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Bonding in Transition Metals and Coordination Complexes

Bonding in Transition Metals and Coordination Complexes. Bonding in Transition Metals and Coordination Complexes. Chemistry of the Transition metals. Properties. Atomic Radius : lanthanide contraction – unusual contraction of lanthanide ions.

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Bonding in Transition Metals and Coordination Complexes

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  1. Bonding in Transition Metals and Coordination Complexes

  2. Bonding in Transition Metals and Coordination Complexes Chemistry of the Transition metals Properties Atomic Radius : lanthanide contraction – unusual contraction of lanthanide ions. Binding energy: higher – more unpaired electrons i.e.) m.p. -- higher in the middle of the row : W ( 3410oC), Hg (-39oC) Oxidation states: higher oxidation state– more covalent bond character lower oxidation state – more ionic bond character Mn(OH)2, Mn(OH)3, H2MnO3, H2MnO4, HMnO4 acidic basic

  3. Chemistry of the Transition metals Coordination complex ; coordination chemistry 배위화학 CuSO4 : greenish white v.s. CuSO4.4H2O : blue Coordination complex : Cu(H2O)42+ coordination Ligand Lewis base Lewis acid Coordination number : total number of metal-to-ligand bond Usually 2 ~ 6

  4. Ligands H- Br- Hydrido Bromo ONO- Cl- Nitrito Chloro SCN- N3- Thiocyanato Azido NCS- CN- Isothiocyanato Cyano NO+ OH- Nitrosyl Hydroxo CO32- NH3 Ammine Carbonato H2O Aqua(o) Oxalate bidentate ligand CO Carbonyl Ethylenediamine (En) NO2- Nitro chelates O2- Oxo

  5. Making coordination complex charge of a complex = sum of charges of metals and ligands charge of a complex + charges of counter ions = 0 coordination number = numbers of donor atoms Nomenclature NH4[Cr(NH3)2(NCS)4] Reinecke’s salt [Co(NH3)5Cl]Cl2 Purpureocobaltic chloride Systematic naming [Co(NH3)5Cl]Cl2 Pentaamminechlorocobalt(III) chloride K4[Fe(CN)6] Potassium Hexacyanoferrate(II)

  6. Rules of Nomenclature Pentaamminechlorocobalt(III) chloride Potassium Hexacyanoferrate(II) 1. Cation Anion b 2. In the complex : names of ligands come first and then name of metal among ligands : alphabetical order 3. Names of ligands : anion – change the last letter to o neutral – same as the original ones 4. Counting number of ligands : di, tri, tetra, penta, hexa, hepta….. if the ligand contains these names in it, use : bis, tris, tetrakis, pentakis…… 5. If the compex is an anion : at the end of the name put ate 6. Oxidation number of metal : in parenthesis with roman letter - (IV)

  7. Influence of Coordination 1. Color Pale yellow orange 2. Reduction potential 3. Chemical reactivity

  8. Structure of coordination complexes Chemical formular(Werner) structure Chemical formular(19thC.) color CoCl3.6NH3 orange-yellow [Co(NH3)6]3+Cl-3 CoCl3.5NH3 pruple [Co(NH3)5Cl]2+Cl-2 octahedral CoCl3.4NH3 green [Co(NH3)4Cl2]+Cl- CoCl3.3NH3 green [Co(NH3)3Cl3]

  9. +2 Cl [Co(NH3)5Cl]2+Cl-2 [Co(NH3)6]3+Cl-3 [Co(NH3)4Cl2]+Cl- [Co(NH3)3Cl3]

  10. [Co(En)2Cl2]+Cl- Geometrical isomers cis trans

  11. Chiral structures [Co(NH3)2(H2O)2Cl2]+ [Pt(En)3]4+

  12. Structure of coordination complexes coordination number Atomic orbital of metal structure d10 [Ag(NH3)2]+ Linear 2 [Zn(NH3)4]2+ Tetrahedral 4 d9 [Pt(NH3)4]2+ Square Planar 4 d8 d6 [Co(NH3)6]3+ Octahedral 6

  13. Super chelating ligand EDTA ( ethylenediaminetetraacetate) Strong affinity to certain metal ions Solubilize metal ions Entropy factor : bigger DS

  14. Transition Metals Coordination complex Structural variety Partially filled d orbitals Low-lying unoccupied orbitals color Unpaired electrons Magnetic property Many oxidation states Catalysts, new reactions Ligands Donates electron pairs coordination Changes color, reactivity, reduction potential Octet rule in transition metal chemistry : 18 electron rule number of electrons in 4s + 3d + 2 x number of ligands = 18

