All contain a transition metal!! All are coordination compounds by vyg10427

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									                    CHEM 1311A Syllabus
• Transition metals and Coordination Chemistry
   – Introduction to coordination compounds; stereochemistry,
      isomerism and nomenclature
   – Coordination compounds: bonding models and energetics
   – Coordination compounds: equilibria and substitution reactions
• Bioinorganic chemistry
Fourth Exam – Friday, November 30




            What do these have in common?
   •   Hemoglobin                    •   “Blue blood”
   •   Myoglobin                     •   Ferredoxins
   •   Automobile paints             •   Rubies
   •   Anti-cancer drugs (some)      •   Emeralds
   •   Industrial catalysts (many)   •   Legumes (nitrogen fixers)
   •   Arthritis drugs               •   Radiopharmaceuticals (some)
   •   Vitamin B12                   •   MRI contrast agents
   •   Cytochromes

  All contain a transition metal!!
 All are coordination compounds
         Many are colored
      Transition metal complex (coordination
              compound) terminology
• Coordination compound, coordination complex, complex - a
    compound containing a metal ion and appended groups, which are
    Lewis bases and may be monatomic or polyatomic, neutral or
    anionic.
•   Ligand - Lewis base bonded (coordinated) to a metal ion in a
    coordination complex.
     –Those with only one point of attachment are monodentate
       ligands.
     –Ligands that can be bonded to the metal through more than one
       donor atom are termed bidentate (two points of attachment),
       tridentate, etc. Such ligands are termed chelating ligands.
•   Coordination number - number of ligands coordinated to a metal
    ion, 2-12.
•   Coordination geometry or stereochemistry (octahedral,
    tetrahedral, square planar) - geometrical arrangements of ligands
    (donor groups) about a metal ion.




Effect of coordination number and geometry
           on absorption spectrum




                   Comparison of electronic absorption
                   spectral intensities for [Co(OH2)6]2+
                   (octahedral) and [CoCl4]2- (tetrahedral)
     Transition metal complex (coordination
             compound) terminology
 • Isomers
    – Constitutional (structural) isomer - one of two or more
      compounds having the same composition but differing in their
      atom connectivities.
    – Stereoisomer - one of two or more compounds having the
      same atom connectivities but different spatial arrangements of
      atoms.
       ○ Diastereoisomer – stereoisomers not related by mirror
         images
       ○ Enantiomer - one of a pair of species that are non-
         superimposable mirror images.




                   Types of Isomerism
                Constitutional
                                        Stereo
                 (structural)

                                             Diastereomers
            Linkage
                                              (geometric)

                                              Enantiomers
           Ionization
                                                (optical)

           Hydration



● Constitutional (structural) isomers – same composition, different
  atom connectivities
● Stereoisomers – same composition, same atom connectivities,
  different spatial arrangements
         Stereoisomers: Diastereoisomers
• Compounds that have the same atom connectivities, but
 which are not mirror images are diastereoisomers.

              X                L                X
                                       M                    cis
              M                L                X
     L
                   X
          L
                               L                X
                                       M                    trans
                               X                L


         tetrahedral               square planar




         Stereoisomers: Diastereoisomers
• Compounds that have the same atom connectivities, but
 which are not mirror images are diastereoisomers.
                           L                        L
  ML4X2                L       X            L           X
                           M                        M         cis
                       L       X           L            X
                           L                        L


                           X                        X
                       L       L           L            L
                           M                        M        trans
                       L       L           L            L
                           X                        X

                           L                        L
                       L       X            L           X
                           M                        M        trans
                       X       L           X            L
                           L                        L
             Stereoisomers: Diastereoisomers
• Compounds that have the same atom connectivities, but
     which are not mirror images are diastereoisomers.


      ML3X3                        X             X
                               L       L   L          L
                                   M            M               mer
                               L       X   L          X
                                   X             X


                                   X             X
                               L       X   L          X
                                   M            M               fac
                               L       X   L          X
                                   L             L




                  Stereoisomers: Enantiomers
• Compounds that have no center or plane of symmetry exist
     in non-superimposable, mirror-image forms.

