Reaction Mechanism of d-metal complexes

Chapter 20

Inorganic Chem 160:371

November 2010

M. Greenblatt

Ligand substitution reactions: one Lewis base (Y) displaces another (X)

Y + M-X → M-Y + X

e.g.,[Co(OH2)6]2+ (aq) + Cl- (aq) → [CoCl(OH2)5]+ (aq) + H2O (l)

Both thermodynamic and kinetic effects determine chemical reaction-A reaction may be thermodynamically possible (∆G < 0), yet kinetically it is not

reactive (nonlabile)

Equilibrium, or Formation constants:

e.g., coordination equilibrium for ligand substitution is

[Fe(OH2)6]3+ (aq) + SCN- → [FeSCN(OH2)5]2+ (aq) + H2O )l)

Kf = [FeSCN(OH2)52+] / [Fe(OH2)6

3+][SCN-]

Kf is the formation constant of the complex; if Kf is large, incoming L binds Stronger than H2O (solvent); if Kf is small, H2O binds stronger than L

Kf spans a huge range, depending on M and L

If more than one L is replaced by substitution, then Kf is more complex:

[Ni(OH2)6]2+ (aq) + 6NH3 → [Ni(NH3)6]2+ (aq) + 6H2O (l)

At least 6 steps are involved:

[Ni(OH2)6]2+ (aq) + NH3 (aq) → [NiNH3(OH2)5]2+ (aq) + H2O (l)

Kf1 =

[NiNH3(OH2)5]2+ (aq) + NH3 (aq) → [Ni(NH3)2(OH2)5]2+ (aq) + H2O (l)

Kf2

etc…..to Kf6

in general

Kf1 > Kf2>Kf3……>Kf6…..

This is understood in terms of ∆G0 = -RTlnKf successive Kf decrease due todecreased # of H2O available for replacement - deminishing statistical factor is reflected in stepwise Kf values for for above reaction

Note Kfn/Kfn+1 is not very large

In general:

M + L → ML Kf1 = [ML]/[M][L]

ML + L → ML2 Kf2 = [ML2/[ML][L]

MLn-1 + L → MLn Kfn = [MLn/[MLn-1][L]

Overall formation constant:

M + nL → MLn βn = [MLn]/[M][L]n

βn = Kf1x Kf2…..Kfn

What are the dissociation constants of MLn?

ML → M + L etc

A reversal of this trend indicates an electronic, or structural change

e.g.: Kfn > Kfn+1

[Fe(bipy)3] 3+ much more stable than [Fe(OH2)2(bipy)2]3+

LFSE of LS, t2g5eg0 >> LFSE of HS t2g

3eg2

[Fe(OH2)6]3+ (aq) + bipy (aq) → [Fe(OH2)4(bipy)]3+ (aq) + 2H2O (l)

log Kf = 4.2

[Fe(OH2)4(bipy)]3+ (aq) + bipy (aq) → [Fe(OH2)2(bipy)2]3+ (aq) + 2H2O (l)

log Kf = 3.7

[Fe(OH2)2(bipy)2]3+ (aq) + bipy (aq) → [Fe(bipy)3]3+ (aq) + 2H2O (l)

log Kf = 9.3

Also if Kfn+1 < Kfn some change has occurred

Note the reaction of [Hg(OH2)6]2+ + Cl-

Kf1 = 6.74; Kf2 = 6.48; Kf3 = 0.85

[HgCl2(OH2)4] (aq) + Cl- (aq) → [HgCl3(OH2)]- (aq) + 3H2O (l)

CN = 6 → CN = 4

Chelate effect

1. [Cd(OH2)6 ]2+ (aq) + en (aq) → [Cd(en)(OH2)4 ]2+ (aq) + 2H2O (l)

2. [Cd(OH2)6 ]2+ (aq) + 2NH3 (aq) → [Cd(NH3)2(OH2)4 ]2+ + 2H2O (l)

∆H are similar, ∆S(1) = +13 J(Kmol)-1, ∆S(2) = -5.2 J(Kmol)-1

Chelate complexes are always more stable due to increase of ∆S & kinetic effect

∆G ≡ ∆H-T∆S

chelate effect has important applications: porphyrin, edta4- complexing agents,biochemical metal sites

very large Kf (1012-1025 indicates chelated complex

Diamine metal comlex

Examples of chelate comlexes M

M

Note five memberedrings-stable

RuII*(bpy)3

RuII(bpy)3/RuIII(bpy)3

hυ

λ-MnO2 Catalyste-

e-

2H2O

O2 + 4H+

Eox = 1.4 V

SO4- + SO4

2-

S2O82-

Photon driven oxidation systemto generate O2 from H2O with Solar energy

Illumination was done using 250W industrial light source with UV filtered by Pyrex and IR with a 12 cm path water filter at intensity of 20 mWcm-2.

