentre
Mean energy of orbitals
How are electrons filled in CFT octahedral split d orbitals?
Depends on delta octahedral vs. pairing energy
Labels for upper/lower split CFT octahedral orbitals
Upper: eg
Lower: t2g
Delta Octahedral
Energy difference between eg and t2g sets
How can do be measured?
Using UV-Vis
Crystal field stabilization energy (CFSE)
Measure of net E of occupation of d-orbitals relative to their mean energy
Ligand Field Theory
Incorporates overlap of metal-based d-orbitals with ligand orbitals of suitable symmetry
What does LFT try to explain?
Effect of different ligands on do
Spectrochemical series
Arranged in order of increasing energy transitions that occur in their complexes
Strong-field ligand
High energy transition, large do
Weak-field ligand
low-energy transition, small do
How does do depend on the metal ion?
-Value of do increases with increasing oxidation number and increases down a group
-Oxidation state reflects smaller size of highly charged ions, therefore closer metal ligand contacts and stronger interaction energy
- Increase down a group because larger size of 4d and 5d orbitals produce better orbital overlap and a stronger metal ligand interaction
How to obtain electron configuration for octahedral complex
- Use orbital E levels obtained in CFT as a basis for applying the Aufbau principle
- Obeys Pauli exclusion principle, maintaining 2 electrons per orbital with opposite spins
- If more than one degenerate orbital is available, obey Hund’s rule of max multiplicity by placing electrons in separate orbitals singly with parallel spins
What results if do is smaller than P?
high spin
What results if do is larger than P?
low spin
Do we typically see low or high spin for 4 and 5d metals?
Low spin; do is higher than for 3d metals
Diamagnetic
No unpaired electrons; repelled by a magnetic field (very slightly)
Paramagnetic
At least one unpaired electron; drawn into a magnetic field. Much stronger than diamagnetism
Measure of magnetism
Magnetic Susceptibility
Types of magnetic susceptibility balances
Gouy balance
Faraday balance
Johnson-Matthey
How can molar susceptibility be found with mass susceptibility?
Xm=Xg * Mm
What does a SQUID do?
Conducts magnetic measurements at various temperatures
Evans NMR MEthod – review
FOO
Magnetic Moment
A result of both orbital and spin angular momenta
Equation relating effective magnetic moment to magnetic susceptibility
ueff = 2.828 (XmT)^1/2 (in uB units)
Bohr Magneton
Units of magnetism
What happens to orbital angular momentum in a complex?
It becomes ‘quenched’ as a result of the interactions of the electrons with their non-spherical environment. True for 3d, not always for 4d and 5d.
Spin Only magnetic moment
Magnetic moment that ignores the orbital contribution
Equation of spin only magnetic moment
uso = (n(n+2))^1/2 in uB
n is number of unpaired electrons
In what case might orbital angular momentum to contribute to the magnetic moment?
There is an unfilled of half-filled orbital similar in E to that of the orbital occupied by the unpaired spins
Draw the CFT splitting diagram for square pyramidal and square planar
See notes
What complexes typically adopt square planar geometries?
d^8 metals, since all orbitals are filled except the high energy dx2-y2
What is the relative comparison of dT to do?
dT is about 4/9 do
Why is there no g labels in the t2 and e sets for tetrahedral?
There is no centre of inversion in a tetrahedron
Draw the CFT splitting diagram for tetrahedral
See Notes
What happens in distortions where the z-axis is compressed or extended?
Changes the energy of the axial orbitals to remove degeneracy
Between 3d8, 4d8, and 5d8 metals, which are more likely to adopt square planar vs. tetrahedral?
4d8 and 5d8 square planar
3d8 tetrahedral
When will a 3d8 metal form square planar?
In a very strong ligand field
Jahn-Teller Effect
If the ground state electronic configuration of a non-linear complex is orbitally degenerate, the complex will distort so as to remove the degeneracy and achieve a lower E.
What octahedral systems may undergo J-T Distortions?
Odd number of electrons in eg orbitals; d9, high spin d4, low spin d7
How does the J-T distortion work?
When there are degenerate orbitals an unpaired electron can fill, it resonates between the two orbitals and destabilizes the compound.
The geometry can distort to remove the degeneracy by forcing one orbital to be higher in energy than the other.
Draw the CFT splitting diagram for z-axis elongation and z-axis compression.