                                               Rotate by 180E


         F             F

Br       C             C   Br
             Cl   Cl
     H                     H
               Stereoisomers: Enantiomers
  • Compounds that have no center or plane of symmetry exist
    in non-superimposable, mirror-image forms.

             H2N
        H2               3+
        N          NH2
            Co
        N          NH2
        H2 H N
            2




How many diastereoisomers can exist for the complex ion
[Co(H2NCH2CH2NH2)(NH3)2Cl2]+ ?




How many of these diastereoisomers have nonsuperimposable mirror
image forms?
How many diastereoisomers can exist for [Co(dien)(Cl)(NO2)2]?

                                                          H
                  H2N                    NH2              N
        dien =                                 =
                              NH
                                                    NH2        H2N




How many of the diastereoisomers that can exist for
[Co(dien)(Cl)(NO2)2] have non-superimposable mirror images, i.e., are
enantiomeric?

       N                     N                     N                     N
   N        NO2          N         NO2         N         Cl          N        NO2
       Co                    Co                    Co                    Co
O2N         N           Cl         N           N         NO2         N        NO2
       Cl                    NO2                   NO2                   Cl
How many stereoisomers (diastereoisomers and enantiomeric forms)
can exist for [Co(H2NCH2CH2O)3]?




The tetradentate ligand shown below forms six-coordinate complexes
with Co(III) having the composition [CoLX2]+ where X is a mondentate
ligand.

               HN      NH2            N      N

                              =
                HN      NH2           N      N




How many diastereoisomers can be formed? How many are
enantiomeric?
 Energy changes for formation of ML6n+
        n+                  n+                                 n+
      M +6L           ML6                               ML6         (octahedral)




 E
                                                      d z 2 d x2 - y 2

                                                                                   )E
                                     e-e replusion   d xz dxy dyz
                                                     differential replusions
                                                         of d orbitals
                     electrostatic
                      attraction




               Magnetic properties
• High spin – maximum number of unpaired electrons for dn
   – Spin pairing energy is greater than ∆E (∆o)
• Low spin – minimum number of unpaired electrons for dn
   – Spin pairing energy is less than ∆E (∆o)
          Dependence of magnetic and spectral
               properties on ligand type

• Spectrochemical series
   – I– < Br– < Cl– < SCN– < F–, OH– < NO2– < H2O < SCN– < NH3 <
     en < NO2– < PR3 < P(OR)3, C2H4< PF3, CO, CN–




           Energy level diagram for complex
                 with F donor ligands

                                       F*

   np
                                      F*

   ns                        eg *
                                      F*
                         )
                             t2g
(n-1) d                               n


                                                         L orbitals

                     F
                     F
                     F
          Metal-ligand B-bonding interactions

                  dB-pB donor interactions; halide, hydroxide


                  dB-pB acceptor interactions (rare)



                  dB-dB acceptor interactions; phosphorus, arsenic


                  dB-B* acceptor interactions; CO, CN-, NO, RNC



                  dB-B* acceptor interactions; olefins (C=C)




          Energy level diagram for complex
             with F and B donor ligands

                                        F*

   np
                                        F*

   ns                         eg *
                                        F*
                           ) t
                               2g
                                        B*
(n-1) d



                   n                                            L orbitals
                   B
                   F
                   F
                   F
 Comparison of level diagrams for complexes
   with F only and F plus B donor ligands



      np


      ns


                                 )
                             )

  (n-1) d



                                               L orbitals




            Energy level diagram for complex
              with F and B acceptor ligands

                                     F*

   np
                                     F*
                                     B*
                                     n     L pi acceptor
   ns                  eg *                orbitals
                                     F*