Irving-William Series: summarizes relative stabilities of M2+ complexesand reflects electrostatic and LSE effects

Ba2+<Sr2+<Ca2+<Mg2+<Mn2+<Fe2+<Co2+<Ni2+<Cu2+>Zn2+

Note sharp increase in Kf forFe2+, d6, to Cu2+ d9 LSE

decrease for Zn2+, d10 (LSE=0)

Why Kf (Cu2+ complex) > Kf (Ni2+ complex)?

Ligand substitution reactions

MLxX + Y → MLxY + X

X is the leaving group and Y is the entering group

Kinetically complexes are inert and labile

Metal complexes that react with t1/2 ≤ 1 min are kinetically labileIf the reaction is significantly longer than this, it is considered kinetically nonlabile, or

inert

No relationship between thermodynamic stability and lability towards substitution.

e.g., ∆hydGº of Cr3+ and Fe3+ are similar,

[Cr(H2O)6]3+ d3 undergoes substitution slowly, while Fe(H2O)6]3+ d5 fast

Overall formation constant of [Hg(CN)4]2- is greater than that of [Fe(CN)6]4-

Hg(II) complex is kinetically labile and exchanges CN rapidly (with isotopicallylabelled CN, while Fe(II) d6 HS slowly; d3 and d6 complexes are extra stable

due to LFSE

Average residence time, τ = 1/k of H2O in first coordination sphere of a metal ion

[Ir(H2O)6]3+ slow, τ = 109

τ = 290 years

rapid

Main groupk for H2O exchange

Increases with:increasing size of M

increasing coordination #decreasing surface charge density (Z/reff)

Lanthanides are large ions, k > 107

Note huge range for d-metals: function of dn

e.g., [Cr(H2O)6]3+ d3, [Rh(H2O)6]3+ LS d6 LFSE

[M(OH2)x]n+ + H2O [M(OH2)x-1 (OH2)]n+ + H2O

Rate of H2O exchange = xk[M(OH2)xn+]

→←k

Some generalizations about reactivity:

1.Metal comlexes without extra stability (e.g., LFSE & chelating L, are labile)

2.Very small ions are less labile, because of strong M-L bond and steric effects(difficult for Y to approach M)

3. All complexes of s (except Be2+ and Mg) are labile)

4. Complexes of M(III) f-block are all very labile

5. Complexes of d10 ions (Zn2+, Cd2+, Hg2+) are normally labile

6. Across d series M2+ are generally labile with distorted Cu2+ most labile

7. Across d series M3+ are distinctly less labile than M2+

8. d-M comlexes with electronic configurations with d3 and d6 (Cr(III), Co(III)are nonlabile due to large LFSE, chelate complexes, like [Fe(dipy)]2+ are even more nonlabile

9. 4d, 5d metal complexes are usually nonlabile, because of large LFSE (LS)

1. Identities of Ls effect rates of reaction; incoming L has greatest effect on rate2. Keq of displacement reaction can rank Ls in order of their strength as Lewis bases3. For kinetics, concept of nuclephilicity is used (instead of equilibrium concept of basicit4. Nuclephilicity: rate of attack on a complex by a Lewis base elative to the rateo attack by a reference Lewis base

Mechanism is usually proposed: it must be consistent with all the experimentalfacts. A mechanism often cannot be proven, since another mechanism may also

beconsistent with the experimental data.

