Inorganic Mechanisms Flashcards
What are the mechanisms of electron transfer (ET)?
Outer sphere
Inner sphere
What is a description of the outer sphere ET reaction?
e- transferred without any apparent changes to coord shells or spheres of each metal centre
What is a brief description of the inner sphere ET reaction?
Formation of a binuclear complex with a bridging ligand
That bridging ligand provided by one of reactants
What is a brief description of the inner sphere ET reaction?
Formation of a binuclear complex with a bridging ligand
That bridging ligand provided by one of reactants
When do reactions in this sector take place?
Aqueous chemistry
NOT organometallic
What is required for a square planar complexes?
d8 configuration
What common ions are square planar?
Rh+, Ir+
Ni2+, Pd2+, Pt2+
Au3+
Why is d8 required for square planar?
x2-y2 orbital is raised very high in energy
How does the config of square planar complexes in substitutions relate?
Retention of config
(cis to cis, trans to trans)
What mechanisms are there for subsitution of square planar complexes?
Solvent pathway
Nucleophilic pathway
What mechanisms are there for subsitution of square planar complexes?
Solvent pathway
Nucleophilic pathway
Why does the rate law for sub at square planar have two components?
Due to the two parallel mechanisms
What is the rate equation for sub at square planar?
-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
What is the solvent pathway in square planar sub?
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
What is the nucleophilic pathway in square planar sub?
- Attack of Nuc directly to vacant metal pz to give square pyramidial
- Rotation
- X eliminated fast
What is ΔS‡ and ΔV‡ in the square planar sub mech?
Large and negative ΔS‡ and ΔV‡
Why are the two pathways for square planar sub associative?
Has two vacant coord sites
What is the reactivity of a metal to square planar sub determined by?
Reactivity depends on willingness to move from square planar to 5-coord system
Controlled by structural preference energy (LFSE, maximise Δ-value)
What is the rel rate of square planar sub for different metals?
Ni2+ > Pd2+ > Pt2+
as Ni2+ maximises Δ
Au3+ > Pt2+
as higher charge attracts incoming nucleophile
How does nucleophilicity of a ligand change rate of square planar sub?
Softer nuc is better due to ΔHsol being lower
I- > Br- > Cl-»_space; F-
PR3»_space; NR3
R2S»_space; R2O
How can a rate graph show if a solvent is nucleophilic?
When intercept zero it is not nucleophilic
How does LG in square planar sub reactions affect rate?
M-LG must distrort from optimum overlap
Broadly follows ligand basicity and M-LG bond strength
NO3- > H2O > Cl- > Br - > I-»_space;> N3- > SCN- > NO2- > CN-
What is the trans effect series?
How ligands change the rate of ligand sub when trans to the leaving group
What is the trans influence series?
Impact of a ligand of bond strength of a bond trans to it in ground state
What is the ordering of the trans effect?
Kinetic measurement of rate of substitution
How does cis and trans effect on rate of sub compare?
Cis has a labilising effect but sig less than a trans ligand
What determines the position of a ligand in trans effect?
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)
What is the σ trans effect?
Easier for LG to leave as lowers Ea
What are the features of ligands high and low in trans effect?
High: powerful trans σ-donors
Low: trans ligand bonding mainly ionic
How can the trans influence be measured?
X-ray crystallography gives M-X bond length
Vib spec gives M-X stretch
NMR gives coupling const
What is the trans influence series?
Bond-weaking effect of trans ligands in sub
Good σ donors are high in influence series
What is the π trans effect?
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
How does π trans effect where π-ligands are in the trans influence and effect?
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
Why are phosphines, SCN-, and NO2- high in the trans effect series?
Both σ and π trans effects together
How do aqueous octahedral complexes undergo ligand replacement?
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
What mechanisms are possible for octahedral complexes?
Associative (A), dissociative (D), and interchange (Ia bond formation, Id eigen-wilkens)
What is the coord in the TS of interchange mech of octahedral complexes?
7-coord transition state
What type of octahedral mechanism is most common?
Interchange mechanisms
(Id or Ia)
What is the rate law of A and I mechanisms for oct sub?
