Chapter 1: Fundamental Concepts - PowerPoint

Document Sample
Chapter 1: Fundamental Concepts - PowerPoint Powered By Docstoc
					             Metal Complexes -- Chapter 24
Metal Complex -- consists of a set of ligands that are bonded to
                 a central metal ion by coordinate covalent
                 bonds.
       e.g. Cu2+ + 4 L --> [CuL4]2+ ( L = NH3, H2O, etc)

Ligands are Lewis Bases and can be:
   – monodentate -- one donor atom
               e.g. H2O, NH3, Cl–, OH–, etc.
   – bidentate -- two donor atoms
               e.g. ethylenediamine, NH2–CH2–CH2–NH2 “en”


                     H2C       CH 2
                                        =   H2N       NH 2
                    H2N          NH 2
                                                  M
                           M
             Polydentate ligands
– polydentate -- more than two donor atoms
           e.g. EDTA -- ethylenediaminetetraacetic acid
                (6 donor atoms)


         O                             O
             H2
    O    C   C      H2C           CH 2 C   O
                          CH 2
                    N        N                 = EDTA4–
    O    C   CH 2                H2C   C   O

         O                             O
                          Chelate Effect
Chelate effect -- complexes with bi- or polydentate ligands are
  more stable than those with similar monodentate ligands

e.g. [Ni(en)3]3+ is more stable than [Ni(NH3)6]3+

                                    3+
                     NH 2
                            NH 2
            H 2N     Ni        N
                               H2
              H 2N
                     NH 2




                            Know Table 24.2!
    Writing Formulas of Complex Ions

Metal ion first, then ligands. Charge outside brackets.
  total charge = sum of metal ion + ligands

       e.g.    metal ion      ligand         complex
               Cu2+           H2O            [Cu(H2O)]2+
               Co3+           NH3            [Co(NH3)6]3+
               Fe3+           CN–            [Fe(CN)6]3–
                        Nomenclature
• 1. Name the ligands, put in alphabetical order.
• 2. Add prefixes for ligands that appear more than once.
• 3. Name the metal
    – Cationic complex; metal name
    – Anionic complex; metal prefix + ate
• 4. Add oxidation state in roman numerals
• 5. As separate word, write “ion” if not neutral, or list
  counterions (positive first, negative second).
Examples:
•        Complex                      Name
    [Ni(CN)4]2–         tetracyanonickelate(II) ion
    [CoCl6]3–           hexachlorocobaltate(III) ion
    [CoCl2(NH3)4]+      tetraaminedichlorocobalt(III) ion
    Na3[Co(NO2)6]       sodium hexanitrocobaltate(III)
    [CrCl2(en)2]2SO4
         dichlorobis(ethylenediamine)chromium(III) sulfate
  Coordination Number and Structure
Coordination # -- number of donor atoms attached to the metal
  center
  (a) Two-Coordinate Complexes -- linear structures
       Rare except for Ag+
       e.g. [Ag(NH3)2]+ and [Ag(CN)2]–

  (b) Four-Coordinate Complexes -- two structural types
       Tetrahedral structures -- common for ions with filled d
       subshells, e.g. Zn2+ as in [Zn(OH)4]2–

                                  OH          2




                                 Zn      OH

                         HO
                                         OH
  More Coordination Number and Structure
(b) Four-Coordinate Complexes -- two structural types
   square planar structures -- common for d8 metal ions (Ni2+, Pd2+,
      Pt2+) and for Cu2+
   e.g.                  2                            2+
           Cl              Cl            H3N                NH 3
                  Pt                              Cu
           Cl                  Cl        H3N                 NH 3



(c) Six-Coordinate Complexes -- the most common!
   “always” octahedral structures, e.g.

                                    3+
                 NH 2                                 Cl
                        NH 2                               NH 3
        H 2N     Ni        N              NH 3        Cr      Cl
                           H2
          H 2N                                 NH 3
                 NH 2                                 Cl
                  Sample Problem
1. Give the name or formula, as required. Draw the structure
   of each complex.
   a) [AgI2]–
   b) [Co(en)3]3+
   c) cis-dichlorobis(ethylenediamine)cobalt(III) chloride
   d) sodium tetracyanonickelate(II)
       Structural Isomers of Coordination Complexes
  • Linkage isomers




   pentaamminenitrocobalt(II) ion     pentaamminenitritocobalt(II) ion

  • Coordination isomers




pentaamminechlorocobalt(II) bromide    pentaamminebromocobalt(II) chloride
Isomerism; Overview




