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Lecture 15 February 15, 2013 Transition metals Nature of the Chemical Bond with applications to catalysis, materials science, nanotechnology, surface science, bioinorganic chemistry, and energy Course number: Ch120a Hours: 2-3pm Monday, Wednesday, Friday William A. Goddard, III, [email protected] 316 Beckman Institute, x3093 Charles and Mary Ferkel Professor of Chemistry, Materials Science, and Applied Physics, California Institute of Technology Teaching Assistants: Ross Fu <[email protected]>; Fan Liu <[email protected]> Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved Ch120a1 Transition metals (4s,3d) Sc---Cu (5s,4d) Y-- Ag (6s,5d) (La or Lu), Ce-Au Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 2 Ground states of neutral atoms Sc (4s)2(3d)1 Sc++ (3d)1 Ti V Cr Mn (4s)2(3d)2 (4s)2(3d)3 (4s)1(3d)5 (4s)2(3d)5 Ti ++ V ++ Cr ++ Mn ++ (3d)2 (3d)3 (3d)4 (3d)5 Fe Co Ni (4s)2(3d)6 (4s)2(3d)7 (4s)2(3d)8 Fe ++ Co ++ Ni ++ (3d)6 (3d)7 (3d)8 Cu (4s)1(3d)10 Cu++ (3d)10 Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 3 Hemoglobin Blood has 5 billion erythrocytes/ml Each erythrocyte contains 280 million hemoglobin (Hb) molecules Each Hb has MW=64500 Dalton (diameter ~ 60A) Four subunits (a1, a2, b1, b2) each with one heme subunit Hb Each subunit resembles myoglobin (Mb) which has one heme Ch120a-Goddard-L17 Mb © copyright 2011 William A. Goddard III, all rights reserved 4 The action is at the heme or Fe-Porphyrin molecule Essentially all action occurs at the heme, which is basically an Fe-Porphyrin molecule The rest of the Mb serves mainly to provide a hydrophobic envirornment at the Fe and to protect the heme Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 5 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 s orbital pointing at it. Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 6 Energies of the 5 Fe2+ d orbitals x2-y2 z2=2z2-x2-y2 yz xz xy Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 7 Exchange stabilizations Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 8 Ferrous FeII x2-y2 destabilized by heme N lone pairs z2 destabilized by 5th ligand imidazole or 6th ligand CO y Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved x 9 Four coordinate FeHeme – High spin case, S=2 or q The 5th axial ligand will destabilize q2 since dz2 is doubly occupied A pi acceptor would stabilize q1 wrt q2 Bonding O2 to 5 coordinate will stabilize q3 wrt q1 Future discuss only q1 and denote as q Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 10 Four coordinate Fe-Heme – Intermediate spin, S=1 or t Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 11 Four coordinate Fe-Heme – Low spin case, S=0 or s Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 12 Out of plane motion of Fe – 4 coordinate Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 13 Add axial base N-N Nonbonded interactions push Fe out of plane is antibonding Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 14 Summary 4 coord and 5 coord states Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 15 Free atom to 4 coord to 5 coord Net effect due to five N ligands is to squish the q, t, and s states by a factor of 3 Ch120a-Goddard-L17 This makes all three available as possible ground states depending 16 onIII,the 6threserved ligand © copyright 2011 William A. Goddard all rights Bonding of O2 with O to form ozone O2 has available a ps orbital for a s bond to a ps orbital of the O atom And the 3 electron p system for a p bond to a pp orbital of the O atom Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 17 Bond O2 to Mb Ch120a-Goddard-L17 Simple VB structures get S=1 or triplet state In fact MbO2 is singlet Why? 18 © copyright 2011 William A. Goddard III, all rights reserved change in exchange terms when Bond O2 to Mb O2ps O2pp Assume perfect 10 K 7 Kdd dd VB spin pairing 5*4/2 up spin 4*3/2 Then get 4 cases + Thus average Kdd is down spin 2*1/2 (10+7+7+6)/4 =7.5 Ch120a-Goddard-L17 7 Kdd 6 Kdd 4*3/2 + 2*1/2 3*2/2 + 3*2/2 © copyright 2011 William A. Goddard III, all rights reserved 19 Bonding O2 to Mb Exchange loss on bonding O2 Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 20 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 DH = -33 + 44 = + 11 kcal/mol Instead 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 Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 21 Bond CO to Mb H2O and N2 do not bond strongly enough to promote the Fe to an excited state, thus get S=2 Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 22 compare bonding of CO and O2 to Mb Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 23 GVB orbitals for bonds to Ti Ti ds character, 1 elect H 1s character, 1 elect Covalent 2 electron TiH bond in Cl2TiH2 Think of as bond from Tidz2 to H1s Csp3 character 1 elect H 1s character, 1 elect Covalent 2 electron CH bond in CH4 Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 24 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{[(Tids)(H1s)+ (H1s)(Tids)](ab-ba)} Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 25 But TM-H bond can also be s-like Cl2TiH+ Ti (4s)2(3d)2 The 2 Cl pull off 2 e from Ti, leaving a d1 configuration Ti-H bond character 1.07 Tid+0.22Tisp+0.71H ClMnH Mn (4s)2(3d)5 The Cl pulls off 1 e from Mn, leaving a d5s1 configuration H bonds to 4s because of exchange stabilization of d5 Mn-H bond character 0.07Ch120a-Goddard-L17 Mnd+0.71Mnsp+1.20H © copyright 2011 William A. Goddard III, all rights reserved 26 Bond angle at a transition metal For two p orbitals expect 90°, HH nonbond repulsion increases it What angle do two d orbitals want H-Ti-H plane 76° Ch120a-Goddard-L17 Metallacycle plane © copyright 2011 William A. Goddard III, all rights reserved 27 Best bond angle for 2 pure Metal bonds using d orbitals Assume that the first bond has pure dz2 or ds character to a ligand along the z axis Can we make a 2nd bond, also of pure ds character (rotationally symmetric about the z axis) to a ligand along some other axis, call it z. 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). Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 28 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 ds with dd because the electrons are farthest apart This favors = 90°, but the bond to the dd orbital is not as good Thus expect something between 53.7 and 90° Seems that ~76° is often best Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 29 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 (3d)2 (4s)(3d)5 Now make bond to less electronegative ligands, H or CH3 Use 4s if available, otherwise use d orbitals Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 30 But TM-H bond can also be s-like Cl2TiH+ Ti (4s)2(3d)2 The 2 Cl pull off 2 e from Ti, leaving a d1 configuration Ti-H bond character 1.07 Tid+0.22Tisp+0.71H ClMnH Mn (4s)2(3d)5 The Cl pulls off 1 e from Mn, leaving a d5s1 configuration H bonds to 4s because of exchange stabilization of d5 Mn-H bond character 0.07Ch120a-Goddard-L17 Mnd+0.71Mnsp+1.20H © copyright 2011 William A. Goddard III, all rights reserved 31 Example (Cl)2VH3 + resonance configuration Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 32 Example ClMometallacycle butadiene Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 33 Example [Mn≡CH]2+ Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 34 Summary: start with Mn+ s1d5 dy2 s bond to H1s dx2-x2 non bonding dyz p bond to CH dxz p bond to CH dxy non bonding 4sp hybrid s bond to CH Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 35 Summary: start with Mn+ s1d5 dy2 s bond to H1s dx2-x2 non bonding dyz p bond to CH dxz p bond to CH dxy non bonding 4sp hybrid s bond to CH Ch120a-Goddard-L17 © copyright 2011 William A. Goddard III, all rights reserved 36