transition - PowerPoint by zhangyun

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									                 Transition Metals

Occupy the d-block of periodic table
Have d-electrons in valence shell

Some characteristics of Transition Metals and their
   compounds

1.   Exhibit more than one oxidation state
2.   Many of their compounds are colored
3.   They exhibit interesting magnetic properties.
4.   They form an extensive series of compounds
     known as metal complexes or coordination
     compounds.
                  Transition Metals
 Exhibit more than one oxidation state
    e.g., Reduction of V5+ by metallic Zn

    VO2(H2O)4+          yellow-orange

    VO(H2O)52+             blue

    V(H2O)63+              green

    V(H2O)62+              violet

Many of their compounds are colored
         ELECTRON CONFIGURATIONS
3d elements: Sc  Zn

Ar 3s23p6                 Sc [Ar]3d14s2
K [Ar]4s1                 Ti [Ar]3d24s2
Ca [Ar]4s2                .        .
                          .        .
                          .        .
                          Zn [Ar]3d104s2

Note: 4s is filled before 3d, but when oxidized, 4s electrons are lost
before 3d.

Ti      [Ar]3d24s2
Ti2+    [Ar]3d24s0
Ti3+    [Ar]3d14s0
Ti4+    [Ar]3d04s0
Ti5+    does not exist!
                   Transition Metals
TRANSITION METALS: ScMn

Oxidation States:
Highest oxidation states of Sc, Ti, V, Cr, Mn = number of
valence (4s + 3d) electrons.

Sc [Ar]3d14s2       Sc3+ [Ar]     maximum
Mn [Ar]3d54s2       Mn7+[Ar]      maximum

Trend from Sc  Mn:
The max. oxidation state becomes increasingly unstable.
Sc3+, Ti4+ are stable (maximum oxidation states).
Sc2O3          Stable oxide.
Mn7+           Exists but is easily reduced.
MnO4-          Strong oxidizing agent.
                Magnetic Properties

Diamagnetic:
  unaffected by a magnetic field
  no unpaired electrons
Paramagnetic:
  influenced by a magnetic field
  unpaired electrons

Transition metals and their compounds are often
  paramagnetic
  Have unpaired d-electrons
Eg.    Ti2+
       Mn2+
            TRANSITION METAL IONS
Transition metal ions are Lewis acids  they accept
electron pairs.

Ligands are Lewis bases  molecules or ions which
donate electron pairs.

Ligands bonded to metal ions  metal complexes or
coordination compounds.

Coordination number:
number of electron donor atoms attached to the metal.

Chelates are ligands possessing two or more donor
atoms.
            COORDINATION COMPOUNDS
• Metals-Lewis acids
• Ligands -Lewis bases.
  Ligand molecules have lone pair electrons.
   – Anions -
     F, Cl, Br, CN, SCN, NO2, etc.
   – Neutral ligands: NH3, H2O, CO
      • mono-dentate -(single claw to hold onto metal d orbital)
          Ex. :NH3, H-:O:-H , CH3-:O:-H
      • Bi-dentate -(has 2 claws to hold onto metal d orbitals). Has 2
        or more functional groups on ligands that have lone pairs
          Example :NH2-CH2-CH2-H2N:
          (= en or ethylenediammine)
          COORDINATION COMPOUNDS
Coordination # = 4

Tetrahedral, e.g. [Zn(NH3)4]2+




    Square Planar, e.g. [Ni(CN)4]2


                            Cl
                                 H       H
                                     C
Square Planar,        Cl    Pt
e.g. [PtCl3(C2H4)]                  C
                                         H
                            Cl H
        COORDINATION COMPOUNDS

Coordination # = 6
                                   F
                               F        F
Octahedral, e.g. [CoF6]3-          Co
                               F        F
                                   F
Octahedral, e.g. [Co(en)3]3+       N
                               N        N
                                   Co
                               N        N
                                   N
           IMPORTANT CHELATING LIGANDS


Porphine




              O                     O
EDTA        HOCCH2               CH2COH
                      NCH2CH2N
                     :




             HOCCH2              CH2COH
                            :

               O                   O
                  CHELATE EFFECT
Chelating ligands form more stable compounds.

[Ni(H2O)6]2+ + 6NH3  [Ni(NH3)6]2+ + 6H2O   Kf = 4x108

[Ni(H2O)6]2+ + 3en  [Ni(en)3]2+ + 6H2O     Kf = 2x1018


      CHELATE EFFECT IS AN ENTROPY EFFECT

Cd2+ + 4CH3NH2  [Cd(CH3NH2)4]2+
 G° = 37.2kJ   H° = 57.3kJ S° = 67.3J/K

Cd2+ + 2en  [Cd(en)2]2+
 G° = 60.7kJ    H° = 56.5kJ      S° = +14.1J/K
     PROPERTIES OF TRANSITION METALS

Transition Metal Complexes have different properties –
• color (all except Zn or Sc3+ white compounds)
• solubility-depends on complex reduction potential
    – lower than free ions

Ag+(aq) + e  Ag(s)      E°1/2= +0.80V

[Ag(CN)2](aq) + e Ag(s)+ 2CN(aq)
                         E°1/2 = 0.31V
                     F-                F
             F-
                                   F        F
Co3+    F-        Co3+        F-       Co
                                   F        F
(3d6)        F-          F-            F
             CRYSTAL FIELD SPLITTING


                           dx2-y2        dz2
d-electron
 energy
                                     


                            dxy       dyz      dxz

      = crystal field splitting energy
     Spectrochemical series:
           CN > NO2 > en > NH3 > H2O > OH- > F > Cl

                         decreasing 
          SPECTROCHEMICAL SERIES

     UV          CN-
                 CO       Strong field
                 NO2-     ligands
                 en
                 NH3
                 H2O
                 Oxalate
                 OH-
                 F-        Weak field
                 SCN -
                           ligands
                 Cl-
     IR
                 Br-
                 I-      Color seen is complementary to
Absorbed light
                         absorbed color
         COLOR WHEEL


             RED
VIOLET
                       ORANGE




  BLUE                 YELLOW
             GREEN
   CRYSTAL FIELD SPLITTING ENERGY


 depends on
      1. Metal
      2. Oxidation state
      3. Ligands
P = spin pairing energy
P does not depend on the ligands


    P <   Low Spin Complex
    P >   High Spin Complex
                   SPIN PAIRING
    OCTAHEDRAL COMPLEXES




E



         CoF63-

    High spin                 Co(CN)63-
    Paramagnetic        Low spin (spin paired)
                        diamagnetic
            USES OF TRANSITION METALS
Ti
Lighter and stronger than steel.
Ti and its alloys are used in jet engines, planes, and in special high
temp applications, e.g. in the reentry shield on the Apollo capsules.
TiO2 is a white pigment in all white paints.

V
Vanadium steel (Fe/V alloy) is the toughest steel known. It is used
in car springs.
V2O5 is a catalyst used in sulfuric acid production.

Cr
Stainless Steel = 73% Fe,18% Cr, 8% Ni, 1% C
Chromium is electroplated to make shiny metal parts.

Mn
Mn steel (Fe/Mn alloy) is very tough and can withstand shock and
abrasion – used in bulldozer blades and armor plates on warships.
              CHROMIUM OXIDES
Cr(III) Oxide, Cr2O3
        Abrasive, Refractory
        Semiconductor, Green pigment
        Amphoteric

Cr(IV) Oxide, CrO2
       Recording tape (magnetic material)

Cr(VI) Oxide, CrO3
       Red
       Chrome plating, corrosion inhibitor

Na2Cr2O7
      Tanning, metal corrosion inhibitor

								
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