Chemistry of Coordination Compounds

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Chemistry ofCoordination Compounds

• Brown, LeMay Ch 24
• AP Chemistry
• Monta Vista High School

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24.1 Structure of Complexes

• Complex species in which a central metal ion
  (usually a transition metal) is bonded to a group
  of surrounding molecules or ions
• Coordination compound compound that contains a
  complex ion or ions.

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• A coordination compound, or complex, consists of
• Metal ion
• Acts as a Lewis acid (e- pair acceptor)
• Electrophile species that is e- poor and seeks
  e- (gets attacked by nucleophile)
• Ligand or complexing agent molecule or ion with
  a lone pair of e- that bonds to a metal ion
• Acts as a Lewis base (e- pair donor)
• Coordinate covalent bond metal-ligand bond
• Nucleophile species that is e- rich and seeks
  an e- poor area of a molecule (seeks an
  electrophile)

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Lewis Structures of common ligands

• NH3 CN-
• S2O32- SCN-
• H2O (not always included in formula, however)

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Complexation reactions

• Ligand usually added in excess on AP
• Usually result in color changes (colors generally
  originate from e- transitions in a partially
  filled d shell)
• Change properties of metal ion
• Thermodynamic (DH, DS, DG)
• Electrochemical (Eº)

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• The golden-orange compound is CoCl36NH3 while
  the purple compound only has 5 ammonia molecules
  in the coordinated compound. As shown in the
  ball-and-stick model, the chlorides serve as
  counter ions to the cobalt/ammonia coordiation
  complex in the orange compound, while one of the
  ammonia molecules is replaced by Cl in the purple
  compound. In both cases, the coordination
  geometry is octahedral around Co.

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Notation

• Write complexes in square brackets, with charge
  on outside
• Ex Cu2 (aq) 4 NH3 (aq) ? Cu(NH3)42 (aq)

NH3
2
H Cu2 (aq) 4 N - H (aq) ?
Cu H
NH3
H3N
NH3
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Coordination number

• Number of positions where a ligand can bond.
• Similar to oxidation state
• Each metal ion has a characteristic (i.e.,
  typical) coordination number, which can be
  predicted according to crystal field theory.
• Ag coordination number 2 (2 ligand bonding
  positions) results in a linear complex
• Ag(NH3)2 (aq)

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• Zn2 Cu2 coordination number 4 tetrahedral
  complex
• Ex Zn(H2O)42 (aq)
• Pt2 coordination number 4 square planar
  complex (d8 e- structure)
• Ex Pt(CN)42- (aq)

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• Al3, Cr3, and Fe3 coordination number 6
  octahedral complex
• Ex Cr(NH3)5Cl2 (aq)

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• Is dependent on
• Charge of ligand
• Ni2 6 NH3 or 4 CN- (since CN- transfers more
  negative charge)
• Size of ligand
• Fe3 6 F- or 4 Cl- (larger ions take up more
  space)

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24.2 Chelates Polydentate ligands

• Ligands with more than one bonding position
• Ethylenediamine (en, C2H4N2), or oxalate,
  C2O42-
• Ex Cr3 (aq) 3 C2O42- (aq) ? Cr(C2O4)33-

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24.3 Nomenclature

• Name cation before anion one or both may be a
  complex. (Follow standard nomenclature for
  non-complexes.)
• Within each complex (neutral or ion), name all
  ligands before the metal.
• Name ligands in alphabetical order
• If more than one of the same ligand is present,
  use a numerical prefix di, tri, tetra, penta,
  hexa,
• Ignore numerical prefixes when alphabetizing.

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• Neutral ligands use the name of the molecule
  (with some exceptions)
• NH3 ammine- H2O aqua-
• Anionic ligand use suffix o
• Br- bromo- CN- cyano-
• Cl- chloro- OH- hydroxo-
• If the complex is an anion, use ate suffix
• Record the oxidation number of the metal in
  parentheses (if appropriate).
• Ex Co(NH3)5ClCl2

pentamminechlorocobalt (III) chloride
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Nomenclature practice
potassium hexacyanoferrate tetrammineaquacyanoc
hromium (III) chloride sodium tetrahydroxoaluminat
e

• 1. K4Fe(CN)6
• 2. Cr(NH3)4(H2O)CNCl2
• 3. NaAl(OH)4

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24.5 Color Magnetism

• Atoms or ions with a partially filled d-shell
  usually exhibit color because the e- transitions
  fall within the visible part of the EM spectrum.
• Ex transition metals such as Cu2 (blue) and
  Fe3 (orange)
• Therefore, those with empty or filled d-shells
  are usually colorless.
• Ex alkali alkaline earth halides, Al3

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24.6 Crystal Field Theory

• Created to explain why transition metal ions in
  complexes (having unfilled d-shells) are not
  necessarily paramagnetic.
• With coordination bonding, valence d-orbitals are
  not truly degenerate. Instead, they split.
• Some are lower in energy (more stable) and some
  higher.

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• The gap between the higher and lower energy
  levels is called the crystal-field splitting
  energy, which varies with each ligand, yielding
  different E, (different l, different colors).
• e- in an unfilled d-shell can actually be all
  paired (i.e., diamagnetic).
• Ex Co3 (has 6 d e-)