ch 15 coordination compounds

7/31/2019 Ch 15 Coordination Compounds

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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.



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.



Chemistry of

Coordination

Compounds

Complexes

• The molecules or ions coordinating to themetal are the ligands.

• They are usually anions or polar molecules.



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.



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.



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.



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.



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.



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.



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.



Chemistry of

Coordination

Compounds

Geometries

• There are twocommon geometries

for metals with acoordination numberof four:

Tetrahedral

Square planar



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.



Chemistry of

Coordination

Compounds

Polydentate Ligands

Ethylenediaminetetraacetate,mercifully abbreviated EDTA,

has six donor atoms.



Chemistry of

Coordination

Compounds

Polydentate Ligands

Chelating agentsgenerally form

more stablecomplexes thando monodentateligands.



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.



Chemistry of

Coordination

Compounds

Chelating Agents

Porphines (likechlorophyll a ) are

tetradentate ligands.



Chemistry of

Coordination

Compounds

Chelating Agents

• Porphyrins arecomplexes containing a

form of the porphinemolecule shown at theright.

• Important biomolecules

like heme andchlorophyll areporphyrins.



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.



Chemistry of

Coordination

Compounds


• 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.



Chemistry of

Coordination

Compounds


• 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.



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).



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.



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.



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.



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.



Chemistry of

Coordination

Compounds

Enantiomers

A molecule or ion that exists as a pair ofenantiomers is said to be chiral.



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.



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.



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.



Chemistry of

Coordination

Compounds

Complexes and Color

Thus, the wavelength of light observed in thecomplex is longer (closer to the red end of thespectrum).



Chemistry of

Coordination

Compounds

Complexes and Color

As the energy gap gets smaller, the lightabsorbed is of longer wavelength, andshorter-wavelength light is reflected.

ch 15 coordination compounds

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