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5/22/2011 D is influenced by: D is influenced by: • The identity of the ligands • The Mn+ oxidation state D (M3+) > D (M2+) > D (M+) Example, [Fe(II)L6]2+ Example, D= Fe(II)(NH3)62+ 12,800 cm-1 vs. Fe(III)(NH3)63+ 26,000 cm-1 L= Δ= H2 O 8,900 CN– 30,000 Cl– 5,900 cm-1 • The row in which Mn+ lies in periodic table D (3rd row) > D (2nd row) > D (1st row) Spectrochemical series The spectrochemical series Ligands influence color Ligands I- < Br- <S2- < SCN- < Cl- < NO2- < N3- < F- < OH- < C2O42- < O2- < H2O < NCS- < CH3C=N < py < NH3 < en < bpy < phen < NO2- < PPh3 < CN- < CO [Ni(H2O)6]2+ [Ni(en)(H2O)4]2+ [Ni(en)2(H2O)2]2+ [Ni(en)3]2+ Appears: green green/blue blue purple Absorbs: red red / orange orange yellow Metal ions Mn2+ < Ni2+ < Co2+ < Fe2+ < V2+ < Fe3+ < Co3+ < Mo3+ < Rh3+ < Ru3+ < Pd4+ < Ir3+ < Pt4+ DO increasing d–orbital splitting 1 5/22/2011 Weak vs. strong field ligands Weak vs. strong field ligands If we need to fill the d orbitals with four electrons, where does the fourth electron go? If we need to fill the d orbitals with four electrons, where does the fourth electron go? d Pairing the electron requires energy – “pairing energy” (P) d Weak vs. strong field ligands Weak vs. strong field ligands If we need to fill the d orbitals with four electrons, where does the fourth electron go? If we need to fill the d orbitals with four electrons, where does the fourth electron go? Occupying an eg orbital requires energy – D d d D < P = Weak field Examples: [Cr(OH2)6]2+ D > P = Strong field [Cr(CN)6]4- 2 5/22/2011 Weak vs. strong field ligands If we need to fill the d orbitals with four electrons, where does the fourth electron go? Demo: Nickel Complexes Ni(H2O)62+(aq) + 6 NH3(aq) → Ni(NH3)62+(aq) + 6 H2O(l) (octahedral) (octahedral) Ni(NH3)62+(aq) + 3 en(EtOH) → Ni(en)32+ + 6 NH3(aq) (octahedral) (octahedral) d Examples: “High-spin” “Low-spin” [Cr(OH2)6]2+ [Cr(CN)6]4- Demo: Ammines Cu(H2O)42+(aq) + 4 NH3(aq) → Cu(NH3)42+(aq) + 4 H2O(l) 2− Spectator Ion: SO4 Ni(H2O)6 2+(aq) 2+(aq) + 6 NH3(aq) → Ni(NH3)6 + 6 H2O(l) Spectator Ion: NO3− Co(H2O)62+(aq) + 6 NH3(aq) → Co(NH3)62+(aq) + 6 H2O(l) − Spectator Ion: Cl Ni(en)32+(aq) + 2 Hdmg(EtOH) + 2 H2O(l) → Ni(dmg)2(s) + 3 en(EtOH) + 2 H3O+(aq) (octahedral) (square planar) Note: If any green precipitate forms, it is Ni(OH)2(s). Chapter 19: Transition Metals and Coordination Chemistry 19.1 Survey of transition metals 19.2 1st-row transition metals 19.3 Coordination compounds 19.4 Isomerism 19.5 Bonding in complex ions: The localized electron model 19.6 The crystal field model 19.7 The molecular orbital model 19.8 The biological importance of coordination complexes 3 5/22/2011 Classes of isomers Fig 19.9 1 Coordination Isomers: [Cr(NH3)5SO4]Br and [Cr(NH3)5Br]SO4 SO4 1 2 3 Br 4 Fig 19.10 2 Linkage Isomers: NO2 can bond to the metal through one of the oxygens or through the nitrogen yellow [Co(NH3)5(NO2)]Cl2 red Pentaamminenitrocobalt(III) chloride [Co(NH3)5(ONO)]Cl2 Pentaamminenitritocobalt(III) chloride 4 5/22/2011 Stereoisomers: 3 3 Cis Chloride ligands Cis Trans Geometrical isomers Cis = together Trans = across, opposite Trans green