For substitution reactions, square planar and octahedral complexeswill be considered only

Mechanism of reactions – sequence of elementary steps by which the reaction takes placeoften not all the steps can be determined, only the slowest step, the rate determining step

First step in elucidating mechanism is determination of therate law - how rate changes with concentration of reactants

e.g., [Ni(OH2)6]2+ (aq) + NH3 (aq) → [Ni(OH2)5]2+ (aq) + H2O

Rate = k[Ni(OH2)62+][NH3]

Generally the slowest elementary step of reaction controls the overall rateof raction and the overall rate law – this is the rate-determining step

Stoichiometric equations often say nothing about mechanism

[(H3N)5Co(CO3)]+ + 2H3O+ → [(H3N)5Co(H2O)]3+ + CO2 + 2H2O

This might suggest direct substitution of CO32- by H2O

However, use of H218O solvent shows that all the O in the [(H3N)5Co(H2O)]3+

complex comes from CO32-

[(H3N)5Co(OCO2)] + H3O+ → [(H3N)5Co— O

CO2

H2+

+ H2O

[(H3N)5Co(OH)]2+ + CO2 + H2O

↓

[(H3N)5Co(H2O)]3+ + H2O

↓ H3O+

Proposed mechanism:

Dissociative (D):

MLxX → {MLx } + Xintermediate leaving group

{MLx } + Y → MLxYentering group

Classification of mechanisms for nucleophilic substitutionsdissociative, associative and interchange

Typical profile of a reaction with D mechanism

e.g., W(CO)6 → W(CO)5 + CO

W(CO)5 + PPh3 → W(CO)5Ph3

The intermediate W(CO)6 has been isolated

Associative:

MLxX + Y → {MLxXY}entering group

{MLxXY} → MLxY + X intermediate leaving group

This mechanism in square planarComplexes of d8 M (Ni(II), Pd(II), Pt(II), Ir(I)

e.g., [Ni(CN)4]2- + 14CN- → [Ni(CN)4 14CN]3-

[Ni(CN)4(14CN)]3- → [Ni(CN)3(14CN)]2- + CN-

Intermediatewas isolated

Interchange (I) mechanism

MLxX +Y → Y……MLx……X → MLxY + X

Transition state

In most substitution pathways: bond formation with Y and bond breaking with X is concurrent. In the I (interchange) mechanism: no intermediate phase but various transition states:

Difference between A and I islife time of intermediate stateif it is long enough, and can be detected, A

e.g., [Ni(CN)5]3- trigonal bipyramid was observed experimentally and isolated in thesolid state – intermediate in square planarsubstitution reaction

trans and cis transformation also evidence fortrigonal bipyramidal intermediate

MLxX +Y → Y……MLx……X → MLxY + X

Transition state

n most substitution pathways: bond formation with Y and bond breakingwith X is concurrent. In the I (interchange) mechanism: no intermediate

phase but various transition states:

dissociative interchange (Id) bond breaking dominates bond formingassociative interchange (Ia) bond formation dominates over bond

breaking

In associative (A) and Ia the reaction rate shows a dependenceon the entering group (Y);

in dissociative (D) and Id very small dependence on entering group.In general it is difficult to distinguish between A and Ia and D and Id.

An interchange mechanism is a concerted process in which there isno intermediate species with a coordination number different from

that of the starting complex

Rate determining step is Associative, if rate strongly depends on the incoming LTypical of square planar substitution reaction of d8 metal complexes

[PtCl(dien)]- (aq) + I- (aq) → [PtI(dien)]+ (aq) + Cl- (aq)

Rate with Br- increases rate by an order of magnitude

Profile of mechanismswith associatively activated steps:

a. Associative mechanism associativelyactivated, Aa - formation of MLnXY is ratedetermining step

b. Dissociative mechanism, associativelyactivated - Da formation of MLnY is ratedetermining step

Profile of mechanismswith dissociatively activated steps: rate is

largely independent on identity of Y (substitution in octahedral comlexes)

Dissociative mechanism associativelyactivated, Ad if loss of X in MLnXY is ratedetermining step

b. Dissociative mechanism, dissociativelyactivated - Dd if initial loss of X from MLnX

is rate determining step

Profile of Interchange mechanismsCan be either associative or dissociative

a. Interchange mechanism associativelyactivated, Ia if rate depends on formation of M….Y bond

b. Interchange mechanism, dissociativelyactivated - Id if rate depends on rate at which M…X bond breaks

Pg 765 transition state

Eyring Eq.- f (T, ∆H#, ∆S#)

k = (k’T/h)e-∆G#/RT = k’T/h)e [-∆H#/RT +∆S#/R]

Linearized form:

ln(k/T) = -∆H#/RT + ln(k’/h) + ∆S#/R

k = rate constant

∆H# =enthalpy of activation (J/mol)

∆S# = entropy of activation (J/mol-K)

k’ = Boltzman const

h= Planck’s const.