                                 )
(n-1) d                t2g
                                     B

                                           L orbitals

                   F
                   F
                   F
 Comparison of level diagrams for complexes
  with F only and F plus B acceptor ligands



     np

                                         L pi acceptor
     ns                                  orbitals


                   )
                           )
  (n-1) d



                                         L orbitals




Energy level diagrams for complexes with F only,
 F plus B donor, and F plus B acceptor ligands



     np

                                         L pi acceptor
     ns                                  orbitals

                       )
                   )
                           )
  (n-1) d



                                         L orbitals
             HOMO and LUMO for cyanide ion




      p

E                                        p


      s

                                        s




         Effect of B-donor and B-acceptor
    interactions on ) in octahedral complexes

                       eg *                         eg *                     eg *
                                                                         )
                                                )   t2g
                                                                                                     )
                       t2g
                                                                             t2g

energy of d-orbitals          F bonding only               F + B donor              F + B acceptor
prior to interaction
with ligands
                               intermediate                 weak field                strong field
                                field ligands                ligands                    ligands
     Dependence of magnetic and spectral
          properties on ligand type
• Spectrochemical series
   – I– < Br– < Cl– < SCN– < F–, OH– < NO2– < H2O < SCN– < NH3 <
     en < NO2– < PR3 < P(OR)3, C2H4< PF3, CO, CN–
• Strong field ligands = low-spin complexes
   – have B-acceptor orbitals: B* as in CO or CN–, B*as in
     CH2=CH2, low lying d as in PR3, PF3
• Weak field ligands = high-spin complexes
   – have B-donor orbitals: usually multiple p orbitals as in X
• Intermediate field ligands = usually high spin for +2 ions,
  low-spin for +3 ions
   – have few, or no, B -donor or acceptor orbitals, or there is a
     poor match in energy of available B orbitals: NH3, H2O, pyridine




        Variation of )O in octahedral Ti(III)
                    complexes
Ti(III) is a d1 ion and exhibits one absorption in the electronic
spectrum of its metal complexes due to transition of the electron
from the t2g (lower energy) orbitals to the eg (higher energy)
orbitals. The energy of the absorption corresponds to )O.

                 Ligand           )O/cm-1*
                 Br-              11,400
                 Cl-              13,000
                 (H2N)2C=O        17,550
                 NCS-             18,400
                 F-               18,900
                 H2O              20,100
                 CN-              22,300
                 *E = h< = hc/8
          Electronic absorption spectra
• Selection rules
   – Transitions that occur without change in number of
       unpaired electrons (spin multiplicity) are allowed
     – Transitions that involve a change in the number of
        unpaired spins are “forbidden” and are therefore of low
        intensity.
         > solutions of high-spin d5, e.g., Mn(II), complexes are
            lightly colored
•   Absorption bands are broad because metal-ligand bonds are
    constantly changing distance (vibration) and since electronic
    transitions occur faster than atomic motions this means that
    there are effectively many values of ∆o.
•   d1 and d9, and high-spin d4 and d6 ions have only one spin-
    allowed transition; high-spin d2, d3, d7 and d8 have three spin-
    allowed transitions




        Allowed vs forbidden transitions


                    dx2-y2 dz2          dx2-y2 dz2


             E     dxz   dxy   dyz    dxz   dxy   dyz




                     dx2-y2 dz2         dx2-y2 dz2


                   dxz   dxy   dyz    dxz   dxy   dyz
       Effect of ligand on absorption
            spectra (and color)




Number of d electrons and spectral intensity




                                        [Mn(OH2)6]2+
       Transitions in d1 and d2 complexes


                                   dx2-y2 dz2


                               dxz     dxy   dyz


      dx2-y2 dz2                                   dx2-y2 dz2
                                                                  dxy1 dx2-y21 dxz1dz21 dyz1dz21
                         dx2-y2 dz2
E
     dxz   dxy   dyz                            dxz   dxy   dyz   dxz1 dx2-y21 dxy1dz21 dyz1dx2-y21
                       dxz   dxy     dyz

       dx2-y2 dz2                  dx2-y2 dz2


     dxz   dxy   dyz           dxz     dxy dyz




    Comparison of crystal field splittings for
    octahedral, square planar and tetrahedral
                  ligand fields
Crystal field splitting in tetrahedral complexes
                            •   Tetrahedral arrangement of four ligands
                                showing their orientation relative to the
                                Cartesian axes and the dyz orbital.
                            •   The orientation with respect to dxz and dxy is
                                identical and the interaction with these
                                orbitals is considerably greater than with the
                                dz2 and dx2- y2 orbitals; therefore the dyz, dxz
                                and dxy orbitals are higher in energy than
                                dz2 and dx2- y2 .
                            •    Because there are only four ligands and
                                the ligand electron pairs do not point
                                directly at the orbitals, ∆t ~4/9 ∆o. As a
                                result the spin-pairing energy is always
                                greater than ∆ and tetrahedral complexes
                                are always high spin.




