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Inorganic Chemistry > Transition metals > Flashcards

Transition metals Flashcards

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1
Q

Complex

A

Central metal ion surrounded by ligands

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2
Q

Ligand

A

An ion which can be bound in a complex

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3
Q

Coordination compound

A

complex with at least one ionic complex

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4
Q

Lewis acid

A

electron pair acceptor

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5
Q

Lewis base

A

electron pair donor

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6
Q

Donor atom

A

atom in the ligand which coordinates with the metal

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7
Q

Shapes

A 
2 = linear
3 = trigonal planar/pyramidal
4 = tetrahedral/square planar
5 = trigonal bipyramid/ square pyramid
6 = octahedral - mer/fac, cis/trans, chirality
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8
Q

Ligand names

A

Ammine - NH3
Aqua = water
Hydroxo = OH
Ethylenediamine (en) C2H4N2

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9
Q

How to name metals?

A

Alphabetical
Cation named first
Ligands are named first then the metal with its oxidation state
Negative = ate

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10
Q

OILRIG

A

Oxidation is lost, reduction is gained

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11
Q

Which elements have accidental degeneracy?

A

Chromium and copper

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12
Q

What is Crystal Field Theory

A

A model for the electronic structure of the metal devised through analysis of spectra
Looks at the symmetry of the orbitals and their filling to determine MOdiagrams

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13
Q

CFT octahedral complexes?

A

Direct interaction between orbitals and ligands = strong repulsion
Less interaction between orbitals and ligands. = weaker repulsion
No longer five fold degeneracy - known as isotropic field
Splitting occurs 3:2 ratio

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14
Q

What are the values of delta o for octahedral complexes?

A

3/5 for 3g and -2/5 for t2g

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15
Q

What is CFSE

A

Crystal Field Stabilisation Energy which is the potential energetic advantage caused by ligand field splitting

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16
Q

What does spectrochemical series depend on?

A

Delta o valyes

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17
Q

Describe spectrochemical series?

A

Weak field ligands cause small splitting so small delta o opposite for strong field ligands

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18
Q

Low spin

A

If delta o is larger than the spin pairing energy there is an energetic advantage to spin pairing

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19
Q

High spin

A

If delta o is smaller than the spin pairing energy spins are arrange in parallel

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20
Q

Which d configurations have high or low spin?

A

d4-d7

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21
Q

Diamagnetic

A

Weakly repelled by a magnetic field, no unpaired electrons

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22
Q

Paramagnetic

A

Weakly attracted to a magnetic field, unpaired electrons

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23
Q

What is S?

A

Spin quantum number = number of unpaired electrons /2

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24
Q

How do you work out spin only magnetic moment?

A

2[S[S+1]^1/2

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25
Q

Describe tetrahedral complexes

A

More distant interaction between orbitals and ligands = weaker repulsion
Tet symmetry affects repulsion in the opposite way to octahedral

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26
Q

Describe tetragonal splitting

A

Moving ligands away along axis changes degeneracy
dz2 have less overlap between the negatively charged ligands and the negative lobes.
- dyz and dxz orbitals experience a similar stabilisation. The coulombic stabilisation is somewhat less because the negative orbitals are not directly pointing along the z-axis.
- Removal of the two ligands means the other 4 ligands feel more of the positive metal centre causing a contraction.
- dx2-y2 orbitals have more orbital overlap and therefore more destabilisation.
- dxy is similarly destabilised however the orbitals do not overlap as efficiently along the x and y axes with the ligands.

27
Q

What does tetragonal splitting do?

A

Increases stability but reduces orbital degeneracy - transitions from t2g to e.g. so observable in emission spectra

28
Q

Square planar

A

dz2 have effectively no overlap/repulsion with ligands and drop in energy.

  • dyz and dxz orbitals experience a similar stabilisation. The coulombic stabilisation is somewhat less because the negative orbitals are not directly pointing along the z-axis.
  • Complete removal of the two ligands means the other 4 ligands feel more of the positive metal centre causing a contraction.
  • dx2-y2 orbitals have more orbital overlap and therefore destabilised.
  • dxy is similarly destabilised however the orbitals do not overlap as efficiently along the x and y axes with the ligands.
29
Q

Describe ligand field theory

A

A combination of CFT and MO theory accounting for complex interaction between metal and ligand

30
Q

End on sigma bonding

A

Consider: which orbitals can overlap end on and have the same symmetry label?