From Eq: ln(k/T) = -∆H#/RT + ln(k’/h) + ∆S#/RPlot of ln(k/T) vs 1/T (Eyring plot)

Large negative ∆S# indicative associative

Pressure dependence of k→ ∆V#

Volume of activation (cm3/mol)

d(lnk)/dP = -∆V#/RT

k = rate const.P = pressure∆V# =volume of activation (Vtran-Vinit)R = molar gas const.T = temperature (K)

integrated form:

Ln[(k(P1)/k(P2)] = (-∆V#/RT)(P1- P2)

negative value of ∆V# ⇒ associativepositive value of ∆V# ⇒ dissociative

Substitution in Square Planar complexes

d8 configuration, Rh(I), Ir(I), Pt(II), Pd(II), Au(III) form squareplanar comlexes

Many kinetic studies on nucleophilic substitution of Pt(II) square planar complexes indicate that the mechanism is associative or Ia :

∆S# and ∆V# are negative

k for the substitution of Cl- by H2O in [PtCl4]2-, [PtCl3(NH3)]-, [PtCl2(NH3)2], and [PtCl(NH3)3]+ are similar,

suggests an associative mechanism

Substitution in a Pt(II) square palanar complex:

PtL3X + Y → PtL3Y + X

Experimental rate law:

Rate = -d[PtL3X]/dt = k1[PtL3X] + k2[PtL3X][Y]

suggests that reaction proceeds simultaneously by two routes:

If reaction is studied under psuedo-first order conditions, S=olventY and S are in large excess: [Y]t ~ [Y]0; [S]t ~ [S]0

Rate= -d(PtL3X)/dt = kobs[PtL3X]; kobs = k1 +k2[Y]

Rate = -d(PtL3X)/dt = k1[PtL3X] + k2[PtL3X][Y]

Rate= -d(PtL3X)/dt = kobs[PtL3X] kobs = k1 +k2[Y]

Here solvent for both is CH3OHWith different solvent, k1 intercept, differen

Effect of solvent on k1: in polar solvents k1 dominates; in apolar Scontribution of k1 is diminished, hence S participates in reaction, so

Rate = -d(PtL3X)/dt = k3[PtL3X][S] + k2[PtL3X][Y]k1 =k3[S]

Study k vs [Y] (Y in large excess; conc. ~const.)

k2 from associative mechanism is effected by Y

fNucleophilicity of Yfor a specific complex is given by the nucleophilicity parameter

nPt = log k2[Y]/k20

k2[Y] is the second-order rate const

for

trans-[PtCl2(py)2] + Y → [PtYCl(py)2]- + Cl-

And k20 is the rate const when Y is CH3OH

Note: if nPt is large, Y is highly nucleophilic

nPt seems to correlate with softness of base

Cl- < I-; NH3 < P

Rate = -d(PtL3X)/dt = k1[PtL3X] + k2[PtL3X][Y] (25.9)

k2 arises from associative mechanism, attack of Y on PtL3X, and is dominant whenY is a good nucleophile;but k1 term, which may indicate a concurrent dissociative pathway; in polar solvents k1becomes dominant, its contribution diminishing in non-polar solvents

Thus solvent participates, and rate eq. (25.9) is better written as:

Rate = -d(PtL3X)/dt = k3[PtL3X][S] + k2[PtL3X][Y]

Since S is in excess, [S] = constant, and k1 = k3[S]

When S is a potential L like H2O, it competes with Y entering group in the rate determiningstep, and X is displaced by either Y or S:

k2PtL3X + Y → PtL3Y + X

competes with:k1

PtL3X + S → PtL3S + X fast

PtL3S + Y → PtL3Y + X this is non-rate determining

Further evidence that both k1 and k2 terms are associative in square planar substitution reactions, is that as bulkiness of Y or S (L) increases, both rate constants decrease

Substitution at square planar complexes is stereo-retentive-entering group takes the coordination site of the leaving group

Further evidence for trigonal bipyramid intermediate

Trigonal bipyramid intermediate

Pt —Cl }2- {Cl —Pt — Cl}- {Cl —Pt — NH3}{Cl

Cl

Cl

→ →3NH

NH3

Cl

→ →3NH

Cl

NH3

{NH3—Pt — NH3}2+ {NH3 —Pt — NH3]+ {NH3 —Pt — NH3}

NH3

NH3

→ − Cl

Cl

NH3

→ − Cl

Cl

Cl

The trans effect: the choice of the leaving group in a square planar complexis determined by the nature of the ligand trans to it, and is kinetic in origin