      Factors affecting the magnitude of )
            (Crystal Field Splitting)
• Charge on the metal. For first row transition elements )O varies
  from about 7,500 cm-1 to 12,500 cm-1 for divalent ions and 14,000
  cm-1 to 25,000 cm-1 for trivalent ions.
• Position in a group. )O values for analogous complexes of
  metal ions in a group increase by 25% to 50% on going from one
  transition series to the next. This is illustrated by the complexes
  [M(NH3)6]3+ where ) values are 23,000 cm-1 for M=Co; 34,000 cm-
  1 for M=Rh and 41,000 cm-1 for M=Ir.

• Geometry and coordination number. For similar ligands )t will
  be about 4/9 )O. This is a result of the reduced number of ligands
  and their orientation relative to the d orbitals. Recall that the
  energy ordering of the orbitals is reversed in tetrahedral
  complexes relative to that in the octahedral case.
• Identity of the ligand. The dependence of ) on the nature of the
  ligand follows a regular order, known as the spectrochemical
  series, for all metals in all oxidation states and geometries.
        Thermodynamic vs kinetic stability
• Stability in a thermodynamic sense refers to the energetics of a
    formation or decomposition reaction )G = )H + T)S
•   Stability in a reactivity sense refers to the rate with which a
    given reaction occurs.
•   Complexes that undergo substitution with half-lives less than
    about one minute are referred to as labile; those that are less
    reactive are termed inert.
•   Complex stability and reactivity do not necessarily correlate with
    ligand field strength; the latter refers to spectroscopic and
    magnetic properties.
•   Thermodynamic and kinetic stabilities sometimes parallel but
    often they do not.
      – [Ni(CN)4]2& illustrates the latter case; the overall equilibrium
        constant its formation is >1030 but the second order rate
        constant for CN& exchange is >5 x 105 M-1 s-1




         Stepwise formation of [Cu(NH3)4]2+

[Cu(OH2)4]2+ + NH3 W [Cu(OH2)3(NH3)]2+ + H2O                         log K1 = 4.22
[Cu(OH2)3(NH3)]2+       + NH3 W   [Cu(OH2)2(NH3)2]2+   + H2O         log K2 = 3.50
[Cu(OH2)2(NH3)2   ]2+   + NH3 W    [Cu(OH2)(NH3)3]2+   + H2O         log K3 = 2.92
[Cu(OH2)(NH3)3   ]2+    + NH3 W [Cu(NH3)4   ]2+   + H2O              log K4 = 2.18

                                                                 [Cu(NH ) 2+ ]
[Cu(OH2)4]2+ + 4 NH3 W [Cu(NH3)4]2+ + 4 H2O                    β =       34
                                                               4 [Cu2+ ][NH ]4
                                                                           3
Speciation is determined by ligand concentration
                                     2+                                    2+
                        [Cu(OH2)4]        + n NH3 = [Cu(OH2)4-n(NH3)n]
                  1.0
                  0.9
                  0.8
                                             n=0             n=4
                  0.7
                  0.6
       Fraction



                                               n=1         n=3
                                                     n=2
                  0.5
                  0.4
                  0.3
                  0.2
                  0.1
                  0.0
                             6                  4                2                0
                                               -log[NH3]




 The Chelate Effect is largely entropic in origin

 [Cu(OH2)4]2+ + en W [Cu(OH2)2(en)]2+ + 2 H2O                                   log K1 = 10.6
                                           )H = -54 kJ     mol-1,    )S = 23 J K-1 mol-1


