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Year 3 - Chemistry > Inorganic Mechanisms > Flashcards

Inorganic Mechanisms Flashcards

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1
Q

What are the mechanisms of electron transfer (ET)?

A

Outer sphere

Inner sphere

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2
Q

What is a description of the outer sphere ET reaction?

A

e- transferred without any apparent changes to coord shells or spheres of each metal centre

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3
Q

What is a brief description of the inner sphere ET reaction?

A

Formation of a binuclear complex with a bridging ligand

That bridging ligand provided by one of reactants

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4
Q

What is a brief description of the inner sphere ET reaction?

A

Formation of a binuclear complex with a bridging ligand

That bridging ligand provided by one of reactants

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5
Q

When do reactions in this sector take place?

A

Aqueous chemistry

NOT organometallic

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6
Q

What is required for a square planar complexes?

A

d8 configuration

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7
Q

What common ions are square planar?

A

Rh+, Ir+

Ni2+, Pd2+, Pt2+

Au3+

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8
Q

Why is d8 required for square planar?

A

x2-y2 orbital is raised very high in energy

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9
Q

How does the config of square planar complexes in substitutions relate?

A

Retention of config

(cis to cis, trans to trans)

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10
Q

What mechanisms are there for subsitution of square planar complexes?

A

Solvent pathway

Nucleophilic pathway

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11
Q

What mechanisms are there for subsitution of square planar complexes?

A

Solvent pathway

Nucleophilic pathway

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12
Q

Why does the rate law for sub at square planar have two components?

A

Due to the two parallel mechanisms

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13
Q

What is the rate equation for sub at square planar?

A

-d[ML3X]/dt = (ks + ky[Y]) [ML3X]

where ks is from solvent and ky from nucleophilic

solvent isnt in rate equation as doesnt directly attack

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14
Q

What is the solvent pathway in square planar sub?

A

1) solvent adds to sq planar to give square pyramidial (slow)
2) Rotation to give correct orientation and X eliminated
3) New group attacks
4) Solvent eliminated

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15
Q

What is the nucleophilic pathway in square planar sub?

A
  1. Attack of Nuc directly to vacant metal pz to give square pyramidial
  2. Rotation
  3. X eliminated fast
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16
Q

What is ΔS‡ and ΔV‡ in the square planar sub mech?

A

Large and negative ΔS‡ and ΔV‡

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17
Q

Why are the two pathways for square planar sub associative?

A

Has two vacant coord sites

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18
Q

What is the reactivity of a metal to square planar sub determined by?

A

Reactivity depends on willingness to move from square planar to 5-coord system

Controlled by structural preference energy (LFSE, maximise Δ-value)

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19
Q

What is the rel rate of square planar sub for different metals?

A

Ni2+ > Pd2+ > Pt2+
as Ni2+ maximises Δ

Au3+ > Pt2+
as higher charge attracts incoming nucleophile

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20
Q

How does nucleophilicity of a ligand change rate of square planar sub?

A

Softer nuc is better due to ΔHsol being lower

I- > Br- > Cl-&raquo_space; F-

PR3&raquo_space; NR3

R2S&raquo_space; R2O

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21
Q

How can a rate graph show if a solvent is nucleophilic?

A

When intercept zero it is not nucleophilic

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22
Q

How does LG in square planar sub reactions affect rate?

A

M-LG must distrort from optimum overlap

Broadly follows ligand basicity and M-LG bond strength

NO3- > H2O > Cl- > Br - > I-&raquo_space;> N3- > SCN- > NO2- > CN-

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23
Q

What is the trans effect series?

A

How ligands change the rate of ligand sub when trans to the leaving group

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24
Q

What is the trans influence series?

A

Impact of a ligand of bond strength of a bond trans to it in ground state

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25
Q

What is the ordering of the trans effect?

A

Kinetic measurement of rate of substitution

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26
Q

How does cis and trans effect on rate of sub compare?

A

Cis has a labilising effect but sig less than a trans ligand

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27
Q

What determines the position of a ligand in trans effect?

A

Based on activation energies

High in trans effect if it raises energy of ground state or lowers energy of TS (both)

Bulky ligand slows sub as (D4h -> C4v)

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28
Q

What is the σ trans effect?

A

Easier for LG to leave as lowers Ea

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29
Q

What are the features of ligands high and low in trans effect?

A

High: powerful trans σ-donors

Low: trans ligand bonding mainly ionic

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30
Q

How can the trans influence be measured?

