Coordination Chemistry I Structures and Isomers by zrn20302

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									Coordination Chemistry I:
 Structures and Isomers
         Chapter 9
        Coordination Compounds
• Coordination compounds –
  compounds composed of a
  metal atom or ion and one
  or more ligands.
  – [Co(Co(NH3)4(OH2)3]Br6
  – Ligands usually donate
    electrons to the metal
  – Includes organometallic
    compounds
                              Werner’s totally inorganic
                              optically active compound.
Werner’s Coordination Chemistry
• Performed systematic studies to understand bonding
  in coordination compounds.
   – Organic bonding theory and simple ideas of ionic charges
     were not sufficient.
• Two types of bonding
   – Primary – positive charge of the metal ion is balanced by
     negative ions in the compound.
   – Secondary – molecules or ion (ligands) are attached directly
     to the metal ion.
      • Coordination sphere or complex ion.
      • Look at complex on previous slide (primary and secondary)
Werner’s Coordination Chemistry
• He largely studied compounds with four or six
  ligands.
   – Octahedral and square-planar complexes.
• It was illustrated that a theory needed to account
  for bonds between ligands and the metal.
   – The number of bonds was commonly more than
     accepted at that time.
      • 18-electron rule.
• New theories arose to describe bonding.
   – Valence bond, crystal field, and ligand field.
                   Chelating Ligands
• Chelating ligands         trisoxalatochromate(III) ion or just [Cr(ox)3]3-
  (chelates) – ligands that
  have two or more points
  of attachment to the
  metal atom or ion.
    – Bidentate, tridentate,
      tetra.., penta…, hexa…
      (EDTA).
     A Hexadentate Ligand, EDTA
• There are six points of
  attachment to the calcium
  metal.
   – Octahedral-type geometry
   ethylene diamine tetraacetic acid
      (EDTA)




                                ethylenediaminetetraacetatocalcium ion or just [Ca(EDTA)]2-
                   Nomenclature
• The positive ion (cation) comes first, followed by the
  name within the coordination sphere, followed by the
  negative ion (anion).
   – These ions are not in the coordination sphere.
   – Diamminesilver(I)chloride and potassium hexacyanoferrate
     (III).
• The inner coordination sphere is enclosed in brackets in
  the formula. Within this sphere, the ligands are named
  before the metal, but in formulas the metal ion is
  written first.
   – Tetraamminecopper(II) sulfate and hexaamminecobalt(III)
     chloride.
                 Nomenclature
• The number of ligands is
  given by the following     2   di      bis
  prefixes. If the ligand
  name includes prefixes     3   tri     tris
  or is complicated, it is   4   tetra   tetrakis
  set off in parentheses     5   penta   pentakis
  and the second set of
  prefixes is used.          6   hexa    hexakis
   – [Co(en)2Cl2]+ and       7   hepta   heptakis
     [Fe(C5H4N-C5H4N)3]2+
                             8   octa    octakis
              Nomenclature
• Ligands are named in alphabetical order
  (name of ligand, not prefix)
  – [Co(NH3)4Cl2]+ and [Pt(NH3)BrCl(CH3NH2)]+2
• Anionic ligands are given an ‘o’ suffix.
  Neutral ligands retain the usual name.
  – Coordianted water is called ‘aqua’.
  – Chloro, Cl-
  – Sulfato, SO42-
                     Nomenclature
• The calculated oxidation number of the metal ion is
  placed as a Roman numeral in parentheses after the
  name of the coordination sphere.
   – [Pt(NH3)4]+2 and [Pt(Cl)4]-2
   – A suffix ‘ate’ is added to the metal ion if the charge is
     negative.
• The prefixes cis- and trans- designate adjacent and
  opposite geometric location, respectively.
   – trans-diamminedichloroplatinum(III) and cis-
     tetraamminedichlorocobalt(III)
                  Nomenclature
• Bridging ligands between two metal ions
  have the prefix ‘’.
   – -amido--hydroxobis(tetraamminecobalt)(IV)