Preparation of cis- and trans-[PtCl2(NH3)2] by different routes, illustrates theTrans effect

cis

trans

One contributing factor to the trans effect is thetrans - influence (Box 23.9)

L |

X—M—X|L’

L and L’ compete for electron density-use same metalorbitals (dz2 and pz) for M-L and ML’ bonds

The existence of ground-state trans influence (the effecof L on M-L’ bond) is established by solid state structure, vibration (IR) and NMR data for a series of relatedcomplexes

Cl }2- H2C = CH2 } _ PMe3 PEtPh2| ↓ | |

Cl—Pt—Cl Cl—Pt—Cl Cl —Pt—PMe3 Cl— Pt —H| b | a | |

Cl Cl Cl Cl

Pt-Cl (pm) 231.6 a=232.7; b=230.5 a = 237 a= 242

IR and 1H NMR data for trans-[PtXH(PEt3)2]X- CN- I- Br- Cl-

ν(Pt-H)cm-1 2041 2156 2178 2183δ(1H for Pt-H) -7.8 -12.7 -15.6 -16.8

Pt-H bond is weakest for X=CN- trans-influence of X-:E = hν (IR; 1/λ)

CN- > I- > Br- >Cl-δ→ higher H, lower νgβH = hν

H- exertsStrong trans-influence

The trans - influence is not the same as the trans –effect

The former is a ground state phenomenon - structural trans-effectthe latter is a kinetic effect – kinetic trans-effect

The second factor, responsible for the kinetic origin of the trans - effect is π bonding in the 5-coordinate transition state, or intermediate as shown:

L2 and M can only communicate electronically via π-bondingif they all lie in the same plane in the transition state.

This implies that the transition state must betrigonal bipyramidal, rather than square pyramidal

If L2 is a strong π-acceptor, such as CO, CN- or H2C=CH2it will stabilize the transition state by accepting electron density that the

incoming nucleophilic ligand (Y) donates to M and thereby facilitate substitution at the site trans to it

General order of the trans - effect: the general ability of ligands to direct trans – substitution spans a factor of ~106 in rates and is:

H2O ≈ OH- ≈ NH3 ≈ py < Cl- < Br- < I- ≈ [NO2 ]- <

Ph- < Me- < PR3 ≈ H- << CO ≈ [CN]- ≈ C2H4

Trans-effect is useful in synthesis of Pt(II) cis or trans complexes (see 26.19, 26.20)

Ligand Nucleophilicity

Substitution in Pt(II) complexes depends on nucleophilicity of Y

the rate constant k2 (Eq. 26.12)

Rate = -d[PtL3X]/dt = k1[PtL3X] + k2[PtL3X][Y] (26.12)

increases in the order of the nucleopholicity sequence:

H2O < NH3 ~ Cl- < py < Br- < I- < CN- < PR3

Nucleophilicity parameternPt = log k2/k’2 or nPt = logk2 – logk’2

k’2 is the rate constant for:trans-[PtCl2(py)2] + CH3OH ⇒ trans-[PtCl(py)2CH3OH]+ + Cl-

nPt = = for Y = CH3OH

The nucleophilicty parameter, nPt describes the dependence of the rate of substitutionin a square planar Pt(II) complex on the nucleophilicy of the entering group, Y

trans-[PtCl2(py)2] + CH3OH ⇒ trans-[PtCl(py)2CH3OH]+ + Cl- 26.21

A linear relation of log k2 vs nPt for Pt(II) square planar comlexesof Y is observed for the generally, reaction:

PtL3X + Y ⇒ PTL3Y + X

Equation of the straight line is defined by:

log k2 = s(nPt) + logk’2

s = nucleophilicity discrimination factor

logk’2 = rate whenY = CH3OH

Ligand substitution in Octahedral complexes

H2O exchange in [M(H2O)6]n+ complexes

[M(H2O)6n+ + H2(17O) → [M(H2O)5{H2(17O)}]n+ + H2O

with M an s, p or d metal, this reaction can be studied with 17O NMR

First order rate constants for the above exchange show the following trends

1. For s and p metals k increases with increasing ionic radius, r+2. For similar r+ (Li+, Mg2+, Ga3+) increase of charge, slows down reaction3. For M2+ d-metals, no correlation of k with r+, but there is with

dn

4. Limited data for M3+ d ions support behavior as in 3

associative

dissociative

Suggests that bond breaking becomes less (and bond makingmore) important for d3 –to-d5 configuration

associative

For first row d-Mn+ ions (all HS) k for H2O exchange varies greatly:

Cr2+ d4 and Cu2+ d9 are kinetically very labile (k ≥ 108 s-1)

Cr3+ d3 is kinetically inert (k ≈ 10-6 s-1)

V 2+ d2 k ≈ 102 s-1, less labile than later M2+ ions.