See Notes
Draw the MOs for O2 and N2
See notes
What is the main difference in the O2 and N2 MO and why does this occur?
The sigma 2p and pi 2p orbitals are swapped in N2. This is because orbital mixing (sp mixing) can occur for N2 due to similarity in s and p energies but not for O2 because the gap is too large.
Draw the MO for CO
See Notes
Which metal d orbitals have sigma bonding character with the ligands (octahedral)?
Axial (dz2 and dx2-y2)
Which metal d orbitals have pi bonding character with ligands (octahedral)?
Usually the interaxial orbitals
What are symmetry-adapted linear combination orbitals?
We consider a combination of all ligand s orbitals. These are not specifically s or p atomic orbitals anymore, but only orbitals which exhibit sigma symmetry with respect to the metal - ligand bond axis.
If we consider ligand group orbitals with respect to sigma symmetry, which metal orbitals are non-bonding?
t2g (interaxial d-orbitals)
Are the bonding orbitals in MH6 typically ligand or metal in character?
Ligand
Draw the MO for ML6 and appropriate SALC - metal orbital combinations for each MO
See Notes
Where do the metal electrons usually reside?
In the t2g non bonding orbitals, so they are largely metal in character
What are the frontier orbitals in ML6 when we only consider sigma bonding?
t2g metal based orbitals and eg antibonding orbitals
Where do most reactions occur in a complex?
The frontier orbitals
What is the HOMO-LUMO gap in ML6?
delta octahedral
What happens if the ligands in a complex have orbitals with local pi symmetry?
They may form pi MOs with the t2g set of the metal orbitals
What are the two options for pi orbitals?
Pi donor or pi acceptor
Another name for pi-acceptor bonding
Synergic
How does a pi donor ligand impact do?
A pi donor ligand has filled orbitals of pi symmetry. This increases the electron density around the metal and therefore increases the energy of the t2g set, decreasing do
What are examples of pi donor ligands
Cl-, OH-, NR2-, H2O
What is the impact of pi-acceptance on do?
Pi-backbonding pulls electron density away from the metal, which lowers the energy of the t2g orbitals and increases do.
Examples of pi-acceptor ligands
CO and N2
Draw the symmetry overlap diagrams for CO and explain how it participates in sigma-donation/pi-acceptance
See Notes
Draw the MO diagram for a complex with ligands that exhibit pi bonding.
See Notes
Explain the difference in pi-donor/pi-acceptor ligands on the MO of ML6 and how this impacts do.
See Notes
Main types of reactions of complexes (3)
- Ligand substitution
- Redox
- Reactions at the ligands
What are ligand substitution reactions important for?
Properties influencing the applications of metal complexes for catalysis
What is the general form of a ligand substitution reaction?
L’ + [ML6]n+ = L + [ML5L’]m+
Labile vs. Inert
Kinetics
Labile is fast reaction
Inert is slow reaction
Thermodynamic stability
Stable is + dG
Unstable is - dG
Generalized rate-law
rate = -d[A]/dt = k[A]^n
Plotting rate law general forms
n=1 : lnA = lnA0 - kt
n not 1 : A^(1-n) = A0^(1-n) - (1-n)kt
Method of Initial Rates
Measuring the initial rate of rxn as a function of initial concentration of A, and plotting ln d[A]/dt vs. ln [A]0 can give the reaction order
Formation Constants
If L is a solvent,
Keq = [L][(ML5L’)m+] / [L’][(ML6)n+]
Large Kf
Incoming ligand binds strongly (Stronger than solvent)
Small Kf
Ligand binds weakly
What is the general trend of Kf with each ligand swapped?
Kf decreases
What happens if Kf increases for a ligand added?
This implies a major change in the electronic structure of complex, and therefore a big increase in LFSE (maybe l.s. to h.s.)
Substitution reactions
exchange one or more ligands for another
What is one way of following a ligand substitution rxn?
Different ligand field strength means different colours; can follow by UV-Vis
What influence ligand substitution reactions?
-solvent; aqueous solutions favour substitution of aquo ligands due to LCP.
-kinetics; inert complex rxts slowly even if thermodynamically favoured. Labile complexes react fast
How can we predict if a complex is labile or inert?
High values of LFSE indicate the complex is inert, with some exceptions
What metal configurations are labile/inert?
Inert: d3, l.s. d4, d5, d6, sq. pl. d8
Labile: d1, d2, h.s. d4, d5, d6, d7, d9, d10
Intermediate: d8 weak field (td)
Are s-block ions labile or inert?
