© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L19 1 Nature of the Chemical Bond with applications to catalysis, materials.

© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L19 1 Nature of the Chemical Bond with applications to catalysis, materials science, nanotechnology, surface science, bioinorganic chemistry, and energy Lecture 19 February 16, 2011 Transition metals:Pd and Pt William A. Goddard, III, wag@wag.caltech.eduwag@wag.caltech.edu 316 Beckman Institute, x3093 Charles and Mary Ferkel Professor of Chemistry, Materials Science, and Applied Physics, California Institute of Technology Teaching Assistants: Wei-Guang Liu wgliu@wag.caltech.edu Caitlin Scott

© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L19 4 Ground state of group 10 column Pt: (5d) 9 (6s) 1 3 D ground state Pt: (5d) 10 (6s) 0 1 S excited state at 11.0 kcal/mol Pt: (5d) 8 (6s) 2 3 F excited state at 14.7 kcal/mol Pd: (5d) 10 (6s) 0 1 S ground state Pd: (5d) 9 (6s) 1 3 D excited state at 21.9 kcal/mol Pd: (5d) 8 (6s) 2 3 F excited state at 77.9 kcal/mol Ni: (5d) 8 (6s) 2 3 F ground state Ni: (5d) 9 (6s) 1 3 D excited state at 0.7 kcal/mol Ni: (5d) 10 (6s) 0 1 S excited state at 40.0 kcal/mol

© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L19 5 Salient differences between Ni, Pd, Pt NiPd Pt 4s more stable than 3d 5s much less stable than 4d 6s, 5d similar stability 3d much smaller than 4s (No 3d Pauli orthogonality) Huge e-e repulsion for d 10 4d similar size to 5s (orthog to 3d,4s Differential shielding favors n=4 over n=5, stabilize 4d over 5s  d 10 2 nd row (Pd): 4d much more stable than 5s  Pd d 10 ground state 3 rd row (Pt): 5d and 6s comparable stability  Pt d 9 s 1 ground state Relativistic effects of 1s huge  decreased KE  contraction  stabilize and contract all ns  destabilize and expand nd

© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L19 6 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?

© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L19 17 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

© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L19 18 Examine CC coupling at transition state Can orient the CH 3 to obtain good overlap with Pd sd hybrid OR can orient the CH 3 to obtain get good overlap with the other CH 3 But CANNOT DO BOTH SIMULTANEOUSLY, thus do NOT get resonance stabilization of TS  high barier

© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L19 19 Examine CH coupling at transition state H can overlap both CH 3 and Pd sd hybrid simultaneously but CH 3 cannot thus get ~ ½ resonance stabilization of TS

© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L19 22 Pt goes from s 1 d 9 to d 10 upon reductive elimination thus product stability is DECREASED by 12 kcal/mol Using numbers from QM

© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L19 23 Pd goes from s 1 d 9 to d 10 upon reductive elimination thus product stability is INCREASED by 20 kcal/mol Using numbers from QM Pd and Pt would be ~ same

© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L19 24 Thus reductive elimination from Pd is stabilized by an extra 32 kcal/mol than for Pt due to the ATOMIC nature of the states The dramatic stabilization of the product by 35 kcal/mol reduces the barrier from ~ 41 (Pt) to ~ 10 (Pd) This converts a forbidden reaction to allowed

© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L19 26 Examine bonding to all three rows of transition metals Use MH+ as model because a positive metal is more representative of organometallic and inorganic complexes M0 usually has two electrons in ns orbitals or else one M+ generally has one electron in ns orbitals or else zero M2+ never has electrons in ns orbitals

© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L19 27 Ground states of neutral atoms Sc(4s)2(3d) 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 Sc + (4s)1(3d)1 Ti + (4s)1(3d)2 V+V+ (4s)0(3d)3 Cr + (4s)0(3d)5 Mn + (4s)1(3d)5 Fe + (4s)1(3d)6 Co + (4s)0(3d)7 Ni + (4s)0(3d)8 Cu + (4s)0(3d)10

© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L19 29 Exchange energies: Get 6*5/2=15 exchange terms 5Ksd + 10 Kdd Responsible for Hund’s rule Ksd Kdd Mn+4.819.8 Tc+8.315.3 Re+11.914.1 kcal/mol Form bond to H, must lose half the exchange stabilization for the orbital bonded to the H A [(d 1  )(d 2  )(d 3  )(d 4  )(d 5  )(s  )] Mn+: s 1 d 5 For high spin (S=3) A {(d 1  )(d 2  )(d 3  )(d 4  )(sd b  )[(sd b )H+H(sd b )](  )} sd b is  half the time and  half the time

© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L19 31 Size of atomic orbitals, M + Valence s similar for all three rows, 5s biggest Big decrease from La(an 57) to Hf(an 72 Valence d very small for 3d

© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L19 32 Charge transfer in MH + bonds electropositive electronegative 1 st row all electropositive 2 nd row: Ru,Rh,Pd electronegative 3 rd row: Pt, Au, Hg electronegative

© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L19 63 Physics behind Woodward-Hoffman Rules For a reaction to be allowed, the number of bonds must be conserved. Consider H 2 + D 2 2 bondsTS ? bonds2 bonds Bonding 2 elect nonbonding 1 elect antibonding 0 elect Have 3 electrons, 3 MO’s Have 1 bond. Next consider 4 th atom, can we get 2 bonds? To be allowed must have 2 bonds at TS How assess number of bonds at the TS. What do the dots mean? Consider first the fragment

© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L19 64 Can we have 2s + 2s reactions for transition metals? 2s + 2s forbidden for organics X Cl 2 Ti ?? 2s + 2s forbidden for organometallics? Cl 2 Ti Me

© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L19 65 Physics behind Woodward-Hoffman Rules Bonding 2 elect nonbonding 1 elect antibonding 0 elect Have 1 bond. Question, when add 4 th atom, can we get 2 bonds? Can it bond to the nonbonding orbital? Answer: NO. The two orbitals are orthogonal in the TS, thus the reaction is forbidden

© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L19 66 Now consider a TM case: Cl 2 TiH + + D 2 Orbitals of reactants GVB orbitals of TiH bond for Cl 2 TiH + GVB orbitals of D 2

© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L19 67 Is Cl 2 TiH + + D 2  Cl 2 TiD + + HD allowed? Bonding 2 elect nonbonding 1 elect antibonding 0 elect when add Ti 4 th atom, can we get 2 bonds? Answer: YES. The two orbitals can have high overlap at the TS orthogonal in the TS, thus the reaction is allowed Now the bonding orbital on Ti is d-like. Thus at TS have

© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L19 73 Are all MH reactions with D2 allowed? No Example: ClMn-H + D2 Here the Mn-Cl bond is very polar Mn(4s-4p z ) lobe orbital with Cl:3pz This leaves the Mn: (3d) 5 (4s+4pz), S=3 state to bond to the H But spin pairing to a d orbital would lose 4*K dd /2+K sd /2= (40+2.5) = 42.5 kcal/mol whereas bonding to the (4s+4pz) orbital loses 5*K sd /2 = 12.5 kcal/mol As a result the H bonds to (4s+4pz), leaving a high spin d5. Now the exchange reaction is forbidden

© copyright 2010 William A. Goddard III, all rights reservedCh120a-Goddard-L21 79 Applications of the olefin metathesis reaction Acc. Chem. Res. 2001, 34, 18-29 http://www.pslc.ws/macrog/pdcpd.htm bulletproof resin Small scale synthesis to industrial polymers

Ch120-L20 13/11/02 GODDARD 81 Well-defined metathesis catalysts Schrock 1991 alkoxy imido molybdenum complex a Bazan, G. C.; Oskam, J. H.; Cho, H. N.; Park, L. Y.; Schrock, R. R. J. Am. Chem. Soc. 1991, 113, 6899-6907 Grubbs 1991 ruthenium benzylidene complex b Grubbs 1999 1,3-dimesityl-imidazole-2-ylidenes P(Cy) 3 mixed ligand system” c Scholl, M.; Trnka, T. M.; Morgan, J. P.; Grubbs, R. H. Tetrahedron Lett. 1999, 40, 2247-2250. Wagener, K. B.; Boncella, J. M.; Nel, J. G. Macromolecules 1991, 24, 2649-2657

© copyright 2010 William A. Goddard III, all rights reservedCh120a-Goddard-L21 83 Schrock and Grubbs catalysts make olefin metathesis practical Schrock catalyst – very active, but destroys many functional groups Grubbs catalyst – very stable, high functional group tolerance, but not as reactive as Schrock Catalysts contain many years of evolutionary improvements

© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L19 1 Nature of the Chemical Bond with applications to catalysis, materials.

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© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L19 1 Nature of the Chemical Bond with applications to catalysis, materials.

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