Transition Metals and Coordination Complexes

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Coordination Compounds

A coordinate covalent bond is a pair of electrons

from a donor shared with an acceptor.

Coordinate covalent bonds frequently are formed in Lewis

acid-base reactions.

Example: ammonia and boron trifluoride.

The boron of BF3 has an empty valence orbital which is able to accept the electron pair donated by NH3.

N

H

H

H B

F

F F

N

H

H

H

B

F

F

F

Ammine Complexes

The ammine complexes contain NH3molecules bonded to metal ions by coordinate covalent bonds, e.g., [Cu(NH3)4]

2+.

Dilute aqueous NH3 reacts with metal ions to form the insoluble metal hydroxides or hydrated oxides.

The exceptions to this trend are metals that form strong, water soluble hydroxides.

Group IA cations and the heavier Group IIA cations, Ca2+, Sr2+, and Ba2+.

Ammine Complexes

Cu and Fe both react with aqueous ammonia to form

hydroxides.

Metal hydroxides dissolve in excess aqueous NH3 to

form ammine complexes.

Properties of Coordination Compounds

The charge on a complex is the sum of its

constituent charges (or oxidation states)

Determine the charge of the metal in

[Pt(NH3 )6 ]4+

[SnCl6 ]2-

[Co(CO)5 NO2]SO4

Dot formula suggest coordination compounds

CrCl3 6H2O also means [CrCl2(OH2)4]Cl 2H2O

PtCl4 6NH3 also means [Pt(NH3)6]Cl4

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Important Terms

A ligand is a Lewis base that coordinates to a

central metal atom or ion.

A donor atom is the atom in a ligand that

donate a lone pair of electrons to form a

coordinate covalent bond.

A unidentate ligand is a ligand that can bind

through only one atom.

Ion/Molecule Name

Name as a

Ligand

NH3 ammonia ammine

COcarbon

monoxidecarbonyl

NOnitrogen

monoxidenitrosyl

PH3 phosphine phosphine

C5H5N pyridine pyridine

CH3NH2 Methylamine Methyl ammine

Typical Neutral Unidentate Ligands

Ion/Molecule Name

Name as a

Ligand

CN- cyanide cyano

F- fluoride fluoro

NO2- nitrite nitrito

OH- hydroxide hydroxo

Typical Anionic Unidentate Ligands Important Terms

A polydentate ligand is a ligand that can bind

through more than one donor atom.

Bidentate oxalate (ox, C2O42-) , ethylenediamine (en)

Tridentate diethylenetriamine (dien)

hexadentate ethylenediamine tetraacetic acid (EDTA)

Chelate complexes are complexes that have a

metal atom or ion and polydentate ligand(s)

that form rings.

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Important Terms

A coordination sphere includes the metal atom or ion and the ligands coordinated to it. The coordination sphere does not include uncoordinated counter ions.

The coordination number is the

number of donor atoms coordinated

to a metal atom or ion.

Nomenclature

Rules for Naming Complex Species

1. Cations (+ ions) are named before anions (- ions)

2. In a complex, name ligands first then metal cation.

3. Ligands are named in alphabetical order.

Prefixes that specify the number of each kind of monodentate ligand (di = 2, tri = 3, tetra = 4, penta = 5, hexa = 6, etc.) are not used in alphabetizing

Prefixes that are part of the name of the ligand, such as in diethylamine, are used to alphabetize the ligands.

How would you name [CrCl2(H2O)4 ]NO3?

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Nomenclature

3. For polydentate chelating ligands, these prefixes are used to specify the number of those ligandsthat are attached to the central atom.

bis = 2

tris = 3

tetrakis = 4

pentakis = 5

hexakis = 6

[Co(en)3]3+ - tris(ethylenediamine) NOT tri(ethylenediamine)

[Co(en)2 Cl2 ]+ - dichlorobis(ethylenediamine) NOT dichlorobi(ethylenediamine)

Nomenclature

4. The names of most anionic ligands end in the suffix -o. Examples of ligands ending in o are: Cl- chloro

S2- sulfido

O2- oxo

OH- hydroxo

CN- cyano

NO3- nitrato

SO42- sulfato

S2O32- thiosulfato

CNS- thiocyanato

Nomenclature

5. The names of most neutral ligands are

unchanged when used in naming the

complex.