  • The s-orbital on the metal may form a bonding/anti-bonding σ- bonds pair with the ligands through a1g symmetry.
  • d-orbitlals with eg symmetry form two bonding/anti-bonding σ-bonds with the ligands.
  • All three p-orbitals have corresponding bonding and antibonding t1u symmetry with the ligands.
  • The t2g on the metal does not have σ symmetry. There is no t2g on the ligand therefore this is non bonding.
31
Q

What is the effect of the ligand in CFT?

A

If its a strong field ligand- low spin

Ligand affects the stabilisation and destabilisation or the e.g. orbitals and therefore the size of delta o

32
Q

Describe pie bonding

A

Ligands have unused p orbitals and depending on d config of the metal there may be available orbitals on metal
If they possess pie symmetry we get side on overlap

33
Q

Pie acceptor ligands

A

If the orbitals on the ligand are higher in energy than the partially filled t2g it causes an increase in Δo.
- The t2g involved is now lower in energy and a bonding MO
π acceptor ligands are high in the spectrochemical series and produce a relatively large Δo.

34
Q

Pie donor ligands

A

If the p-orbitals on the ligand are lower in energy than the partially filled t2g on the metal, it causes a decrease in Δo.
- The t2g involved is now higher in energy and an antibonding MO.
π-donor ligands are low in the spectrochemical series and produce a relatively small Δo.

35
Q

Describe pie donors or acceptors in a spectrochemical series

A

Pie donors< weak pie donors< no pie< pie acceptors

36
Q

Lower frequency =

A

weaker bond

37
Q

Shorter bond =

A

stronger bond

38
Q

What causes a change in Δo.

A

Consider a d1 metal (only HOMO is shown for simplicity).
σ bonding produces 10 MOs.
The t2g, comprising of the dxy, dxz and dzy are non-bonding…
but have π symmetry with the ligand p-orbitals - which are higher in energy than the s- orbtials.

39
Q

Why are transition metals coloured?

A

Atoms can be raised to excited states through absorption of specific frequencies of radiation
The same or different frequencies may be emitted as atoms return to ground

40
Q

What is bond length?

A

Eqm distance at which positive attractive interaction between two atoms are maximised and negative destabilising repulsive interactions between electrons are minimised
Excitation of a bond produces another different curve and optimised bond length, emission is a transition between the energy levels on the curves

41
Q

What are the two relationships to know for molecular spectroscopy?

A

Energy is proportional to wavenumber

Absorbance is proportional to amount

42
Q

Describe ligand spectra

A

Part of the spectrum due to transitions within ligand

43
Q

Describe charge-transfer spectra

A

Electronic transitions between molecular orbitals where one has mainly M character and one has L
L–> M and M–> L

44
Q

L –> M transitions

A 
M must be readily reduced, L
must be readily oxidised.
- The easier it is to reduce M (and oxidise L) the lower the energy of the transition.
e.g. MX6n- where X is a π donor ligand. Transitions observed are:
π→π* (both t2g)
- π→σ* (t2g→eg)
- e.g. [TiCl6]2-
π→π* (31850 cm-1) π→σ* (42500 cm-1)
45
Q

M–> L transitions

A 
M must be readily oxidised, L must be readily reduced
   πL*
     eg*
ΔO
ΔO -
e.g. With π conjugated ligands such as pyridine
              t2g
ML6
σ
- Generally stronger field ligands.
- Transitions observed are:
π→π* (both t2g) σ*→π* (eg→t2g)
46
Q

Describe d-d spectra

A

Electrons are transferred between d orbitals eg of different energies.

  • An understanding of both crystal field and ligand field theory is required to understand the d-d transitions observed in spectra.
  • The energy gap between orbitals depends on electron repulsion and hence the identity of both the metal and the ligand.
  • There is only one electron so there is only one option for exciting the electron. Therefore there is only one absorption.
47
Q

When do electrons interact more or less strongly?

A

If they are in the same plane

48
Q

How does high/low spin affect transition possibilities?