A

X-ray crystallography gives M-X bond length

Vib spec gives M-X stretch

NMR gives coupling const

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31
Q

What is the trans influence series?

A

Bond-weaking effect of trans ligands in sub

Good σ donors are high in influence series

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32
Q

What is the π trans effect?

A

π-acceptors remove e- density from metal which stabilises TS
Most effective when shares same d-orbital as X and Y in eq plane of intermediate species

Therefore increases rate of substitution

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33
Q

How does π trans effect where π-ligands are in the trans influence and effect?

A

π-acceptors are high in trans effect series as stab TS

π-accpetors are low in trans influence series, do not weaken M-X bond

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34
Q

Why are phosphines, SCN-, and NO2- high in the trans effect series?

A

Both σ and π trans effects together

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35
Q

How do aqueous octahedral complexes undergo ligand replacement?

A

Two steps:

1) Hydrolysis - ligand replaced by H2O
2) Water sub - ligand replaced by new ligand

Different mech under acidic and basic cond
Rate determined by M-X bond strength

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36
Q

What mechanisms are possible for octahedral complexes?

A

Associative (A), dissociative (D), and interchange (Ia bond formation, Id eigen-wilkens)

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37
Q

What is the coord in the TS of interchange mech of octahedral complexes?

A

7-coord transition state

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38
Q

What type of octahedral mechanism is most common?

A

Interchange mechanisms
(Id or Ia)

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39
Q

What is the rate law of A and I mechanisms for oct sub?

A

2nd order

Rate = kobs [ML6][X]

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40
Q

What is the rate law of D mechanism for oct sub?

A

Use steady-state approx to derive

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41
Q

What are the classes of water exchange rates in oct complexes?

A

Class 1 - v fast, diffusion controlled, group 1/2 ions except Be2+ & Mg2+, group 12 except Zn2+, Cr2+ & Cu2+, lanthanides except late ones

Class 2 - fast, includes Mg2+, late lanthanide 3+, and most of first row TM divalent cations

Class 3 - slower, smaller main group ions (Be2+, Al3+, some 1st row M3+)

Class 4 - v slow, 2nd or 3rd row TM ions

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42
Q

How does rate of water exchange change down a group (in general)?

A

Increases down a group

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43
Q

What is the equation for ΔV‡?

A

(dlnk/dP)T = -ΔV‡/RT

where
ΔV‡ = V(transition state) - V(reactants)

44
Q

What does the sign of ΔV‡ indicate for water exchange reactions?

A

Dissociative: Positive, ΔV‡ > 0

Associative: Negative, ΔV‡ < 0

45
Q

How does rate of water exchange change across a series?

A

Decreases across a series (so descreases as ionic radius decreases)
Due to charge increasing

46
Q

What are the water exchange mech for groups 1,2,12,13?

A

ΔS‡ and ΔV‡ > 0

Kinetic parameters support Id (Eigen-Wilkins) mechanism
Bond-breaking important in RDS but key factor is strength of M-OH2

47
Q

Why does Hg2+ undergo rel fast water exchange?

A

sdz2 for d10
Results in 2-short and 4-long Hg-OH2

48
Q

What are primary and secondary coord states?

A

Exchange between 1° and bulk solvent requires:
2° and bulk exchange fast as weakly held
1° and 2° more crucial

49
Q

What is the water excange mechanism for lanthanide ions?

A

Early (La3+ to Nd3+) - Id

Late (Gd3+ to Lu3+) - Ia

As ionic radius decreases, so high coord numbers not as supported

50
Q

What are the configs in lanthanide water exchange?

A

Eqm between 9-coord (tricapped trig prism, D3h) and 8-coord (square anti-prism, D4d)

Ia: CN = 8 -> 9 -> 8
Id: CN = 9 -> 8 -> 9

51
Q

Why does Eu2+ undergo fast exchange?

A

In eqm with 8-coord form and undergoes associative mech

52
Q

What is the mechanism for early and later TM?

A

Early (V to Mn) - Ia activation, vacant t2g
Late (Fe to Ni) - Id activation

2nd row - larger ion, stronger R-OH2 and Ia

53
Q

Why do Cr2+ and Cu2+ have fast diss water exchange?

A

Cr2+ is HS d4, Cu2+ has d9

Both susceptible to JT which elongates 2xM-OH2 bonds

54
Q

How does LFSE effect rate of water exchange?