 [Cu(OH2)4]2+ + 2 NH3 W [Cu(OH2)2(NH3)2]2+ + 2H2O                                log K2 = 7.7
                                            )H = -46 kJ    mol-1,    )S = -8.4 J K-1 mol -1
Ligand substitution in coordination complexes

 • Arguably the most important reaction of coordination complexes
     is ligand substitution.
 •   There are two limiting mechanisms for substitution reactions
      – associative parallels the SN2 reaction in organic chemistry;
          the reaction involves an intermediate of higher
          coordination number. rate = k[complex][L]
            > associative reactions are more important for larger metal
              ions and for those that have vacancies in the t2g orbitals
      – dissociative parallels the SN1 reaction in organic chemistry;
          the reaction involves an intermediate of lower
          coordination number. rate = k[complex]
 •   The simplest substitution is ligand exchange which is not
     complicated by thermodynamics since ∆G = 0.
      – exchange rates of water have been most extensively studied
      – rate constants for water exchange range from 1.1x10-10 s-1 to
          5x109 s-1




          Observations on water exchange

 • An increase in oxidation state for the metal reduces the rate of
     exchange
 •   Early (larger) elements in a period tend to have a greater
     contribution from associative processes
 •   Heavier (larger) elements in a family have a greater contribution
     from associative processes; also greater bond strengths
     decrease rate of dissociative processes
 •   Occupancy of (antibonding) eg orbitals increases the rate for all
     oxidation states
Water exchange rates in aquo metal ions
          Rate constantsa for water exchange
             [MLn(OH2)]n+
                n      n+           k/s-1
                                       -1                 [MLn (OH2)]n+
                                                             n    2 n+                    k/s-1
                                                    [Ti(OH2) 6]3+                 1.8 x 105
          [V(OH2)6] 2+      8.7 x 101               [V(OH2)6] 3+                  5.0 x 102
          [Cr(OH2)6] 2+     >108                    [Cr(OH2)6] 3+                 2.4 x 10-6
          [Mn(OH2)6] 2+     2.1 x 107
          [Fe(OH2)6]2+      4.4 x 106               [Fe(OH2 )6]3+                 1.2 x 102
          [Ru(OH ) ]
                  2 6 2+    1.8 x 10   -2           [Ru(OH ) ]2 6 3+              3.5 x 10-6
          [Co(OH ) ]
                  2 6 2+    3.2 x 10   6


          [Ni(OH2)6] 2+     3.2 x 104
          [Pd(OH2) 4]2+
                            5.6 x 10-2
          [Pt(OH2)4]2+      3.9 x 10-4
          [Cu(OH2)6]2+      >107
          [Zn(OH2)6]2+      >107
                                                    [Cr(NH3)5OH2]3+               5.2 x 10-5
                                                    [Co(NH3) 5OH2]3+              5.7 x 10-6
                                                    [Rh(NH3) 5OH2]3+              8.4 x 10-6
                                                    [Ir(NH3)5OH2]3+               6.1 x 10-8
          aAll
             rate constants are expressed as first order rate constants for
          comparative purposes even though some reactions are associative.




    Electron transfer reactions: importance of
     orbital occupancy and spin state on rate
•   Electron transfer is second only to substitution in importance as a
    characteristic reaction of coordination complexes and especially in
    biological systems.
•   Again the simplest reaction is outer-sphere electron exchange where
    )G=0
•   Rates of electron exchange vary enormously across the transition series,
    but two things are invariably true:
     – The rate of electron transfer is greatest when electrons are transferred
       from a t2g orbital on the reductant to a t2g orbital on the oxidant.
     – There is minimal change in bond distance in either oxidant or reductant
       upon electron transfer.
                                              Complex               e- config            M-L BD, D     kex, M-1 s-1
                                           [Co(NH3)6]2+                t2g5eg2                 2.114     # 10-9
                                           [Co(NH3)6]3+                 t2g6                   1.936
                                           [Ru(NH3)6]2+                 t2g   6                2.144    8.2 x 102
                                           [Ru(NH3)6]3+                 t2g   5                2.104

								
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