A

Low spin - can transition

High spin cannot as unalloyed spin change

49
Q

What are the 3 selection rules?

A
  1. Transitions between d orbitals are forbidden.
  2. Transitions between d orbitals are forbidden for complexes with a centre of symmetry.
  3. Transitions which change the direction of the spin of an electron are forbidden ( high spin d5 only)
50
Q

Why do these transitions still occur?

A

Because the rules are based on idealised models

51
Q

Describe e

A

measure of how likely a transition is to occur
Independent of concentration and cuvette
dependant upon material and wavelength
Large when there is high prib of transitions occurring

52
Q

Why are transition metal complex peaks broad?

A
  1. Metal-ligand vibrations lengthening/shortening the bond slightly and therefore changing the magnitude of e-e repulsion and hence ΔO.
  2. Jahn-Teller effects in the excited state splitting otherwise degenerate orbitals and giving rise to more than one possible transition.
53
Q

Describe ligand substitution reactions

A

A general equation for ligand substitution is:
Ligand Substitution Reactions
Y + M-X → M-Y + X Y = entering group X = leaving group
- Often the leaving group is a solvent molecule, e.g. H2O.
Demonstrate the chelate effect: a complex containing coordinated polydentate ligands has greater stability than the analogous monodentate ligands

54
Q

What are the factors affecting ligand substitution?

A

a) nucleophilicity (Lewis base strength) of the entering group.
b) identity of the metal.
c) π acceptor/donor character of the leaving group.
d) π acceptor/donor character of the other ligands on the metal.
e) steric effects.
f) ‘spectator Ligands’ and the trans effect.
g) temperature and pressure.

55
Q

Inner sphere

A

occurs when A ligand is shared in inner sphere reactions. The ligand may also be transferred in the process. The sharing bridging occurs in a transition state.

56
Q

Outer sphere

A

electron transfer, there is no bridging ligand between species in an outer sphere redox reaction. Electron tunnelling between two complexes. It may also be accompanied by a change of spin state.

57
Q

Describe organometallic

A

18 electron rule is useful indicator of complex stability

Almost all octahedral complexes are 18 electrons causing larger splitting

58
Q

Describe square planar complexes in terms of organometallic basics

A

Electron deficient as they have 16e - only 8 ligand electrons so 8 metal electrons are required

59
Q

Define hapticity

A

how many atoms in the ligand are bound to the metal

60
Q

Describe CO ligand

A
  • CO is a good bridging ligand.
  • HOMO has C character and is σ donating.
  • LUMO has C character and is π accepting - sometimes referred to as back bonding.
  • Refer back to lecture 4: Back bonding weakens CO bond therefore lowers the stretching frequency.
  • M-C bond is shorter where we see increased back bonding.
61
Q

Describe Phosphines

A

Not strictly organometallic but very relevant.

  • Often sterically crowded.
  • Phosphines are σ-donors and π-acceptors, as with carbonyls
  • Phosphines are good σ-donors and bad π-acceptors, or vice versa, depending on the electronegativity of the substituents.
  • Bond strength depends on the metal character too.
62
Q

Describe hydrogen and hydrides

A

A hydride is a single hydrogen bonded directly to a metal. It is not necessarily H– as some TM hydrides are acidic.
- Bonding is simple. H only has s orbitals available therefore only σ bonded.
- Hydrides can be bridging
Dihydrogen may also bind side on.
- σ donation occurs from the bond to the metal.
- π donation from the metal to the σ* on the ligand.
- This weakens the σ bond between the two H and the bonding picture tends towards dihydride-like bonding.

63
Q

Describe alkyls and associated

A 
η1- alkyl, alkenyl, alkynyl, aryl
-
Simple σ donation from carbon to metal centre to form M-C single bond
Treat like any other 2 electron donor (formally R–) e.g. Me–, Ph–
η2 alkene and alkyne
-
Side on bonding mode like H2.
  -
σ donation from the bond to the
-
metal.
π donation from the metal to the π* This weakens the π bond.
64
Q

Why are organometallics useful in biological systems?

A 
Flexible coordination number.
Flexible coordination geometry.
Variable oxidation state.
Labile (more easily replaceable) ligands.
Easily ‘tuneable’ ligand environment.

Inorganic Chemistry (3 decks)

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