Complex ions

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Complex ions
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• Review transition metals
• Metals that have more than one oxidation state
• We have displayed them in simple compounds such
  as CuSO4 and CrCl3

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15.1 composition of complex ions

• Complex ion
• Charged molecular aggregate (ion), consisting of
  a metallic atom or ion to which is attached one
  or more electron-donating molecules
• In some complex ions, such as sulfate, SO4-2 ,
  the atoms are so tightly bound together that they
  act as a single unit
• Many complex ions, however, such as tetrammine
  zinc (II), Zn(NH3)42 , are only loosely
  aggregated and tend to dissociate in a water
  solution until an equilibrium is established
  between the complex ion and its components
• Such complex ions, or coordinated complexes as
  they are also called, generally consist of a
  positively charged central metal atom or ion,
  like the zinc in tetramine zinc, surrounded by
  electron-donating, or basic, groups called
  ligands

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Coordination compounds

• The transition metals also form a variety of
  ionic compounds with more complex formulas known
  as coordination compounds
• Examples
• Cu(NH3)4SO4
• In these compounds, the transition metal is
  present as a complex ion, enclosed within
  brackets
• In our example, the complex ion was Cu(NH3)42
• The charge of the complex ion was canceled by the
  simple anion it bonded to
• In our example, the simple anion was SO42-

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Coordinate covalent bonds

• In our example, we had the Cu(NH3)42 ion formed
• This ion is held together by a coordinate
  covalent bond
• These bonds involve electrons being contributed
  by the same atom
• This ion is commonly referred to as a complex ion
• A charged species in which a central metal cation
  is bonded to molecules and/or anions
• The anions are referred to collectively as
  ligands
• The number of atoms bonded to the central metal
  cation is referred to as its coordination number

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In our example

• Cu(NH3)42
• The central metal cation is Cu2
• The ligands are the NH3 molecules
• The coordination number is 4

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Coordination number

• Page 442 table 15.2
• Most common is 6
• 4 is less common
• 2 is restricted to Cu, Ag, and Au
• A few cations have only one coordination number
• Other cations have variable coordination numbers
  depending on the ligands

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Composition of complex ions

• Commonly formed by transition metals
• Usually those towards the right of a transition
  series
• Nontransition metals form a more limited number
  of stable complex ions
• Examples Al, Sn, Pb
• Cations of these metals exist in aqueous solution
  as complex ions

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Lewis acid-base reactions

• Reactions in which one species donates an
  electron pair to another
• When a complex ion is formed from a simple cation
• The electron pairs required for bond formation
  come solely from the ligands
• A Lewis base species that donates a pair of
  electrons
• Usually ligands
• Also classified as Bronsted-Lowry bases because
  they can accept a proton
• A Lewis acid accepts a pair of electrons
• Usually the metals
• Nee not be Bronsted-Lowry acids (proton donors)

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example

• Cu2 (aq) 4 NH3 ? Cu(NH3)42
• Lewis acid Lewis base

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Ligand

• Any molecule or anion with an unshared pair of
  electrons can act as a Lewis base
• It can donate a lone pair to a metal cation to
  form a coordinate covalent bond
• A ligand usually contains an atom of one to the
  more electronegative elements
• C, N, O, S, F, Cl, Br, I
• Several hundred are known
• Most common are NH3 and H2O molecules and CN-,
  Cl-, and OH- ions

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Charges of complexes

• Charge of complex is determined by
• Charge of complex
• oxida. no. central metal charges of ligands
• Example
• Cu(NH3)42
• Oxidation number of copper 2
• Charge of ligands 0
• Total charge of molecule 2

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Chelates

• Ligands that have more than one atom with
  unshared pairs of electrons
• Must have at least two lone pairs of electrons
• The electron pairs must be far enough from one
  another to give a chelate ring with a stable
  geometry
• Can form more than one bond with the central
  metal atom
• Most common are oxalate anion and the
  ethylenediamine molecule

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15.2 - Geometry of complex ions

• The physical and chemical properties of complex
  ions and of the coordination compounds they form
  depend on the spatial orientation of ligands
  around the central metal atom

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Coordination number 2

• Linear shape
• Two bonds are at a 180o angle
• Examples CuCl2-, Ag(NH3)2, Au(CN)2-

