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 M +2/+3 aq except Co(III) and Cr(III) are labile, t 1/2 < 1 minute Al 3+ < Fe 3+ < V 2+ Ni 2+ < Co 2+, Fe 2+ < Mn 2+ < Cr 2+ Cu 2+ d n 0 5 3 8 7 6 5 4 9 log k 0.3 2 4 4 5.5 6 6.5 8.2 8.5 Inert Co 3+ ls d 6 log k = -6 cfse = 24Dq Cr 3+ d 3 log k = -5.5 cfse = 12Dq Ir 3+ is low spin d 6 k = 10-10 s -1 t 1/2 water exchange 200 yrs
Trends in lability 1. charge/size ratio 2. CFSE Co 3+ not shown as the aqua species is unstable and oxidizes water. Co(NH 3 ) 5 (H 2 O) 3+ k = 10-6 s -1 d 3 and ls d 6 complexes inert square d 8 Pt(II) inert Co 3+ Richens, Chem Rev. 2005
OCTAHEDRAL SUBSTITUTION Dissociative mechanism predominant (bond breaking) acid hydrolysis L 5 CoX 2+ L 5 Co L 5 Co(H 2 O) 3+ Co(NH 3 ) 5 X 2+ + H 2 O Co(NH 3 ) 5 (H 2 O) 3+ + X - rate = k -x [Co] first order rate law -steric effects L = NMe 3 >>> NH 3 -leaving group k (s -1 ) decreases with strength of Co-X I - > Br - > Cl - Anation L 5 Co(H 2 O) 3+ + X - L 5 CoX 2+ + H 2 O rate = k 2 [Co] [ X - ] second order rate law k 2 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. K E ML 6 + Y {ML 6,Y} ML 5 Y + L k i rate = k i K E [ML 6 ] total [Y] / ( 1 + K E Y ) = k i K E [ML 6 ] [ML 6 ] total = ML 6 + {ML 6,Y} fraction ML 6 = 1/ ( 1 + K E Y ) fraction {ML 6,Y} = K E Y/ ( 1 + K E Y ) for 1 >> K E Y we have only ML 6 : rate is second order for K E Y >>1 we have only {ML 6,Y} : rate is independent of Y when ML 6 is a cation and Y an anion {ML 6,Y} is an ion-pair
anation (reverse of aquation) rate = k [Co] [X] Does 2 nd order kinetics mean Sn 2 or A mechanism? NO A L 5 Co(H 2 O) 3+ + X - 7 Coordinate L 5 CoX 2+ + H 2 O rate = k 2 [Co][X - ] X attacks giving 7 CN intermediate D L 5 Co(H 2 O) 3+ L 5 Co L 5 CoX 2+ W and X compete for I rate = k -w k +x [Co][X] / { k +w [W] + k +x [X] } Ion Pair L 5 Co(H 2 O) 3+ + X - IP L 5 CoX 2+ + H 2 O rate = K IP [Co][X] / { 1 + K IP [X]} fast preeq followed by interchange step All 3 mechanisms could give second order kinetics. The D mechanism explains why k is only slightly dependent on nature of entering nucleophile.
