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  • 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 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 n = [MLn]/[M][L]n

    n = 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 t2g3eg2

    [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-TS

    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


    Note five memberedrings-stable

  • RuII*(bpy)3



    -MnO2 Catalyste-



    O2 + 4H+

    Eox = 1.4 V

    SO4- + SO42-


    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


  • 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


    No relationship between thermodynamic stability and lability towards substitution.

    e.g., hydG 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, = 1/k of H2O in first coordination sphere of a metal ion

    [Ir(H2O)6]3+ slow, = 109 = 290 years


    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 dne.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+]


  • 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



    + H2O

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

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


    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 YMLxX 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 YMLxX 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


    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



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