                      10
          Stereoisomers of Coordination Complexes
       (a) Geometrical Isomers
         Cl                      Cl
                NH 3                  NH 3
NH 3     Cr        Cl   NH 3     Cr      NH 3
  NH 3                    NH 3
         NH 3                    Cl

       cis                     trans            fac              mer
       (b) Enantiomers




       Three common ways to get enantiomers:
       --chiral ligand
       --tetrahedral complex with 4 different groups
       --octahedral complexes with chelating ligands (and no mirror plane)
                 Sample Questions
Draw a clear, 3-dimensional structure of the geometrical
  isomer of Co(en)(NH3)2Cl2 that is optically active. (define any
  abbreviations)




Excess silver nitrate is added to a solution containing 0.0522
  mol of [Co(NH3)4Cl2]Cl. How many g of AgCl (FW = 143.3)
  will precipitate?
                 Sample Questions
Draw a clear, 3-dimensional structure of the geometrical
  isomer of Co(en)(NH3)2Cl2 that is optically active. (define any
  abbreviations)


                     NH 2                      NH 2

                NH 2                                  NH 2
                Cl   Co        NH 3   NH 3     Cr        Cl
                            NH 3        NH 3
                     Cl                        Cl




Excess silver nitrate is added to a solution containing 0.0522
  mol of [Co(NH3)4Cl2]Cl. How many g of AgCl (FW = 143.3)
  will precipitate?

                            7.48 g
                    Crystal Field Theory
           (Bonding in Transition Metal Complexes)
Metal complexes are usually highly colored and are often
  paramagnetic -- such facts can be explained by a “d-orbital
  splitting diagram”



                                               eg
                                    dz2   dx2-y2
 energy                            D = crystal field splitting energy
                                                   t2g
                                    dxy dxz dyz

                                    d orbitals of the metal ion in
                                    an octahedral field of ligands

d orbitals of the free metal ion
Shape and Directionality of the d Orbitals




      eg



  D


      t2g
                 Crystal Field Strength
The size of D depends on…
• The nature of the ligand (ligand-metal bond strength!)
       “spectrochemical series” -- D decreases:
     CN– > NO2– > en > NH3 > H2O > OH– > F– > Cl– > Br–
    “strong field ligands”               “weak field ligands”


• The oxidation state of the metal (charge!)
      D is greater for M3+ than for M2+

• The row of the metal in the periodic table (size!)
      for a given ligand and oxidation state of the metal, D
      increases going down in a group
      e.g. D is greater in Ru(NH3)63+ than in Fe(NH3)63+

Colors of metal complexes are due to electronic transition
  between the t2g and eg energy levels
 d Orbital Splitting Diagrams for Octahedral
                   Complexes


                                                eg
                                dx2    dx2–y2



                    eg
  dx2      dx2–y2                         large D
                                        “low spin”
                 small D
               “high spin”


                         t2g                          t2g
dxy      dxz      dyz          dxy     dxz      dyz

      Fe(H2O)62+                      Fe(CN)64–
             High Spin vs. Low Spin
CN– is a stronger field ligand than is H2O which leads to a
  greater D value (i.e. a greater d orbital splitting)
As a result,
• Fe(H2O)62+ is a “high spin” complex and is paramagnetic (4
               unpaired electrons)
while,
• Fe(CN)64– is a “low spin” complex and is diamagnetic (no
               unpaired electrons)

The CN– complex with the larger D value absorbs light of higher
  energy (i.e. higher frequency but shorter wavelength)



OMIT … d orbital splitting diagrams for other geometries (i.e.
  tetrahedral and square planar)
                  Sample Questions
A complex [CoA6]3+ is red. The complex [CoB6]3+ is green.
   a) Which ligand, A or B, produces the larger crystal field
   splitting D?
   b) If the two ligands are ammonia and water, which is A and
   which is B?
   c) Draw the d orbital diagram of either complex, assuming it
   is “high spin.” How many unpaired electrons will it have?




Give the order of increase of D in the following sets:
   a) Cr(NH3)63+, CrCl63–, Cr(CN)63–
   b) Co(H2O)62+, Co(H2O)63+, Rh(H2O)63+

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:47
posted:7/27/2012
language:English
pages:19