Mechanism of Inorganic Reactions. 1. Octahedral Substitution. Water exchange
rates correlate with charge/size and cfse. All M aq. +2/+3 except Co(III) and ...
Mechanisms of Inorganic Reactions HS -26 A. Ligand Substitution Reactions Octahedral Co(III), Cr(III) Dissociative mechanism Square Planar Pt(II) Associative mechanism trans effect in Pt(II) complexes B. Electron Transfer Reactions inner sphere (Taube 1952) outer sphere (Marcus theory) electron transfer proteins * review of kinetics, rate laws, transition state theory
Mechanism of Inorganic Reactions 1. Octahedral Substitution Water exchange rates correlate with charge/size and cfse All Maq+2/+3 except Co(III) and Cr(III) are labile, t1/2 < 1 minute Al3+ < Fe3+ < V2+ Ni2+ < Co2+, Fe2+ < Mn2+ < Cr2+Cu2+ dn
0
log k
0.3
5 2
3
8 4
4
7
6
5.5
6
5
4
6.5
8.2
9 8.5
Inert Co3+ ls d6 log k = -6 cfse = 24Dq Cr3+ d3 log k = -5.5 cfse = 12Dq Ir3+ is low spin d6 k = 10-10 s-1 t1/2 water exchange 200 yrs
Trends in lability 1. charge/size ratio 2. CFSE Co3+ not shown as the aqua species is unstable and oxidizes water. Co(NH3)5(H2O)3+ k = 10-6 s-1 d3 and ls d6 complexes inert square d8 Pt(II) inert Co3+
-steric effects L = NMe3 >>> NH3 -leaving group k (s-1) decreases with strength of Co-X I- > Br- > ClAnation L5Co(H2O)3+ + X- → L5CoX2+ + H2O rate = k2 [Co] [ X-]
second order rate law
k2 is relatively independent of the nature of X- except OH-
Eigen Wilkins Mechanism Rapid outer sphere association to form an encounter complex followed by slow interchange to give product. ki KE ML6 + Y {ML6 ,Y} → ML5Y + L rate = ki KE [ML6]total [Y] / ( 1 + KE Y ) = ki KE [ML6] [ML6]total = ML6 + {ML6 ,Y} fraction ML6 = 1/ ( 1 + KE Y ) fraction {ML6 ,Y} = KE Y/ ( 1 + KE Y ) for 1 >> KEY we have only ML6 : rate is second order for KEY >>1 we have only {ML6 ,Y} : rate is independent of Y when ML6 is a cation and Y an anion {ML6 ,Y} is an ion-pair
anation (reverse of aquation) rate = k [Co] [X] Does 2nd order kinetics mean Sn2 or A mechanism ? NO A L5Co(H2O)3+ + X- → 7 Coordinate → L5CoX2+ + H2O rate = k2 [Co][X-]
Nitrate Catalysis. Swaddle. Inorg Chem 13,61 (1974) Rate Coefficients kn+(n-l) (sec-1) for the Aquation of [Cr(NH3)nW6-n]3+ in Nitrate and Perchlorate Media at 40o >0.1 M HN03 1.0 M HCl04 ratio k6-5 1.4E-6 1.4 E-6 1 effect absent if no W k 5-4 2.5 E-5 1.3 E-5 19 4 N are cis 1 trans to W k 4c-3 2.8 E-5 6.1E-7 ~ 46 k 4t-3 2.8 E-4 1.2 E-6 233 all NH3 are cis to W k l-0 2.5 E-6 7.6 E-9 330 N gets more inert stepwise data in HNO3 make no sense because something fishy is going on Nitrate exchanges with water -reduced +charge labilizes NH3 Max effect is with nitrate cis to leaving group- chelation stabilizes intermediate In HClO4 trend is that as the better donor NH3 is replaced by water, the remaining NH3’s are more tightly held
∆V* + for D -for A solvation effects may complicate matters for charged ions
similar for ∆S*
Square Planar Substitution in Pt(II) Two term rate law rate = kS [Pt] + kY [Pt] [Y] Associative mechanism kS is independent of Y - solvent attack path kY depends on nucleophillicity of Y , direct attack
kobs = kS + kY [Y] Intercept = kS slope = kY
Nucleophilicity SCN- > I- > RSH > Br- > N3> NO2soft bases ( polarizable) are the best nucleophiles.
105 X kobs vs. 10-2 [Y] for Pt(py)2Cl2 + Y- in methanol Basolo JACS 87, 241 (1965).
A Bio-Inorganic Example. Myoglobin While the pentacoordinate intermediate Co(NH3)5 cannot be detected in water, hemoproteins nicely illustrate the classic D mechanism, a pentacoordinate Fe(II) heme is seen in the X-ray structure of deoxy Myoglobin. O2 binds to the vacant coordination site on Fe in a bent geometry. CO binds to the same site in a linear geometry.
Kinetic data for O2 and CO binding is obtained by stopped-flow and flash methods. Equilibria can be computed from the rate data or independently by measuring the visible spectrum at various pressures of CO and O2.
Mb + CO MbCO KCO = [MbCO] / [Mb] [CO] rate f = k+CO [Mb] [CO] rate r = k-co [MbCO] k+CO = 5 x 105 M-1s-1 k-CO = 0.017 s-1 K CO = 3 x 107 M-1 K = k+/k-
Mb + O2 MbO2 KO2 = [MbO2 ] / [Mb] [O2] rate f = k+O2 [Mb] [O2] rate r = k-O2 [MbO2] k+O2 = 1.4 x 107 M-1s-1 k -O2 = 11 s-1 KO2 = 1.27 x 106 M-1
Flash Photolysis - Gibson technique for MbCO or FeN4LCO note that k+O2 is 24 X larger than k+CO while KCO is 23 X larger than KO2 pentacoordinate intermediate
FeN4L(CO) FeN4L FeN4L(O2) hv FeN4L rapidly adds O2 (microsec) and then slowly (millisecs) reverts back to the CO complex via a classic D mechanism. (steady state neglecting k-co ) kobs = k-o2 k+CO [CO] / {k+o2 [O2] + k+CO [CO] } rate constants are obtained from plot of 1/kobs vs [O2]/[CO] .
Kinetics of CO Dissociation FeN4(PY)(CO) + PY → FeN4(PY)2 + CO 1.2 1.1 1
