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Chemistry of
Coordination
Compounds
Chapter 15
Chemistry of CoordinationCompounds
Chemistry, The Central Science , 10th editionTheodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten
John D. BookstaverSt. Charles Community College
St. Peters, MO
2006, Prentice Hall, Inc.
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Chemistry of
Coordination
Compounds
Complexes
• A central metal atombonded to a group ofmolecules or ions is a
metal complex.• If the complex bears a
charge, it is a complex ion.
• Compounds containing
complexes arecoordination compounds.
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Chemistry of
Coordination
Compounds
Complexes
• The molecules or ions coordinating to themetal are the ligands.
• They are usually anions or polar molecules.
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Chemistry of
Coordination
Compounds
Coordination Compounds
• Many coordination compounds are brightly
colored.• Different coordination compounds from the same
metal and ligands can give quite differentnumbers of ions when they dissolve.
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Chemistry of
Coordination
Compounds
Werner’s Theory
Werner proposed putting all molecules and ionswithin the sphere in brackets and those “free”
anions (that dissociate from the complex ion whendissolved in water) outside the brackets.
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Chemistry of
Coordination
Compounds
Werner’s Theory
• This approach correctlypredicts there would be two
forms of CoCl3 ∙ 4 NH3.The formula would be written
[Co(NH3)4Cl2]Cl.
One of the two forms has the two
chlorines next to each other.The other has the chlorines
opposite each other.
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Chemistry of
Coordination
Compounds
Metal-Ligand Bond
• This bond is formed between a Lewis acidand a Lewis base.
The ligands (Lewis bases) have nonbondingelectrons.
The metal (Lewis acid) has empty orbitals.
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Chemistry of
Coordination
Compounds
Oxidation Numbers
Knowing the charge on a complex ion and the
charge on each ligand, one can determinethe oxidation number for the metal.
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Chemistry of
Coordination
Compounds
Oxidation Numbers
Or, knowing the oxidation number on the
metal and the charges on the ligands, onecan calculate the charge on the complex ion.
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Chemistry of
Coordination
Compounds
Coordination Number
• Some metals, such aschromium(III) andcobalt(III), consistently
have the samecoordination number (6in the case of these twometals).
• The most commonlyencountered numbersare 4 and 6.
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Chemistry of
Coordination
Compounds
Geometries
• There are twocommon geometries
for metals with acoordination numberof four:
Tetrahedral
Square planar
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Chemistry of
Coordination
Compounds
Polydentate Ligands
• Some ligands have twoor more donor atoms.
• These are calledpolydentate ligands orchelating agents.
• In ethylenediamine,NH2CH2CH2NH2,
represented here as en,each N is a donor atom.
• Therefore, en isbidentate.
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Chemistry of
Coordination
Compounds
Polydentate Ligands
Ethylenediaminetetraacetate,mercifully abbreviated EDTA,
has six donor atoms.
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Chemistry of
Coordination
Compounds
Polydentate Ligands
Chelating agentsgenerally form
more stablecomplexes thando monodentateligands.
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Chemistry of
Coordination
Compounds
Chelating Agents
• Therefore, they canrender metal ionsinactive without actually
removing them fromsolution.
• Phosphates are used totie up Ca2+ and Mg2+ in
hard water to preventthem from interferingwith detergents.
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Chemistry of
Coordination
Compounds
Chelating Agents
Porphines (likechlorophyll a ) are
tetradentate ligands.
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Chemistry of
Coordination
Compounds
Chelating Agents
• Porphyrins arecomplexes containing a
form of the porphinemolecule shown at theright.
• Important biomolecules
like heme andchlorophyll areporphyrins.
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Chemistry of
Coordination
Compounds
Nomenclature of CoordinationCompounds
• The basic protocol in coordination nomenclature
is to name the ligands attached to the metal asprefixes before the metal name.
• Some common ligands and their names arelisted above.
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Chemistry of
Coordination
Compounds
Nomenclature of CoordinationCompounds
• As is the case with ionic compounds, the name ofthe cation appears first; the anion is named last.
• Ligands are listed alphabetically before the metal.
Prefixes denoting the number of a particular ligandare ignored when alphabetizing.
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Chemistry of
Coordination
Compounds
Nomenclature of CoordinationCompounds
• The names of anionic ligands end in “o”; the
endings of the names of neutral ligands are notchanged.
• Prefixes tell the number of a type of ligand in thecomplex. If the name of the ligand itself has sucha prefix, alternatives like bis -, tris -, etc., are used.
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Chemistry of
Coordination
Compounds
Isomers
Isomers have the same molecular formula, but
their atoms are arranged either in a different order(structural isomers) or spatial arrangement(stereoisomers).
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Chemistry of
Coordination
Compounds
Structural Isomers
If a ligand (like the NO2 group at the bottom of thecomplex) can bind to the
metal with one or anotheratom as the donor atom,linkage isomers areformed.
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Chemistry of
Coordination
Compounds
Structural Isomers
• Some isomers differ in what ligands arebonded to the metal and what is outsidethe coordination sphere; these are
coordination-sphere isomers.• Three isomers of CrCl3(H2O)6 are
The violet [Cr(H2O)6]Cl3,
The green [Cr(H2O)5Cl]Cl2 ∙ H2O, andThe (also) green [Cr(H2O)4Cl2]Cl ∙ 2 H2O.
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Chemistry of
Coordination
Compounds
Stereoisomers
• With these geometricisomers, two chlorinesand two NH3 groupsare bonded to the
platinum metal, but areclearly different.
cis -Isomers have like groups on the same side.trans -Isomers have like groups on opposite sides.
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Chemistry of
Coordination
Compounds
Stereoisomers
• Other stereoisomers, called optical isomers or
enantiomers, are mirror images of each other.• Just as a right hand will not fit into a left glove,
two enantiomers cannot be superimposed oneach other.
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Chemistry of
Coordination
Compounds
Enantiomers
A molecule or ion that exists as a pair ofenantiomers is said to be chiral.
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Chemistry of
Coordination
Compounds
Complexes and Color
Interactions between electrons on a ligandand the orbitals on the metal causedifferences in energies between orbitals inthe complex.
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Chemistry of
Coordination
Compounds
Complexes and Color
Some ligands (such as fluoride) tend to makethe gap between orbitals larger, some (likecyano groups) tend to make it smaller.
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Chemistry of
Coordination
Compounds
Complexes and Color
The larger the gap, the shorter thewavelength of light absorbed by electrons
jumping from a lower-energy orbital to ahigher one.
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Chemistry of
Coordination
Compounds
Complexes and Color
Thus, the wavelength of light observed in thecomplex is longer (closer to the red end of thespectrum).
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Chemistry of
Coordination
Compounds
Complexes and Color
As the energy gap gets smaller, the lightabsorbed is of longer wavelength, andshorter-wavelength light is reflected.