violet Fig 19.11 Fig 19.12 a facial isomer (fac) where the three identical ligands are mutually cis a meridional isomer (mer) where the three ligands are coplanar 5 5/22/2011 4 Optical Isomers Figure 19.15 Mirror image of hand Objects that are not superimposable until you make a mirror image are called chiral. Zumdahl: hands are “nonsuperimposable mirror images” 4 Figure 19.16 Isomers I and II for [Co(en)3]3+ Geometric Isomers not always Optical Isomers 3 [Co(en)2Cl2]+ Trans isomer Achiral Complex 4 Cis isomer Chiral Complex Nonsuperimposable mirror images! Fig 19.17 6 5/22/2011 Chiral Amino Acids C N C C * C O O D-Alanine (unnatural) N C * O C O L-Alanine (natural in proteins) * denotes “chirality center”, where the C noted has 4 different substituents (-CH3, -H, -COOH, -NH2) Achiral Complex Chiral Complex (I and III are enantiomers) BIOINORGANIC CHEMISTRY TMs serve as the active site within many large biological molecules. Key is ability of TM metals to Coordinate with and release ligands Easily undergo oxidation and reduction Human body contains only 0.01% TM by mass, divided among 3d Cr, Mn, Fe, Co, Ni, Cu, Zn and 4d Mo. Nature has used the most abundant TMs: 3d abundance >> 4d/5d. Fe is most abundant 3d element and the most used biologically. Mo is the most abundant 4d/5d element. 7 5/22/2011 BIOINORGANIC CHEMISTRY Functions of these trace metals: Electron Carriers. TM have >1 stable oxidation state. Oxidized form can pick up electrons; reduced form can release electrons elsewhere as pH or other conditions change. Example: Iron-Sulfur Proteins. Ancient; found in all organisms from bacteria to mammals. Tetrahedral FeS4 active site. Catalyze metabolic redox reactions. Cycle between Fe+3 and Fe+2, which are much closer in stability in proteins (Eo = 0.3 V) than in H2O (Eo = +0.8 V); hence inter-conversion requires less energy. BIOINORGANIC CHEMISTRY BIOINORGANIC CHEMISTRY Oxygen Carriers. TM have >1 stable CN. At different O2 partial pressures, can bind or release this metabolically crucial small molecule. Example: Hemoglobin, Myoglobin (Hb, Mb). Recently evolved proteins that carry O2 so efficiently that warmblooded birds and mammals can exist. Blood Hb picks up O2 in lungs, transfers to Mb in cells. Hb (M = 64,500 g/mol) is 4 Mb units stitched together; binds 4 O2. Myoglobin…storage of O2 Hemoglobin…transport of O2 Hemoglobin Molecule Catalysts (Enzymes). Flexibility of both oxidation state and CN allows TM to bond reactants close together, allowing reaction under milder conditions than normal. Critical for organisms, which must carry out all metabolic reactions near STP. Example: Nitrogenase. Mo-Fe enzyme. Reduces N2(air) RNH2(soil) at STP, within bacteria on roots of legumes (industrial process requires 400 oC, 250 atm). Converts dead-organism protein decomposition product (inert N2) into reactive form suitable for making new proteins. Hard since N2 is so stable. Fe, Mo together coordinate, then give electrons to N2, weakening and eventually severing NN bond. Figures 19.33,19.36 Heme • Sickle cell anemia (importance of structure) • High-altitude sickness (how hemoglobin works) • Toxicity of CO and CN- (ligand strength) 8