ph Effects ( involve conjugate acid and conjugate base) A. Base Hydrolysis SN 1 CB Co(NH 3 ) 5 Cl 2+ Co(NH 3 ) 4 (NH 2 )Cl + + H + Co(NH 3 ) 4 (NH 2 )Cl + Co(NH 3 ) 4 (NH 2 ) 2+ + Cl - Co(NH 3 ) 4 (NH 2 ) 2+ + H 2 O Co(NH 3 ) 5 (OH) 2+ B. Acid Catalysis H + + Co(NH 3 ) 5 F 2+ Co(NH 3 ) 5 (HF) 3+ Co(NH 3 ) 5 (HF) 3+ Co(NH 3 ) 5 3+ + HF Co(NH 3 ) 5 3+ + H 2 O Co(NH 3 ) 5 (H 2 O) 3+ Example from PS #5 Basolo,
Chelation Effects BP page 160, 162 relative rate of hydrolysis [Co(NH 3 ) 5 Cl 2+ 100 [Co(en) 2 (NH 3 )Cl] 2+ 21 [Co(tetraen)(NH 3 )Cl] 2+ 10 [Co(dien)(en)Cl] 2+ 7.8 [Co(tetraethylenepentaamine)Cl] 2+ 3.8 trans-[co(aa) 2 Cl 2 ] + + H 2 O trans-[co(aa) 2 Cl(H 2 O)] +2 AA kobs s -1 NH 2 CH 2 CH 2 NH 2 3.2 e -5 NH 2 CH(Me)CH 2 NH 2 6.2 e-5 NH 2 CH(Me)CH(Me)NH 2 42 e -5 NH 2 C(Me) 2 C(Me) 2 NH 2 over on mixing
Nitrate Catalysis. Swaddle. Inorg Chem 13,61 (1974) Rate Coefficients kn+(n-l) (sec-1) for the Aquation of [Cr(NH 3 )nw 6-n ] 3+ Nitrate and Perchlorate Media at 40 o >0.1 M HN0 3 1.0 M HCl0 4 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 NH 3 are cis to W k l-0 2.5 E-6 7.6 E-9 330 N gets more inert stepwise data in HNO 3 make no sense because something fishy is going on Nitrate exchanges with water -reduced +charge labilizes NH 3 Max effect is with nitrate cis to leaving group- chelation stabilizes intermediate In HClO 4 trend is that as the better donor NH 3 is replaced by water, the remaining NH 3 s are more tightly held in
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 = k S [Pt] + k Y [Pt] [Y] Associative mechanism k S is independent of Y - solvent attack path k Y depends on nucleophillicity of Y, direct attack
kobs = k S + k Y [Y] Intercept = k S slope = k Y Nucleophilicity SCN - > I - > RSH > Br - > N 3 - > NO 2 - soft bases ( polarizable) are the best nucleophiles. 10 5 X kobs vs. 10-2 [Y] for Pt(py) 2 Cl 2 + Y - in methanol Basolo JACS 87, 241 (1965).
A Bio-Inorganic Example. Myoglobin While the pentacoordinate intermediate Co(NH 3 ) 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. O 2 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 O 2 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 O 2. Mb + CO MbCO Mb + O 2 MbO 2 K CO = [MbCO] / [Mb] [CO] K O2 = [MbO 2 ] / [Mb] [O 2 ] rate f = k +CO [Mb] [CO] rate f = k +O2 [Mb] [O 2 ] rate r = k -co [MbCO] rate r = k -O2 [MbO 2 ] k +CO = 5 x 10 5 M -1 s -1 k +O2 = 1.4 x 10 7 M -1 s -1 k -CO = 0.017 s -1 k -O2 = 11 s -1 K CO = 3 x 10 7 M -1 K O2 = 1.27 x 10 6 M -1 K = k + /k -
Flash Photolysis - Gibson technique for MbCO or FeN 4 LCO note that k +O2 is 24 X larger than k +CO while K CO is 23 X larger than K O2 pentacoordinate intermediate FeN 4 L(CO) FeN 4 L FeN 4 L(O 2 ) hv FeN 4 L rapidly adds O 2 (microsec) and then slowly (millisecs) reverts back to the CO complex via a classic D mechanism. (steady state neglecting k -co ) k obs = k -o2 k +CO [CO] / {k +o2 [O2] + k +CO [CO] } rate constants are obtained from plot of 1/kobs vs [O 2 ]/[CO].
1.2 1.1 Kinetics of CO Dissociation FeN 4 (PY)(CO) + PY FeN 4 (PY) 2 + CO Absorbance 1 0.9 0.8 0.7 0.6 0.5 0.4 1 k -co k +py [py] k obs = k +py [py] k +co [CO] k-co = 4.22 x 10-4 sec -1 0.3 0.2 23 0.1 0 350 400 450 500 550 600 650 700 750 800 Wavelength (nm)