Coordination Chemistry I (1)

7/29/2019 Coordination Chemistry I (1)

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Coordination Chemistry I:

Structures and Isomers

Chapter 9


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Coordination Compounds

Coordination compounds

compounds composed of a

metal atom or ion and oneor more ligands.

[Co(Co(NH3)4(OH2)3]Br6

Ligands usually donate

electrons to the metal Includes organometallic

compoundsWerners totally inorganic

optically active compound.


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Werners Coordination Chemistry

Performed systematic studies to understand bondingin coordination compounds.

Organic bonding theory and simple ideas of ionic chargeswere not sufficient.

Two types of bonding

Primarypositive charge of the metal ion is balanced bynegative ions in the compound.

Secondarymolecules or ion (ligands) are attached directlyto the metal ion.

Coordination sphere or complex ion.

Look at complex on previous slide (primary and secondary)


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Werners Coordination Chemistry

He largely studied compounds with four or sixligands.

Octahedral and square-planar complexes.

It was illustrated that a theory needed to accountfor bonds between ligands and the metal.

The number of bonds was commonly more thanaccepted at that time.

18-electron rule.

New theories arose to describe bonding.

Valence bond, crystal field, and ligand field.


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Chelating Ligands

Chelating ligands

(chelates)ligands that

have two or more pointsof attachment to the

metal atom or ion.

Bidentate, tridentate,

tetra.., penta, hexa(EDTA).

trisoxalatochromate(III) ion or just [Cr(ox)3]3-


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A Hexadentate Ligand, EDTA

There are six points of

attachment to the calcium

metal. Octahedral-type geometry

ethylene diamine tetraacetic acid

(EDTA)

ethylenediaminetetraacetatocalcium ion or just [Ca(EDTA)]2-


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Nomenclature

The positive ion (cation) comes first, followed by thename within the coordination sphere, followed by thenegative ion (anion).

These ions are not in the coordination sphere. Diamminesilver(I)chloride and potassium hexacyanoferrate

(III).

The inner coordination sphere is enclosed in brackets in

the formula. Within this sphere, the ligands are namedbefore the metal, but in formulas the metal ion iswritten first.

Tetraamminecopper(II) sulfate and hexaamminecobalt(III)chloride.


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Nomenclature

The number of ligands isgiven by the following

prefixes. If the ligandname includes prefixesor is complicated, it isset off in parenthesesand the second set of

prefixes is used. [Co(en)2Cl2]

+ and[Fe(C5H4N-C5H4N)3]

2+

2 di bis

3 tri tris4 tetra tetrakis

5 penta pentakis

6 hexa hexakis7 hepta heptakis

8 octa octakis


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Nomenclature

Ligands are named in alphabetical order(name of ligand, not prefix)

[Co(NH3)4Cl2]+ and [Pt(NH3)BrCl(CH3NH2)]+2

Anionic ligands are given an o suffix.Neutral ligands retain the usual name.

Coordianted water is called aqua.

Chloro, Cl-

Sulfato, SO42-


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Nomenclature

The calculated oxidation number of the metal ion isplaced as a Roman numeral in parentheses after thename of the coordination sphere.

[Pt(NH3)4]+2 and [Pt(Cl)4]

-2

A suffix ate is added to the metal ion if the charge isnegative.

The prefixes cis- and trans- designate adjacent andopposite geometric location, respectively.

trans-diamminedichloroplatinum(III) and cis-tetraamminedichlorocobalt(III)


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Nomenclature

Bridging ligands between two metal ions

have the prefix .

-amido--hydroxobis(tetraamminecobalt)(IV)

There is an error in this picture. What is it?


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Isomerism

Our discussion of isomers will be largely

limited to those with the same ligands arranged

in different geometries. This is referred to asstereoisomers.


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Isomerism

Four-coordinate complexes

Square-planar complexes may have

cis and trans isomers. No chiral

isomers (enantiomers) are possiblewhen the molecule has a mirror

plane.

cis- and trans-

diamminedichloroplatinum(II)

How about tetrahedral complexes?

Chelate rings commonly impose a

cis structure. Why


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Chirality

Mirror images are nonsuperimposable.

A molecule can be chiral if it has no rotation-reflection

axes (Sn) Chiral molecules have no symmetry elements or only

have an axes of proper rotation (Cn).

CBrClFI, Tetrahedral molecule (different ligands)

Octahedral molecules with bidentate or higher chelatingligands

Octahedral species with [Ma2b2c2], [Mabc2d2], [Mabcd3],[Mabcde2], or [Mabcdef]


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Six-Coordinate Octahedral

Complexes ML3L3

Fac isomers have three

identical ligands on thesame face.

Mer isomers have three

identical ligands in a plane

bisecting the molecule.


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Six-Coordinate Octahedral

Complexes The maximum number of isomers can be

difficult to calculate (repeats).

Placing a pair of ligands in the notation indicates that a and b are trans to each other.

[M], [Pt]

How many diastereoisomers in the aboveplatinum compound (not mirror images)?

Identify all isomers belonging to Ma3bcd.


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Determining the Number of

Isotopes Bailar method

With restrictions (such as chelating agents)

some isomers may be eliminated.

Determine and identify the number if

isomers.

