lecture 18 february 14, 2011 transition metals:pd and pt
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Lecture 18 February 14, 2011 Transition metals:Pd and Pt. Nature of the Chemical Bond with applications to catalysis, materials science, nanotechnology, surface science, bioinorganic chemistry, and energy. William A. Goddard, III, [email protected] 316 Beckman Institute, x3093 - PowerPoint PPT PresentationTRANSCRIPT
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Nature of the Chemical Bond with applications to catalysis, materials
science, nanotechnology, surface science, bioinorganic chemistry, and energy
Lecture 18 February 14, 2011
Transition metals:Pd and Pt
William A. Goddard, III, [email protected] Beckman Institute, x3093
Charles and Mary Ferkel Professor of Chemistry, Materials Science, and Applied Physics,
California Institute of Technology
Teaching Assistants: Wei-Guang Liu <[email protected]>Caitlin Scott <[email protected]>
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Last time
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Transition metal atoms
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Transition metals
Aufbau
(4s,3d) Sc---Cu
(5s,4d) Y-- Ag
(6s,5d) (La or Lu), Ce-Au
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Transition metals
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Ground states of neutral atoms
Sc (4s)2(3d)1
Ti (4s)2(3d)2
V (4s)2(3d)3
Cr (4s)1(3d)5
Mn (4s)2(3d)5
Fe (4s)2(3d)6
Co (4s)2(3d)7
Ni (4s)2(3d)8
Cu (4s)1(3d)10
Sc++ (3d)1
Ti ++ (3d)2
V ++ (3d)3
Cr ++ (3d)4
Mn ++ (3d)5
Fe ++ (3d)6
Co ++ (3d)7
Ni ++ (3d)8
Cu++ (3d)10
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The heme group
The net charge of the Fe-heme is zero. The VB structure shown is one of several, all of which lead to two neutral N and two negative N.
Thus we consider that the Fe is Fe2+ with a d6 configuration
Each N has a doubly occupied sp2 orbital pointing at it.
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Energies of the 5 Fe2+ d orbitals
x2-y2
z2=2z2-x2-y2
xy
xz
yz
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Exchange stabilizations
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Ferrous FeII
x
y
z2 destabilized by 5th ligand imidazole
or 6th ligand CO
x2-y2 destabilized by heme N lone pairs
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Summary 4 coord and 5 coord states
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Out of plane motion of Fe – 4 coordinate
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Add axial base
N-N Nonbonded interactions push Fe out of plane
is antibonding
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Net effect due to five N ligands is to squish the q, t, and s states by
a factor of 3
This makes all three available as possible ground states depending
on the 6th ligand
Free atom to 4 coord to 5 coord
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Bonding of O2 with O to form ozone
O2 has available a p orbital for a bond to a p orbital of the O atom
And the 3 electron system for a bond to a p orbital of the O atom
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Bond O2 to Mb
Simple VB structures get S=1 or triplet state
In fact MbO2 is singletWhy?
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change in exchange terms when Bond O2 to Mb
O2p
O2p
10 Kdd
5*4/2
7 Kdd
4*3/2 +
2*1/2
6 Kdd
3*2/2 +
3*2/2
7 Kdd
4*3/2 +
2*1/2
Assume perfect VB spin pairing
Then get 4 cases
up spin
down spinThus average Kdd is (10+7+7+6)/4
=7.5
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Bonding O2 to Mb
Exchange loss on
bonding O2
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Modified exchange energy for q state
But expected t binding to be 2*22 = 44 kcal/mol stronger than q
What happened?
Binding to q would have H = -33 + 44 = + 11 kcal/molInstead the q state retains the high spin pairing so that there is no exchange loss, but now the coupling of Fe to O2 does not gain the full VB strength, leading to bond of only 8kcal/mol instead of 33
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Bond CO to Mb
H2O and N2 do not bond strongly enough to promote the Fe to an excited state, thus get S=2
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compare bonding of CO and O2 to Mb
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GVB orbitals for bonds to Ti
Covalent 2 electron TiH bond in Cl2TiH2
Covalent 2 electron CH bond in CH4
Ti d character, 1 elect H 1s character, 1 elect
Csp3 character 1 elect H 1s character, 1 elect
Think of as bond from Tidz2 to H1s
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Bonding at a transition metaal
Bonding to a transition metals can be quite covalent.
Examples: (Cl2)Ti(H2), (Cl2)Ti(C3H6), Cl2Ti=CH2
Here the two bonds to Cl remove ~ 1 to 2 electrons from the Ti, making is very unwilling to transfer more charge, certainly not to C or H (it would be the same for a Cp (cyclopentadienyl ligand)
Thus TiCl2 group has ~ same electronegativity as H or CH3
The covalent bond can be thought of as Ti(dz2-4s) hybrid spin paired with H1s
A{[(Tid)(H1s)+ (H1s)(Tid)]()}
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But TM-H bond can also be s-like
Cl2TiH+
ClMnH
Ti (4s)2(3d)2
The 2 Cl pull off 2 e from Ti, leaving a d1 configuration
Mn (4s)2(3d)5
The Cl pulls off 1 e from Mn, leaving a d5s1 configurationH bonds to 4s because of exchange stabilization of d5
Ti-H bond character1.07 Tid+0.22Tisp+0.71H
Mn-H bond character0.07 Mnd+0.71Mnsp+1.20H
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Bond angle at a transition metal
For two p orbitals expect 90°, HH nonbond repulsion increases it
H-Ti-H plane
76°
Metallacycle plane
What angle do two d orbitals want
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Best bond angle for 2 pure Metal bonds using d orbitals
Assume that the first bond has pure dz2 or d character to a ligand along the z axis
Can we make a 2nd bond, also of pure d character (rotationally symmetric about the axis) to a ligand along some other axis, call it
For pure p systems, this leads to = 90°
For pure d systems, this leads to = 54.7° (or 125.3°), this is ½ the tetrahedral angle of 109.7 (also the magic spinning angle for solid state NMR).