A

LFSE lost going to the TS

So higher LFSE cuses slower rate for both associative and diss

Called ligand field activation energy (LFAE)

55
Q

Which divalent 1st row TM have high or low LFAE?

A

Higher LFAE: V2+&Cr2+-d3, d8-Ni2+, low spin d4,5,6

No LFAE: d0 and hs d5,10

56
Q

How does rate of water exchange change down TM?

A

2nd and 3rd row TMs should be slower

Stronger bonds and larger Δ, and more associative

57
Q

What is the comparative rates of associative and dissociative water exchange?

A

Dissociative sig faster than associative

58
Q

What is the water exchange mech for trivalent ions?

A

For Al3+ and Ga3+ - Id, no vacant orbitals

Other M3+ - Ia, Stronger M-OH2, vacant orbitals

Still consider LFAE etc

59
Q

What is a hydrolysed trivalent metal ion?

A

M(OH2)5(OH)2+

60
Q

What is the mech when a hydrolysed trivalent metal ion (M(OH2)5(OH)2+) undergoes water exchange?

A

Id

Rates sig higher than unhydrolysed forms, trans-labilising effect by OH-

Only Ga3+ remains Id

61
Q

What is the rate equation for hydrolysis?

A

Rate = kA[ML5X] + kB[ML5X][OH-]

where 1st term is acid and 2nd alkali

kB&raquo_space;> kA

62
Q

What is the general rates of acid hydrolysis?

A

Id
Correlates with the stability of complexes

63
Q

What is the rate of acid hydrolysis when compelx has carboxylate ligand?

A

Correlation with the basicity of the carboxylate

64
Q

How does bulk change the rate of acid hydrolysis?

A

Rate increases as bulk of bidentate ligand
Indicates dissociative

65
Q

What occurs to stereochem in acid and base hydrolysis?

A

Scrambled as goes via a 5-coord intermediate which is trig bipyramidal

66
Q

What is the mechanism for base hydrolysis?

A

Conjugate base mech

67
Q

Why does the conjugate base hydrolysis mech occur?

A

Conj base has strong σ and π donation

This accelerates loss of trans ligand

68
Q

Why does a trig bipyramidial intermediate form in acid/base hydrolysis?

A

When loses ligand - square pyramidial

Then will move to tbp and scrambles the stereochem

69
Q

Why does a trig bipyramidial intermediate form in base hydrolysis?

A

When loses ligand - square pyramidial

Then will move to tbp and scrambles the stereochem

70
Q

How do cis and trans complexes undergo acidic hydrolysis?

A

trans - stereochem change, as has trans pπ lone pair which can only overlap when tbp

cis - retention, faster as has a π-donating group cis which overlaps when square pyr

71
Q

What is the hydrolysis mech when OH- group?

A

tbp intermediate when OH- only
will therefore give cis product

72
Q

What is the stereochem of conjugate base hydrolysis?

A

deprotonated conj base long-lived enough to adopt tbp

therefore both cis and trans complexes show scrambling

73
Q

Which complexes undergo outer-sphere electron transfer?

A

Substitutionally inert complexes

For oh: d3,8, ls d4,5,6 are inert due to LFAE
Strong-field and chelating ligands (CN,bipy,etc.) give iner

74
Q

What is the rate of outer-sphere electron transfer?

A

Rate = kobs [Ox][Red]

kobs = KA ket

75
Q

What is the mech of the outer sphere of electron transfer?

A

1) precursor formation, by eqm const Ka
Ox + Red <-> Ox-Red

2) chem activation and ET, by rate const ket
Ox-Red <-> -Ox-Red+

3) dissociation to prod
-Ox-Red+ <-> Ox- + Red+

76
Q

When is outer sphere e- transfer faster?

A

Faster when change in π* (t2g)

Called the π*->π* requirement

77
Q

What occurs in a self-exchange reaction?

A
78
Q

What is the Franck-Condon principle?

A

e- transfer occurs with nuclei being stationary because elec motion is faster than nuclear motion

79
Q

What is the Franck-Condon barrier for product formation?

A

Allowed when goes via T
Not allowed when goes via A

80
Q

What is the ΔG‡ in franck-condon barrier?

A

ΔG‡ = λ/4

where λ is the reorganisation energy (depending on change in r(M-L))

81
Q

How does the change in r(M-L) change the rate of self-exchange?

A

If large r(M-L) then large λ

leads to a larger rate

82
Q

What causes a change in r(M-L)?