There is an error in this picture. What is it?
                 Isomerism
• Our discussion of isomers will be largely
  limited to those with the same ligands arranged
  in different geometries. This is referred to as
  stereoisomers.
                           Isomerism
• Four-coordinate complexes
   – Square-planar complexes may have
     cis and trans isomers. No chiral
     isomers (enantiomers) are possible
     when the molecule has a mirror
     plane.
   – cis- and trans-
     diamminedichloroplatinum(II)
   – How about tetrahedral complexes?
   – Chelate rings commonly impose a
     ‘cis’ structure. Why
                       Chirality
• Mirror images are nonsuperimposable.
• A molecule can be chiral if it has no rotation-reflection
  axes (Sn)
• Chiral molecules have no symmetry elements or only
  have an axes of proper rotation (Cn).
   – CBrClFI, Tetrahedral molecule (different ligands)
   – Octahedral molecules with bidentate or higher chelating
     ligands
   – Octahedral species with [Ma2b2c2], [Mabc2d2], [Mabcd3],
     [Mabcde2], or [Mabcdef]
        Six-Coordinate Octahedral
               Complexes
• ML3L3’
  – Fac isomers have three
    identical ligands on the
    same face.
  – Mer isomers have three
    identical ligands in a plane
    bisecting the molecule.
       Six-Coordinate Octahedral
              Complexes
• The maximum number of isomers can be
  difficult to calculate (repeats).
• Placing a pair of ligands in the notation <ab>
  indicates that a and b are trans to each other.
  – [M<ab><cd><ef>], [Pt<pyNH3><NO2Cl><BrI>]
• How many diastereoisomers in the above
  platinum compound (not mirror images)?
• Identify all isomers belonging to Ma3bcd.
Determining the Number of
         Isomers
    Determining the Number of
            Isotopes
• Bailar method
• With restrictions (such as chelating agents)
  some isomers may be eliminated.
• Determine and identify the number if
  isomers.
  – [Ma2b2cd] and [M(AA)bcde]
 Combinations of Chelate Rings
• Propellers and helices
   – Left- and right-handed propellers
• Examine the movement of a propeller required to
  move it in a certain direction.
   – For a left-handed propeller, rotating it ccw would cause
     it to move away ().
   – For a right-handed propeller, rotating it cw would cause
     it to move away ().
   This is called ‘handedness’. Many molecules possess it.
 Tris(ethylenediamine)cobalt(III)
• This molecule can be treated like a three-
  bladed propeller.
• Look down a three fold axis to determine
  the ‘handedness’ of this complex ion.
  – The direction of rotation required to pull the
    molecule away from you determines the
    handedness ( or ).
• Do this with you molecule set and rubber
  bands.
   Determining Handedness for
        Chiral Molecules
• Complexes with two or more nonadjacent chelate
  rings may have chiral character.
   – Any two noncoplanar and nonadjacent chelate rings can
     be used.
   – Look at Figure 9-14 (Miessler and Tarr).
• Molecules with more than one pair of rings may
  require more than one label.
   – Ca(EDTA)2+
      • Three labels would be required.
      • Remember that the chelate rings must be noncoplanar,
        nonadjacent, and not connected at the same atom.
Linkage (ambidentate) Isomerism
• A few ligands may bond to the metal through
  different atoms.
   – SCN- and NO2-
• How would you expect hard acids to bond to the
  thiocyanate ligand?
• Solvents can also influence bonding.
   – High and low dielectric constants.
• Steric effects of linkage isomerism
• Intramolecular conversion between linkages.
   – [Co(NH3)5NO2]+2, Figure 9-19.
   Separation and Identification of
               Isomers
• Geometric isomers can be separated by fractional
  crystallization with different counterions.
  – Due to the slightly different shapes of the isomers.
  – The ‘fit’ of the counterion can greatly influence
    solubility.
     • Solubility is the lowest when the positive and negative
       charges have the same size and magnitude of charges
       (Basolo).
   Separation and Identification of
           Chiral Isomers
• Separations are performed with chiral
  counterions. The resulting physical properties
  will differ allowing separation.
• Rotation of polarized light will be opposite for
  two chiral isomers at a specific wavelength.
  – The direction of optical rotation can change with
    wavelength.
  Circular Dichroism Meaurement
• The difference in the absorption of right and left
  circularly polarized light is measured.
             Circular dichroism   l   r
  – Where l and r are the molar absorption
    coefficients for left and right circularly polarized
    light.
• The light received by the detector is presented
  as the difference between the absorbances.
Figure 9-20.
   Plane-Polarized Light Measurement