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Best bond angle for 2 pure Metal bonds using d orbitals
Problem: two electrons in atomic d orbitals with same spin lead to 5*4/2 = 10 states, which partition into a 3F state (7) and a 3P state (3), with 3F lower. This is because the electron repulsion between say a dxy and dx2-y2 is higher than between sasy dz2 and dxy
.
Best is d with d because the electrons are farthest apart
This favors = 90°, but the bond to the d orbital is not as good
Thus expect something between 53.7 and 90°
Seems that ~76° is often best
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How predict character of Transition metal bonds?
Start with ground state atomic configuration
Ti (4s)2(3d)2 or Mn (4s)2(3d)5
Consider that bonds to electronegative ligands (eg Cl or Cp) take electrons from 4s
easiest to ionize, also better overlap with Cl or Cp, also leads to less reduction in dd exchange
(4s)(3d)5(3d)2
Now make bond to less electronegative ligands, H or CH3
Use 4s if available, otherwise use d orbitals
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But TM-H bond can also be s-like
Cl2TiH+
ClMnH
Ti (4s)2(3d)2
The 2 Cl pull off 2 e from Ti, leaving a d1 configuration
Mn (4s)2(3d)5
The Cl pulls off 1 e from Mn, leaving a d5s1 configurationH bonds to 4s because of exchange stabilization of d5
Ti-H bond character1.07 Tid+0.22Tisp+0.71H
Mn-H bond character0.07 Mnd+0.71Mnsp+1.20H
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Example (Cl)2VH3
+ resonance configuration
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Example ClMo-metallacycle butadiene
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Example [Mn≡CH]2+
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Summary: start with Mn+ s1d5
dy2 bond to H1sdx2-x2 non bondingdyz bond to CHdxz bond to CHdxy non bonding4sp hybrid bond to CH
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Summary: start with Mn+ s1d5
dy2 bond to H1sdx2-x2 non bondingdyz bond to CHdxz bond to CHdxy non bonding4sp hybrid bond to CH
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new
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Compare chemistry of column 10
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Ground state of group 10 column
Pt: (5d)9(6s)1 3D ground statePt: (5d)10(6s)0 1S excited state at 11.0 kcal/molPt: (5d)8(6s)2 3F excited state at 14.7 kcal/mol
Pd: (5d)10(6s)0 1S ground statePd: (5d)9(6s)1 3D excited state at 21.9 kcal/molPd: (5d)8(6s)2 3F excited state at 77.9 kcal/mol
Ni: (5d)8(6s)2 3F ground stateNi: (5d)9(6s)1 3D excited state at 0.7 kcal/molNi: (5d)10(6s)0 1S excited state at 40.0 kcal/mol
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Salient differences between Ni, Pd, Pt
2nd row (Pd): 4d much more stable than 5s Pd d10 ground state
3rd row (Pt): 5d and 6s comparable stability Pt d9s1 ground state
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Ground state configurations for column 10
Ni Pd Pt
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Mysteries from experiments on oxidative addition and reductive elimination of CH and CC bonds on Pd and Pt
Why are Pd and Pt so different
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Mysteries from experiments on oxidative addition and reductive elimination of CH and CC bonds on Pd and Pt
Why is CC coupling so much harder than CH coupling?
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Step 1: examine GVB orbitals for (PH3)2Pt(CH3)
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Analysis of GVB wavefunction
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Alternative models for Pt centers
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energetics
Not agree with experiment
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Possible explanation: kinetics
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Consider reductive elimination of HH, CH and CC from Pd
Conclusion: HH no barrier
CH modest barrierCC large barrier
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Consider oxidative addition of HH, CH, and CC to Pt
Conclusion: HH no barrier
CH modest barrierCC large barrier
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Summary of barriers
But why?
This explains why CC coupling not occur for Pt while CH and HHcoupling is fast
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How estimate the size of barriers (without calculations)
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Examine HH coupling at transition state
Can simultaneously get good overlap of H with Pd sd hybrid and with the other H
Thus get resonance stabilization of TS low barrier
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Examine CC coupling at transition state
Can orient the CH3 to obtain good overlap with Pd sd hybrid OR can orient the CH3 to obtain get good overlap with the other CH3
But CANNOT DO BOTH SIMULTANEOUSLY, thus do NOT get resonance stabilization of TS high barier
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Examine CH coupling at transition state
H can overlap both CH3 and Pd
sd hybrid simultaneously but CH3 cannot
thus get ~ ½ resonance
stabilization of TS
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Now we understand Pt chemistry
But what about Pd?
Why are Pt and Pd so dramatically different
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stop