2nd order
Rate = kobs [ML6][X]
What is the rate law of D mechanism for oct sub?
Use steady-state approx to derive
What are the classes of water exchange rates in oct complexes?
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
How does rate of water exchange change down a group (in general)?
Increases down a group
What is the equation for ΔV‡?
(dlnk/dP)T = -ΔV‡/RT
where
ΔV‡ = V(transition state) - V(reactants)
What does the sign of ΔV‡ indicate for water exchange reactions?
Dissociative: Positive, ΔV‡ > 0
Associative: Negative, ΔV‡ < 0
How does rate of water exchange change across a series?
Decreases across a series (so descreases as ionic radius decreases)
Due to charge increasing
What are the water exchange mech for groups 1,2,12,13?
ΔS‡ and ΔV‡ > 0
Kinetic parameters support Id (Eigen-Wilkins) mechanism
Bond-breaking important in RDS but key factor is strength of M-OH2
Why does Hg2+ undergo rel fast water exchange?
sdz2 for d10
Results in 2-short and 4-long Hg-OH2
What are primary and secondary coord states?
Exchange between 1° and bulk solvent requires:
2° and bulk exchange fast as weakly held
1° and 2° more crucial
What is the water excange mechanism for lanthanide ions?
Early (La3+ to Nd3+) - Id
Late (Gd3+ to Lu3+) - Ia
As ionic radius decreases, so high coord numbers not as supported
What are the configs in lanthanide water exchange?
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
Why does Eu2+ undergo fast exchange?
In eqm with 8-coord form and undergoes associative mech
What is the mechanism for early and later TM?
Early (V to Mn) - Ia activation, vacant t2g
Late (Fe to Ni) - Id activation
2nd row - larger ion, stronger R-OH2 and Ia
Why do Cr2+ and Cu2+ have fast diss water exchange?
Cr2+ is HS d4, Cu2+ has d9
Both susceptible to JT which elongates 2xM-OH2 bonds
How does LFSE effect rate of water exchange?
LFSE lost going to the TS
So higher LFSE cuses slower rate for both associative and diss
Called ligand field activation energy (LFAE)
Which divalent 1st row TM have high or low LFAE?
Higher LFAE: V2+&Cr2+-d3, d8-Ni2+, low spin d4,5,6
No LFAE: d0 and hs d5,10
How does rate of water exchange change down TM?
2nd and 3rd row TMs should be slower
Stronger bonds and larger Δ, and more associative
What is the comparative rates of associative and dissociative water exchange?
Dissociative sig faster than associative
What is the water exchange mech for trivalent ions?
For Al3+ and Ga3+ - Id, no vacant orbitals
Other M3+ - Ia, Stronger M-OH2, vacant orbitals
Still consider LFAE etc
What is a hydrolysed trivalent metal ion?
M(OH2)5(OH)2+
What is the mech when a hydrolysed trivalent metal ion (M(OH2)5(OH)2+) undergoes water exchange?
Id
Rates sig higher than unhydrolysed forms, trans-labilising effect by OH-
Only Ga3+ remains Id
What is the rate equation for hydrolysis?
Rate = kA[ML5X] + kB[ML5X][OH-]
where 1st term is acid and 2nd alkali
kB»_space;> kA
What is the general rates of acid hydrolysis?
Id
Correlates with the stability of complexes
What is the rate of acid hydrolysis when compelx has carboxylate ligand?
Correlation with the basicity of the carboxylate
How does bulk change the rate of acid hydrolysis?
Rate increases as bulk of bidentate ligand
Indicates dissociative
What occurs to stereochem in acid and base hydrolysis?
Scrambled as goes via a 5-coord intermediate which is trig bipyramidal
What is the mechanism for base hydrolysis?
Conjugate base mech
Why does the conjugate base hydrolysis mech occur?
Conj base has strong σ and π donation
This accelerates loss of trans ligand
Why does a trig bipyramidial intermediate form in acid/base hydrolysis?
When loses ligand - square pyramidial
Then will move to tbp and scrambles the stereochem
Why does a trig bipyramidial intermediate form in base hydrolysis?