• The plane of polarization is rotated when passing
  through a chiral substance.
  – Caused by a difference in the refractive indices of the
    right and left circularly polarized light.
                         l  r
                      
                            
  – The optical rotation illustrates positive value on one
    side of the adsorption maximum and negative side on
    the other. This is termed as the Cotton effect.
      Coordination Numbers and
             Structures
• Factors considered when determining structures.
  – The number of bonds. Bond formation is
     exothermic; the more the better.
  – VSEPR arguments
  – Occupancy of d orbitals.
  – Steric interference by large ligands.
  – Crystal packing effect.
  It may be difficult to predict shapes.
  Low Coordination Numbers (C.N.)
• C.N. 1 is rare except in ion pairs in the gas phase.
• C.N. 2 is also rare.
   – [Ag(NH3)2]+, Ag is d10 (how?)
   – VSEPR predicts a linear structure.
   – Large ligands help force a linear or near-linear arrangment.
      • [Mn(N[SiMePh2]2)2] in Figure 9-22.
• C.N. 3 is more likely with d10 ions.
   – Trigonal-planar structure is the most common.
   – [Cu(SPPh3)3]+, adopts a low C.N. due to ligand crowding.
        Coordination Number 4
• Tetrahedral and square planar complexes are
  the most common.
  – Small ions and/or large ligands prevent high
    coordination numbers (Mn(VII) or Cr(VI)).
• Many d0 or d10 complexes have tetrahedral
  structures (only consider bonds).
  – MnO4- and [Ni(CO)4]
  – Jahn-Teller distortion (Chapter 10)
       Coordination Number 4
• Square-planar geometry
  – d8 ions (Ni(II), Pd(II), and Pt(III))
     • [Pt(NH3)2Cl2]
  – The energy difference between square-planar
    and tetrahedral structures can be quite small.
     • Can depend on both the ligand and counterion.
     • More in chapter 10.
        Coordination Number 5
• Common structures are trigonal bipyramid and
  square pyramid.
   – The energy difference between the two is small. In
     many measurements, the five ligands appear identical
     due to fluxional behavior.
   – How would you modify the experiment to differentiate
     between the two structures?
• Five-coordinate compounds are known for the full
  range of transition metals.
   – Figure 9-27.
       Coordination Number 6
• This is the most common C.N. with the
  most common structure being octahedral.
  – If the d electrons are ignored, this is the
    predicted shape.
     • [Co(en)3]3+
• This C.N. exists for all transition metals (d0
  to d10).
      Distortions of Complexes
         Containing C.N. 6
• Elongation and compression (Fig. 9-29).
   – Produces a trigonal antiprism structure when the angle
     between the top and bottom triangular faces is 60.
   – Trigonal prism structures are produced when the faces
     are eclipsed.
      • Most trigonal prismatic complexes have three bidentate ligands
        (Figure 9-30).
      •  interactions may stabilize some of these structures.
   The Jahn-Teller effect (Ch. 10) is useful in predicting
     observed distortions.
  Higher Coordination Numbers
• C.N. 7 is not common
• C.N. 8
  – There are many 8-coordinate complexes for
    large transition elements.
     • Square antiprism and dodecahedron
• C.N.’s up to 16 have been observed.

								
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