  15. 18-electron rule for transition metal complexes Octet rule : Lewis structure consider a transition metal : Cr Chromium: [Ar] (4s)2(3d)4 6 valence electrons 18-electron rule Chromium need 18 electrons in its most outer shell. Therefore the complex of Cr with CO will look like i.e. CO provides 2 x 6 = 12 electrons Cr provides 6 electrons Total 18 electrons

  16. Using the 18-electron rule Given that H2Fe(CO)x exists, what does x equal? Iron: [Ar] (4s)2(3d)6 8 valence electrons 8 hydrogen: 1s1 1 valence electrons x 2 = 2 CO: 2 valence electrons x n = 2n Total : 10 + 2n = 18 electrons n = 4 H2Fe(CO)4

  17. Understanding of metal-ligand binding mode facts • Color : only for partially filled d orbitals • i.e. d0, d10 : colorless [CrF6]3- [Cr(H2O)6]3+ [Cr(NH3)6]3+ [Cr(CN)6]3- green violet yellow yellow 2. magnetism: paramagetic v.s. diamagnetic unpaired electrons [Co(NH3)6]3+ diamagnetic – no unpaired electrons [CoF6]3- paramagnetic – 4 unpaired electrons 3. tetrahedral or square planar [Ni(CN)4]2- [NiCl4]2- tetrahedral square planar

  18. Crystal Field Theory How to explain Color, magnetic properties, and choice of tetrahedral, square planar & octahedral Crystal field theory : ionic description of the metal-ligand bonds Consider only the energy changes of d orbitals of metal during coordination Consider only electrostatic interaction with ligands : charge-charge, charge-dipole Begin with octahedral geometry

  19. Low spin complex : when Do is large High spin complex : when Do is small

  20. magnetism d1 ~ d5: always paramagnetic d6: depending on the ligands d7 ~ d9: always paramagnetic d10: always diamagnetic

  21. Square planar & tetrahedral complexes Tetrahedral Reversal of octahedral !

  22. Square planar & tetrahedral complexes Tetrahedral Reversal of octahedral !

  23. Square planar & tetrahedral complexes Square planar Removal of axial ligands from octahedral

  24. Square planar Removal of axial ligands from octahedral

  25. Spectrochemical Series Color of complexes and magnetic properties are determined by Do Do can be determined by ligands I- < Br- < Cl- < F-, OH- < H2O < NCS- < NH3 < en < CO, CN- Weak field Ligands Strong field Ligands large Do small Do Low spin complex High spin complex [CoF6]3- v.s. [Co(CN)6]3- Weak point of crystal field theory 1. Coordination is not fully ionic. Ligand field theory 2. Spectrochemical series is all empirical.

  26. Ligand Field Theory Crystal field theory : Consider ionic interaction only Modification : addition of covalent aspect of coordination. Ligand field theory : How? : construction of molecular orbitals. Using 4s, 4p, 3d orbitals of metals & coordinating orbitals of ligands

  27. For an octahedral complex • Orbital overlap is 0 for • dxy, dyz, dzx • become nonboinding orbitals 2. Varying overlapping ligand orbitals for Cl- : p NH3 : sp3 3. MO’s can be formed from 6 ligand orbitals and 6 metal orbitals (4s, 4p, 3d)

  28. For [CoF6]3-

  29. Now, we can explain Spectrochemical Series I- < Br- < Cl- < F-, OH- < H2O < NCS- < NH3 < en < CO, CN- Weak field Ligands Strong field Ligands large Do small Do pback-bonding Interaction between dxyof metal and py of halide : ionic ---- increases energy level of t2g p bonding of ligand can overlap with dxyorbital Lowers the energy level of t2g Makes smaller Do for I- and less smaller one for F- Makes larger Do for CO, CN-

  30. For [CoF6]3-

  31. Organometallic compounds and Catalysis Catalytic converter : Pt catalyst Haber process Olefin metathesis reaction “Grubbs catalyst” 2005 Nobel Prize in Chemistry

  32. Coordination complexes and Life We need transition elements for life : V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo……. Important for oxygen transfer, detoxification, photosynthesis, nitrogen fixation Porphine structure

  33. 숙제 18장 : 6, 12, 22, 26, 34, 42, 46 제출일 : 10월 19일

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