When loses ligand - square pyramidial
Then will move to tbp and scrambles the stereochem
How do cis and trans complexes undergo acidic hydrolysis?
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
What is the hydrolysis mech when OH- group?
tbp intermediate when OH- only
will therefore give cis product
What is the stereochem of conjugate base hydrolysis?
deprotonated conj base long-lived enough to adopt tbp
therefore both cis and trans complexes show scrambling
Which complexes undergo outer-sphere electron transfer?
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
What is the rate of outer-sphere electron transfer?
Rate = kobs [Ox][Red]
kobs = KA ket
What is the mech of the outer sphere of electron transfer?
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+
When is outer sphere e- transfer faster?
Faster when change in π* (t2g)
Called the π*->π* requirement
What occurs in a self-exchange reaction?
What is the Franck-Condon principle?
e- transfer occurs with nuclei being stationary because elec motion is faster than nuclear motion
What is the Franck-Condon barrier for product formation?
Allowed when goes via T
Not allowed when goes via A
What is the ΔG‡ in franck-condon barrier?
ΔG‡ = λ/4
where λ is the reorganisation energy (depending on change in r(M-L))
How does the change in r(M-L) change the rate of self-exchange?
If large r(M-L) then large λ
leads to a larger rate
What causes a change in r(M-L)?
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
How does solvent reorganization change ΔG‡?
ΔG‡ larger when there is extensive solvent reorganistion
Hydrophobic ligands such as bipy lower the solvent reorg energy
What is the rate of electron-transfer?
ket = C(HAB)2 exp[-λ/4kT]
where HAB is donor-acceptor overlap
What is HAB?
(donor-acceptor overlap)
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
What is the Marus equation for a redox reaction?
What is the graph of -ΔG against lnk in Marcus equation?
Normal region - where rate constant increases with |ΔG| as expected
Inverted region - rate constant decreases as |ΔG|, which is weird
What is the marcus cross relation?
Find K, eq const, from -nFE = -RTlnK
How do hydrophobic ligands in self exchange have an effect on the rate?
More hydrophobic ligands have a lesser solvent reorganisation contribution to λ
(such as en)
Means a larger rate
Why are Co(bipy)32+ self-exchange reactions fast??
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
Why is the Co(bipy)32+/+ exchange very fast?
- No change in eg
*or M-L bond length - π-acceptor bipy gives larger HAB
- Hydrophobic bipy reduces solvent organisation
How does NH3 and H2O compare in terms of self-exchange rates?
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)
What is the rel speed of outer and inner sphere ET reactions?
Inner sphere ET reactions sig faster due to Franck-Condon barrier in outer
What is required for a bridging ligand?
Requires a lone pair to form an inner sphere precursor complex
What are the steps of inner sphere ET?
What is the inner sphere ET rate?
From SS approx
What is requried for the inner-sphere ET mechanism?
- Bridging ligand must have extra lp, NH3/ H2O cannot be a bridging ligand, but OH- and halides are excellent
- Other reactant is substitutionaly labile
What are examples of sub labile reactants for inner sphere ET?
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
What is the dependence of ET mechanism on pH?
Outer sphere has little pH dependence
Inner sphere accelerated as pH increases, conjugate base is a better bridging ligand
What is the ΔV‡ for outer and inner sphere ET?
Outer-sphere: ΔV‡ < 0, formation of loosely associated precursor complex
Inner-sphere: ΔV‡ > 0, water ligand expelled on precursor complex formation
What is the azide/thiocyanate test for outer/inner sphere mechanisms?
Azide can form a more stable bridged intermediate
What energetic factors increase the rate of inner sphere ET?
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*
Why is py a good bridging ligand from inner sphere ET?
Better DA coupling as π-acceptor
More hydrophobic and lower λ
Are hard/soft ligands the best for bridging inner sphere ET?
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
What is a complementary reaction?
Two centres donate or accept the same number of electrons
Reactions can occur by a single electron transfer step
What is a non-complementary reaction?
Oxidant and reductant requires different changes in oxn states
How do non-complementary reactions occur?
Formation of reactive intermediates in unstable oxn states