[Ma2b2cd] and [M(AA)bcde]


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Combinations of Chelate Rings

Propellers and helices

Left- and right-handed propellers

Examine the movement of a propeller required tomove it in a certain direction.

For a left-handed propeller, rotating it ccw would cause

it to move away ().

For a right-handed propeller, rotating it cw would causeit to move away ().

This is called handedness. Many molecules possess it.


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Tris(ethylenediamine)cobalt(III)

This molecule can be treated like a three-bladed propeller.

Look down a three fold axis to determinethe handedness of this complex ion.

The direction of rotation required to pull themolecule away from you determines the

handedness ( or).

Do this with you molecule set and rubberbands.


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Determining Handedness for

Chiral Molecules Complexes with two or more nonadjacent chelate

rings may have chiral character.

Any two noncoplanar and nonadjacent chelate rings canbe used.

Look at Figure 9-14 (Miessler and Tarr).

Molecules with more than one pair of rings mayrequire more than one label.

Ca(EDTA)2+

Three labels would be required.

Remember that the chelate rings must be noncoplanar,nonadjacent, and not connected at the same atom.


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Linkage (ambidentate) Isomerism

A few ligands may bond to the metal throughdifferent atoms.

SCN- and NO2-

How would you expect hard acids to bond to thethiocyanate ligand?

Solvents can also influence bonding.

High and low dielectric constants. Steric effects of linkage isomerism

Intramolecular conversion between linkages.

[Co(NH3)5NO2]+2, Figure 9-19.


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Separation and Identification of

Isomers Geometric isomers can be separated by fractional

crystallization with different counterions.

Due to the slightly different shapes of the isomers.

The fit of the counterion can greatly influence

solubility.

Solubility is the lowest when the positive and negativecharges have the same size and magnitude of charges

(Basolo).


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Separation and Identification of

Chiral Isomers Separations are performed with chiral

counterions. The resulting physical properties

will differ allowing separation. Rotation of polarized light will be opposite for

two chiral isomers at a specific wavelength.

The direction of optical rotation can change withwavelength.


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Circular Dichroism Meaurement

The difference in the absorption of right and leftcircularly polarized light is measured.

Where land

rare the molar absorption

coefficients for left and right circularly polarizedlight.

The light received by the detector is presentedas the difference between the absorbances.

Figure 9-20.

rldichroismCircular


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Plane-Polarized Light Measurement

The plane of polarization is rotated when passing

through a chiral substance.

Caused by a difference in the refractive indices of theright and left circularly polarized light.

The optical rotation illustrates positive value on oneside of the adsorption maximum and negative side on

the other. This is termed as the Cotton effect.

rl


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Coordination Numbers and

Structures Factors considered when determining structures.

The number of bonds. Bond formation is

exothermic; the more the better.VSEPR arguments

Occupancy ofdorbitals.

Steric interference by large ligands.Crystal packing effect.

It may be difficult to predict shapes.


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Low Coordination Numbers (C.N.)

C.N. 1 is rare except in ion pairs in the gas phase.

C.N. 2 is also rare.

[Ag(NH3)2]+, Ag is d10 (how?) VSEPR predicts a linear structure.

Large ligands help force a linear or near-linear arrangment.

[Mn(N[SiMePh2]2)2] in Figure 9-22.

C.N. 3 is more likely with d10 ions.

Trigonal-planar structure is the most common.

[Cu(SPPh3)3]+, adopts a low C.N. due to ligand crowding.


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Coordination Number 4

Tetrahedral and square planar complexes are

the most common.

Small ions and/or large ligands prevent highcoordination numbers (Mn(VII) or Cr(VI)).

Many d0 ord10 complexes have tetrahedral

structures (only consider bonds).MnO4

- and [Ni(CO)4]

Jahn-Teller distortion (Chapter 10)


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Square-planar geometry

d8 ions (Ni(II), Pd(II), and Pt(III))

[Pt(NH3)2Cl2]

The energy difference between square-planar

and tetrahedral structures can be quite small.

Can depend on both the ligand and counterion.

More in chapter 10.


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Common structures are trigonal bipyramid andsquare pyramid.

The energy difference between the two is small. In

many measurements, the five ligands appear identicaldue to fluxional behavior.

How would you modify the experiment to differentiatebetween the two structures?

Five-coordinate compounds are known for the fullrange of transition metals.

Figure 9-27.


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This is the most common C.N. with the

most common structure being octahedral.

If the delectrons are ignored, this is thepredicted shape.

[Co(en)3]3+

This C.N. exists for all transition metals (d0

to d10).


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Distortions of Complexes

Containing C.N. 6 Elongation and compression (Fig. 9-29).

Produces a trigonal antiprism structure when the angle

between the top and bottom triangular faces is 60.

Trigonal prism structures are produced when the faces

are eclipsed.

Most trigonal prismatic complexes have three bidentate ligands

(Figure 9-30).

interactions may stabilize some of these structures.

The Jahn-Teller effect (Ch. 10) is useful in predicting

observed distortions.


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Higher Coordination Numbers

C.N. 7 is not common

C.N. 8

There are many 8-coordinate complexes for

large transition elements.

Square antiprism and dodecahedron

C.N.s up to 16 have been observed.

Coordination Chemistry I (1)

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