5.111 Lecture 27 Notes

27.1 5.111 Lecture 27 Transition Metals Topics: Formation of coordination complexes, coordination number, coordination complex notation, structures of coordination complexes, chelate effect, isomers, delectron counting, and d-orbitals. Chapter 16 p. 669-681 (p. 620-631 in 3rd ed). From Wednesday’s handout Now the answer to the biochemical question How is vitamin B12 reduced in the body? Vitamin B12 is reduced by a protein called flavodoxin. E° for vitamin B12 is -0.526 V E° for flavodoxin is -0.230 V Is the reduction of vitamin B12 by flavodoxin spontaneous? ∆E°(cell) = E°(reduction) - E° (oxidation) = E° (vitamin B12) - E° (flavodoxin) = -0.526 V -(-0.230 V) = -0.296 V ∆G° = -nℑ∆E° = -(1)(96485 Cmol-1)(-0.296 V) = +28.6 kJ/mol Vitamin B12 is a better reducing agent than flavodoxin. Vitamin B12 should reduce flavodoxin not the other way around. So why don't we all have heart disease and megaloblastic anemia? Answer: S-adenosylmethionine provides the energy to drive the reaction. The ∆G° for the cleavage of S-adenosylmethionine is -37.6 kJ/mol Today’s material 21 22 23 24 25 26 27 28 29 30 Sc Ti V Cr Mn Fe Co Ni Cu Zn 39 Y 40 41 42 43 44 45 46 47 48 Zr Nb Mo Tc Ru Rh Pd Ag Cd 74 W 78 79 80 Pt Au Hg Developed from Lippard & Berg 1994 d-block metals naturally occurring in biology – V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Cd, W. d-block metals used as probes of biological systems and/or drugs include Cr, Co, Y, Tc, Ag, Cd, Pt, Au, Hg. 27.2 Roles of metals in biology include global cycling of nitrogen, carbon, hydrogen radical reactions biosynthesis of vitamins biosynthesis of deoxynucleotides etc Formation of coordination complexes A key feature of transition metals is their ability to form complexes with small molecules and ions. Positive metals ions can attract electron density, usually a lone pair of electrons from another atom or molecule to form a coordination complex. Donor atoms are called ligands (Lewis _________ –typically _______ one lone pair of electrons) Examples of ligands: NO2- OCO2-2 CN- SCN NCS 2e- 2e- 2e- 2e OH- OH2 NH3 CO NO+ 2e- 2e- 2e- 2e- 2e 2e- Acceptor atoms are transition metals (Lewis acid –accept lone pair electrons) Examples of transition metals: Ti, Cr, Mn, Fe, Co, Ni, Zn, Ir, Pt, etc Coordination complexes ≡ metals surrounded by ligands. Example: 3+ NH3 NH3 H3N Co H3N NH3 NH3 Coordination number (CN) is the number of ligands bonded to the metal ion. Here CN = 6. Six ligands comprise the primary coordination sphere. Typical CN’s range from 2-12. Six is the most common. 27.3 3+ NH3 Coordination Complex Notation NH3 H3N [Co(NH3)6]+3 3Cl-1 ↓ [Co(NH3)6]Cl3 Co H3N NH3 NH3 NH3 within bracket is bound to Co, Cl outside bracket is a counter ion. Structures of coordination complexes (M = Metal, L = Ligand) L L L L M L L M L L CN=6 __________ L L L L M L L L L L L M M L L L CN=5 trigonal bipyramidal square pyramidal L L M L L CN=4 square planar tetrahedral L L M L L CN=3 trigonal planar CN=2 linear Chelate Effect in Coordination Complexes Ligands that bind a metal at one site are called unidentate or monodentate (one tooth). Ligands that have two or more points of attachment to the metal are called chelating ligands and the coordination complexes are called chelates (greek for claws). Bidentate – two points of attachment Tridentate – Tetradentate - ex. corrin ring of B12 Hexadentate – ex. EDTA Metal chelates are unusual stable. This is partly due to the favorable entropic factor accompanying release of non-chelating ligands (usually H2O) from the coordination sphere. Examples 27.4 1. Vitamin B12. Cobalt is coordinated by a planar tetradentate ligand (corrin ring system). It is also coordinated by an upper axial ligand (5’deoxyadenosine) and a lower axial ligand (dimethylbenzimidazole) 2. Ethylenediamine tetraacetic acid (EDTA). O O HO O CH2 O O O H2C H2C N HO CH2 CH2 CH2 OH H2C N CH2 N CH2 M O OH N CH2 CH2 O O CH2 O O Free EDTA O EDTA in complex with metal (M) Binding of EDTA is entropically favorable. Six molecules of H2O are released for every 1 molecule of EDTA bound. Uses 27.5 Geometric Isomers Geometric isomers can have vastly different properties. [Pt(NH3)2Cl2] has two geometric isomers H 3N Cl Pt H 3N Cl potent anti-cancer drug H 3N Cl Cl Pt NH3 no known function transplatin cisplatin Optical isomers (enantiomers) are non-superimposable mirror images of each other. A complex that is not identical to its mirror image is also called a chiral complex. Chiral molecules have different properties in chiral environments (such as a human body). NH3 H 3N NH3 Cl Cl Co OH2 Co OH2 OH2 Cl NH3 OH2 Cl mirror plane d-Electron Counting in Coordination Complexes d-electron count of metal = group number (periodic table) -oxidation number of metal 1. find oxidation number: For Co in [Co(NH3)6]3+ NH3 is neutral, so Co must be +3 2. d-count is 9-3 = 6 Practice with d-counts d6 27.6 Complexes [Ni(CO)4] Oxidation number of metal d-count [Co(H2O)2(NH3)Cl3] d Orbitals There are five d orbitals: dxy, dxz, dyz, dx -y , dz . 2 2 2 You need to be able to draw their shapes. Z Z Y Y X X dz has maximum amplitude along z and doughnut in xy plane dx -y has maximum amplitude along x and y axes. 2 2 2 Z Z Z Y X Y Y X X dyz has maximum amplitude 45° to y and z axes dxz has maximum amplitude 45° to x and z axes dxy has maximum amplitude 45° to x and y axes