Labile, except for Be2+ and Mg2+
Are M (III) f-block ions labile or inert?
Labilte
Are M (II) d-block ions more or less labile than M (III) ions?
More
How do speeds of reaction compare for 4d/5d to 3d?
4d and 5d are more inert than 3d
What are the 3 classifications of substitution mechanisms?
- Dissociative
- Associative
- Interchange
Dissociative mechanism
Loss of one ligand before new ligand binds, leading to a potentially isolable intermediate
Associative mechanism
gain of ligand leads to a highly coordinated IM followed by displacement of a weakly bound ligand
Interchange mechanism
no real IM; rate of ligand loss and nucleophilic attack are nearly identical
What are D mechanisms typical for?
6 coordinate complexes
How can we stabilize an IM?
Donor solvents; solvents like THF are good Lewis bases that can act as place-holders, giving solvento complexes.
How may ligand loss be encouraged for inert complexes?
Trick it to be labile via an excited state (heat or photochemically induce)
Rate law for D mech
d[ML5AY]/dt = k2k1[ML5X][Y] / k-1[X] + k2[Y]
*Review derivation
What must be true for a D mech?
The IM, ML5, must be detectable. Detection of IM is difficult at low concentrations, thus there are not very many D mech.
What is most common for A-type mech?
4- or 5- coordinate complexes
Rate Laws for A mech
d[ML5Y] / dt = kobs[ML5X][Y] where kobs = k2k1 / k-1 + k2
*review derivation
What are interchange mechanisms typical for?
6 coordinate complexes
How do interchange mechanisms work?
They have no detectable IM but occur in one step, leading to a high E TS
What is a dissociative interchange mech?
Has early TS, (+) TS entropy, little dependence on entering group and good leaving group required
What is an associative interchange mech?
Late TS, large dependence on entering group, only moderate LG required, TS entropy is (-)
How do we differentiate A and D mechanisms?
A: rate depends strongly on incoming ligand (SN2), stronger nucleophiles give faster rates.
D: rate is independent of the incoming ligand (like SN1). If changing the incoming ligand has little effect on rate, rds is dissociative.
Can you ever prove a mechanism?
No, can only disprove it
Chelate Effect
Net increase in product molecules vs. reactants; dS increase; favoured
Macrocycle effect
A chelating ligand which has to distort/move/rotate to bind requires more E than one that is fixed in shape (cyclic). This enthalpically favours the macrocycle.
What is the issue with using macrocycles?
They are very hard to make
Where are we most likely to see trans effect / trans influence?
Square planar or octahedral
What is the trans effect?
Spectator ligands, T (not undergoing exchange) trans to the LG affect the rate of displacement (trans effect) or strength of the bond (trans influence) of the leaving group, X.
What spectator ions change substitution rates?
Strong sigma-donor/pi-acceptor ligands accelerate the rate of substitution.
Draw the orbitals that show how ethylene, C2H4, can act as a sigma donor/pi acceptor ligand.
See notes
How do the rate constants appear for strong trans effect ligands?
They should be high
How does trans influence change the mech?
Raises the GS E of the initial complex; thermodynamic effect
How does the trans effect change the mech?
Makes the LG more labile; kinetic effect
When would a ligand cis to the LG have significant influence on the trans effect?
When sterically bulky
What do large ligands do in mechanisms?
Prevent associative reactions and favour dissociative rds
How does the steric effect influence square planar systems?
Slows down the ligand exchange and improves kinetic inertness
Draw diagrams showing the trans influence and trans effect (rxn coordinate diagrams)
SEE NOTES
What should we do to favour D mechanisms and why?
‘Heat up to speed up’
For D mechanisms, dHTS is + and dSTS is +. Higher T decreases dG TS and favours the spontaneity of the rxn
What should we do to favour A mechanisms?
‘Speed up, cool down’
dHTS is - and dS TS -. To lower dG TS, decrease the T.
When is the Arrhenius eqn completely true?
Gas phase
What is the limitation of the Arrhenius eqn?
It only works over narrow T ranges (usually dT has to be less than 100).
Eyring Eqn
k = kBT / h * exp( - dGTS / RT)
Explain the eta-1 (bent) and eta-2 (side-on) bonding modes of O2 by drawing the overlap diagrams
See Notes
Draw MO for NH3
See Notes