The rates of water exchange in HS hexa-aquao complexes follows the sequence:

V2+ < Ni2+ < Co2+ < Fe2+ < Mn2+ < Zn2+ < Cr2+ < Cu2+

and

Cr3+< V3+ < Fe3+ < Ti3+

For a series of ions of similar charge, size, undergoing the same reaction of thesame mechanism, it can be reasonably assumed that the collision frequencies and

∆S# are approx. constant and the variation in rate is attributed to ∆H# ,which is dependent on loss or gain of LFSE.

Model for change of LFSE in going to 5- (dissociative)or 7- (associative) coordinated complex

V2+ < Ni2+ < Co2+ < Fe2+ < Mn2+ < Zn2+ < Cr2+ < Cu2+

Despite the qualitative nature of CFT/LFSE, good agreement between labalityand LFSEs for example for either model, Cr2+ and Cu2+ are most labile due JT

loss of LFSE means increase in ∆H# and slower k

The Eigen - Wilkinson Mechanism of ligand substitutionin an octahedral complex

Water exchange is always more rapid than substitution with other ligands:

ML6 +Y → products

For most ligand substitutions in octahedral complexes, experimental evidencesupports dissociative mechanism; two limiting cases are observed:

•at high conc of Y, rate is independent of [Y], pointing to disociative mechanism•at low conc of Y, the rate depends on [Y] and [ML6], indicativeof associative mechanism

These apparent contradictions are explained by The Eigen - Wilkinson Mechanism:

An encounter complex is formed between the substrate and entering ligandin a pre-equilibrium step, followed by loss of a leaving ligand in a

rate-determining step

[Ni(OH2)6]2+ + NH3 → [Ni(OH2)5 (NH3)]2+ + H2O

1st step diffusion of reactants to form a pre-equilibrium pair

[Ni(OH2)6]2+ + NH3 ↔ {[Ni(OH2)6]2+, NH3} τ ~1 ns

Eigen-Wikinson mechanism

and in general:

KEML6 +Y ⇔ {ML6,Y}

Encounter complex

KE = {ML6,Y}/ [ML6][Y]

KE can be rarely determined experimentally, sometimes can be calculated/estimated theoretically

2nd step is the rate determining step in the Eigen - Wilkinson Mechanism

{[Ni(OH2)6]2+, NH3} → [Ni(OH2)5(NH3)]2+ + H2O

in generalk

{ML6 ,Y} → ML5Y + L

Rate = k[{ML6,Y}]

Cannot just use from

KE = {ML6,Y}/ [ML6][Y]

[{ML6,Y}] = KE [ML6][Y] as [ML6] is less by the amount that is in the encounter pair

Rate = k [{ML6,Y}]

[{ML6,Y}] cannot be measured directly, but KE can be estimated

KE = [{ML6,Y}] / [ML6] [Y]

[M]total is the total concentration of the complex

[M]total = [ML6] + [{ML6,Y}]

[M]total = [ML6] + KE [ML6] [Y]

= [ML6] (1+KE[Y])

[ML6] = [M]total / (1+KE[Y])

thus substituting in the rate expression above[{ML6,Y}] =KE[ML6][Y]

Rate = k KE [M]total [Y] / (1+KE [Y])

Rate = k KE [M]total[Y] / (1+KE[Y])

at low conc. [Y] << 1, K[Y] << 1

Rate = k KE [M]total[Y] = kobs [M]total[Y]

kobs = kKE kobs is measured experimentally, KE can be measued, oris estimated by theory

sok = kobs/KE

Rate = k KE [M]total [Y] / (1+KE [Y])

At high conc. of Y, e. g., when Y is the solvent (e.g., H2O exchange)

KE[Y]) >> 1Rate = k [M]total (first order rate eq.)