A

Large if change in σ* population upon ET

For Oh: eg is σ*, so if change in population will give larger barrier so larger rate

83
Q

How does solvent reorganization change ΔG‡?

A

ΔG‡ larger when there is extensive solvent reorganistion

Hydrophobic ligands such as bipy lower the solvent reorg energy

84
Q

What is the rate of electron-transfer?

A

ket = C(HAB)2 exp[-λ/4kT]

where HAB is donor-acceptor overlap

85
Q

What is HAB?
(donor-acceptor overlap)

A

HAB higher if d e- in extended orbitals and if delocalised out ot the periphery of the ligand

π-acceptors increase HAB so increases ET

86
Q

What is the Marus equation for a redox reaction?

A
87
Q

What is the graph of -ΔG against lnk in Marcus equation?

A

Normal region - where rate constant increases with |ΔG| as expected

Inverted region - rate constant decreases as |ΔG|, which is weird

88
Q

What is the marcus cross relation?

A

Find K, eq const, from -nFE = -RTlnK

89
Q

How do hydrophobic ligands in self exchange have an effect on the rate?

A

More hydrophobic ligands have a lesser solvent reorganisation contribution to λ
(such as en)

Means a larger rate

90
Q

Why are Co(bipy)32+ self-exchange reactions fast??

A

Co(bipy)32+ is high spin

  • change in r(M-L) similar to NH3 complexes which decreases rate but
  • Higher donor-acceptor coupling as bipy is a π-acceptor
  • A lower λ due to hydrophobic ligand
91
Q

Why is the Co(bipy)32+/+ exchange very fast?

A
  • No change in eg* or M-L bond length
  • π-acceptor bipy gives larger HAB
  • Hydrophobic bipy reduces solvent organisation
92
Q

How does NH3 and H2O compare in terms of self-exchange rates?

A

NH3 is less polar than water, so lower λ

NH3 is a pure σ donor so t2g is non-bonding and low r(M-L)

93
Q

What is the rel speed of outer and inner sphere ET reactions?

A

Inner sphere ET reactions sig faster due to Franck-Condon barrier in outer

94
Q

What is required for a bridging ligand?

A

Requires a lone pair to form an inner sphere precursor complex

95
Q

What are the steps of inner sphere ET?

A
96
Q

What is the inner sphere ET rate?

A

From SS approx

97
Q

What is requried for the inner-sphere ET mechanism?

A
  • Bridging ligand must have extra lp, NH3/ H2O cannot be a bridging ligand, but OH- and halides are excellent
  • Other reactant is substitutionaly labile
98
Q

What are examples of sub labile reactants for inner sphere ET?

A

Cr aquo - reductant has JT distortion
Fe aquo and Eu2+ - good inner sphere reductants
V aquo - t2g3 with slower sub but can do inner sphere charge is low
Cr bipy 2+ - only outer sphere reductant

99
Q

What is the dependence of ET mechanism on pH?

A

Outer sphere has little pH dependence

Inner sphere accelerated as pH increases, conjugate base is a better bridging ligand

100
Q

What is the ΔV‡ for outer and inner sphere ET?

A

Outer-sphere: ΔV‡ < 0, formation of loosely associated precursor complex

Inner-sphere: ΔV‡ > 0, water ligand expelled on precursor complex formation

101
Q

What is the azide/thiocyanate test for outer/inner sphere mechanisms?

A

Azide can form a more stable bridged intermediate

102
Q

What energetic factors increase the rate of inner sphere ET?

A

Transfer of e- from orbital of M-L being formed into orbital of oxidant (M-L bond being broken)

σ*->σ* faster for inner sphere ET

[Cr(OH2)6]2+ is good as σ*->σ*, but V/Fe2+ less so as π*->σ*

103
Q

Why is py a good bridging ligand from inner sphere ET?

A

Better DA coupling as π-acceptor

More hydrophobic and lower λ

104
Q

Are hard/soft ligands the best for bridging inner sphere ET?

A

Bond to softer weaker but more covalent overlap which speeds up ET in bridged complex

This decreases bridged complex stability - overcome for softer ions (Cr2+ not Fe2+)
V ionic metals cannot promote ET

105
Q

What is a complementary reaction?

A

Two centres donate or accept the same number of electrons

Reactions can occur by a single electron transfer step

106
Q

What is a non-complementary reaction?

A

Oxidant and reductant requires different changes in oxn states

107
Q

How do non-complementary reactions occur?

A

Formation of reactive intermediates in unstable oxn states

Year 3 - Chemistry (12 decks)

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