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Coordination number 4

• Two different geometries
• Tetrahedral
• Four bonds from the central metal may be directed
  toward the corners of a regular tetrahedron
• Zn(NH3)4 2
• Square planar
• Four bonds are directed toward the corners of a
  square
• More common
• Pt(NH3)42

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Coordination number 6

• All complex ions here are octahedral
• The metal ion or atom is at the center of the
  octahedron
• The six ligands are at the corners
• The six ligands can be considered to be
  equidistant from the central metal
• An octahedral complex can be regarded as a
  derivative of a square planar complex
• The two extra ligands are located above and below
  the square, on a line perpendicular to the square
  at its center

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Geometric isomerism

• Two or more complex ions have the same chemical
  formula, but different properties because of
  their different geometries
• Called isomers of each other
• Geometric isomers are ones that differ only in
  the spatial orientation of ligands around the
  central metal atom
• Found in square planar and octahedral complexes
• Cannot occur in tetrahedral complexes where all
  four positions are equivalent

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Square planar

• Pt(NH3)2Cl2 (see page 444)
• This complex can be written two different ways
• Pt Pt
• The one that has the similar molecules on the
  same side is called the cis isomer
• The one that has the similar molecules diagonally
  across from each other is called the trans isomer

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Octahedral

• With octahedral, for any given position of a
  ligand, four other positions are at the same
  distance from that ligand and a fifth is at a
  greater distance (page 445)
• Look at position 1
• 1 and 2, 1 and 3, 1 and 4, and 1 and 5 are cis to
  each other
• Positions 1 and 6 are trans (opposite corners
  from each other, as far away as possible)

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Example

• Co(NH3)4Cl2
• The cis form
• The two Cl- ions are at adjacent corners
• As close together as possible
• The trans form
• The two Cl- ions are at opposite corners
• As far away from one another as possible

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15.3 - Electronic structure of complex ions

• Crystal field model
• Hans Bethe and J.H. can Vleck
• Used the magnetic properties and brilliant colors
  of coordination compounds
• Explained in terms of the effect of ligands on
  the energies of electronic levels in transition
  metal cations

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Review transitional metal cations

• No outer s electrons
• Electrons beyond the preceding noble gas are
  located in an inner d sublevel
• Electrons are distributed among the five d
  orbitals in accordance with Hunds rule, giving
  the maximum number of unpaired electrons

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Octahedral complexes

• As six ligands approach a central metal ion to
  form an octahedral complex, they change the
  energies of electrons in the d orbitals
• Split the five d orbitals into two groups of
  different energy
• A higher energy pair, dx2-y2 and dz2 orbitals
• A lower energy trio, the dx, dyz, and dxz
  orbitals
• The difference in energy between the two groups
  is called the crystal field splitting energy and
  given the symbol ro
• O octahedral

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Color

• The energy difference between two sets of d
  orbitals in a complex is ordinarily equal to that
  of a photon in the visible region
• By absorbing visible light, an electron may be
  able to move from the lower energy set of d
  orbitals to the higher one
• This removes some of the component wavelengths of
  white light, so that the light reflected or
  transmitted by the complex is colored
• Show opposite color from what is absorbed on the
  color wheel

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Crystal filed splitting energy

• From the color of the complex spectrum, we can
  deduce the value of ro
• E hc/ ?
• Example absorbs purple
• (6.62 x 10-34 Js)(2.998 x 108 m/s) / 510 x 10-9
  m 3.90 x 10-19 J
• So just plug in the wavelength and you will be
  able to convert to energy in kilojoules per mole
  for the ro
• E 3.90 x 10-19 J x 1Kj/1000 J x 6.022 x 1023 /
  1 mol
• 2.35 102 kJ/mol
• The smaller the value of ro, the longer the
  wavelength of the light absorbed

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15.4 - Formation constants of complex ions

• Formation constant the equilibrium constant for
  the formation of a complex ion
• Kf
• Large numbers strongly favor complex formation
• Large Kf value the forward reaction goes
  virtually to completion
• The larger the Kf value, the more stable the
  complex
• Example
• Cu2 (aq) 4NH3(aq) ?? Cu(NH3)42 (aq)
• Kf Cu(NH3)42
• -----------------
• Cu2 x NH3 4