kobs = kThus reaction with solvent can be compared with other Y, without

needing to know KE

Further exp. evidence that ligand substitution in octahedral complexes is Dor Id is supported by many examples:

The rate of ligand substitution depends on the nature of the ligand leaving

[Co(NH3)5X]2+ + H2O → [Co(NH3)5H2O]3+ + X-

For above reaction, rate of substitution increases as: OH- < NH3 ≈ NCS- < MeCO2

- < Cl- < Br- < I- < NO3-

Rate

constnts

for

t

Rate constant, k for the substitution reaction:

[Ni(H2O)6]2+ + Y → [Ni(H2O)5Y]2+ + H2O

The fact that k varies so little is consistent with a Id mechanismif the path was associative, k would depend on nature of Y more

The substitution of an uncharged ligand (H2O) by an anionic one (F-)is called anation

OH- < NH3 ≈ NCS- < MeCO2- < Cl- < Br- < I- < NO3

-

This trend correlates with M-X bond strength; the stronger the bond,lower the rate and consistent with dissociative mechanism

with bond breaking the rate-determining step

[Co(NH3)5X]2+ + H2O → [Co(NH3)5H2O]3+ + X-

k is rate const for forward react., K is equilibrium const

∆G# ∝ -log k∆G0 ∝ -log K∆G# = p∆G0 + cP ≈ 1, i.e., slope ≈ 1

this linear relationship between log k and log K representslinear relationship between ∆G# and ∆G0 called

linear free energy relationship (LFER)

slope = 1.0

LFER with a slope < 1 has associative characteras indicated for analogous Rh(III) complexes

For Co(III) rate goes as I- > Br- > Cl- for For Rh(III) rate is reversed I- < Br- < Cl-

Co(III) is hard, forms weaker bond with I- than Cl-Rh(III) is softer, forms weaker bond with Cl- than I-

Leaving group effect -the identity of X has large effect on dissociatively activated reaction – rate depends on M….X : lnk = lnK + c; plot of lnk vs lnK

∆G# = p∆G0 + c linear f(X) withslope, p ~ 1

and shows that for D or IdX has the same effect on the

formation of M…X transition stateas it has on ∆G0 for the

complete elimination of X-

Effect of changing X to X’ on ∆G# & p∆G0

Spectator ligands

effect the rate of substitution in octahedral complexes by thestrength M – L interactions

trongest donor ligands increasing the rate, by stabilizing the transition state

In Co(III) and Cr(III) both cis and trans L effect rate of substitution inproportion to strength of M-L bond, but no important trans effect

e.g., [NiXL5]+ + H2O → [NiL5OH2] + H2O

Much faster with L = NH3 than L = H2O, because NH3 is betterσ donor, large electron density on M enhences M….X break more readily

alsolarger electron density on M stabilizes reduced coordination

of transition state

Steric crowding favors dissociative activation formation of transition state relieves crowding

Effect of Spectator Ligand

Stereochemistry of substitution-square-pyramidal intermediate leadsretention of original geometrytrygonal-bipyramidal intermediatecan lead to isomerization

[CoX(en)2A]+ + H2O ⇒ [Co(H2O)(en)2A]2+ + X-

both cis and trans were studied

Table shows that

cis comlexes remain cis

trans comlexes undergo isomerizationto cis in order:

A = NO2- < Cl- < NCS- < OH-

This can be understood in terms of Id forthe two different 5-coordinate

intermediate possible

Reaction through a square-pyramidal complex leadsto retention of geometry

Reaction through trigonal-bipyramidal complexcan lead to isomerization

cis octahedral complex leads to square-pyrmidal transition statetrans octahedral complex leads to trigonal-bipyramidal transition stateGood π -donor L in equatorial position favor trigonal-bipyramidal

Good π -donor L(A) trans toX, favor isomerization

Electron -Transfer Processes

simplest involve only transfer of electrons and can be monitored by isotopic tracers

[56Fe3+(CN)6]3- + [59Fe2+(CN)6]4- → [56Fe2+(CN)6]4- + [59Fe3+(CN)6]3-

two classes of electron transfer reactions were defined by Taube (Nobel Prize inchemistry 1983)

in an outer sphere mechanism, electron transfer occurs without a covalentlinkage formed between the reactants

in an inner-sphere mechanism, electron transfer occurs via acovalently bonded bridging ligand

Kinetic data can sometimes distinguish between these two cases, but often, it is not possible to distinguish between inner- and outer-sphere

mechanism