There are several important exceptions to this

rule including:

NH3 ammine

H2O aqua

CO carbonyl

NO nitrosyl

Nomenclature

6. The oxidation number of a metal that exhibits variable oxidation states is designated by a Roman numeral in parentheses following the name of the complex ion or molecule.

7. If a complex is an anion, the suffix "ate" ends the name. No suffix is used in the case of a neutral or cationic

complex. Usually, the English stem is used for a metal; the Latin

stem is substituted for those in Latin. ferrate instead of ironate plumbate instead of leadate Argentate for Ag Stannate for Sn

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Name the following compounds:

[Ni(NH3)4(OH2)2](NO3)2tetraamminediaquanickel(II) nitrate

K4[Fe(CN)6]

potassium hexacyanoferrate (IV)

[Co(SCN)(NH3)5]Cl2pentaamminethiocyanato-S-cobalt(III) chloride

Write formula for the following compounds:

potassium hexacyanochromate(III)

K3[Cr(CN)6]

tris(ethylenediammine) cobalt(III) nitrate

[Co(en)3] (NO3)3

sodium hexachloroferrate (III)

Na3[Fe(Cl)6]

Structures

The structures of coordination compounds are

controlled primarily by the coordination

number of the metal.

Usually the structures can be predicted by

VSEPR theory

The geometries and hybridizations for common

coordination numbers are summarized in this

table.

Coordination

Number Geometry

Metal

Hybridization Example

2 linear sp + dz2 Ag[NH3)2]+

4 tetrahedral sp3 + dyz + dxz [Zn(CN)4]2-

4 square planarsp3d2 + (dx2-y2 +

dz2)[Ni(CN)4]

2-

5trigonal

bipyramidsp3d

Fe(CO)5

5Square

pyramidal

sp3d2 + (dx2-y2 +

dz2)

[Ni(CN)5]3-

6 octahedral sp3d2 [Fe(CN)6]4-

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Structure and Function

Isomerism in Coordination Compounds

Structural (Constitutional) Isomers

Structural isomers involve different atom to

ligand bonding sequences.

Ionization or Ion-Ion Exchange Isomers

[Pt(NH3)4Cl2]Br2 compared to [Pt(NH3)4Br2]Cl2

Note where the Cls and Brs are in the structures,

that is what makes these two species isomers.


[Pt(NH3)4Cl2]Br2 [Pt(NH3)4Br2]Cl2

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Hydrate isomers (Solvation isomerism) are a special case of ionization isomers in which water molecules may be changed from inside to outside the coordination sphere.

For example:

[Cr(OH2)6]Cl3 vs.

[Cr(OH2)5Cl]Cl2. H2O vs.

[Cr(OH2)4Cl2]Cl2. 2H2O

Note whether the water molecule(s) are inside or outside the coordination sphere.


[Cr(OH2)6]Cl3 [Cr(OH2)5Cl]Cl2.

H2O [Cr(OH2)4Cl2]Cl2.

2H2O


Coordination isomers denote an exchange of

ligands between the coordination spheres of

the cation and anion.

For example look at these two isomers:

[Pt(NH3)4][PtCl6] vs [Pt(NH3)4Cl2][PtCl4]

The isomeric distinction is whether the ligands

are on the cation or the anion.


[Pt(NH3)4][PtCl6]

[Pt(NH3)4Cl2][PtCl4]

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Linkage isomers have ligands that bind to the

metal in more than one way.

cyano -CN- compared to isocyano -NC-

nitro -NO2- compared to nitrito -ONO-

For example:

[Co(NH3)5ONO]Cl2 vs. [Co(NH3)5NO2]Cl2

Note which atom in the ligand is bound to the

central metal atom.

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[Co(NH3)5ONO]Cl2 [Co(NH3)5NO2]Cl2

Stereoisomers

Stereoisomers are isomers that have different spatial arrangements of the atoms relative to the central atom.

Complexes with only simple ligands can occur as stereoisomers only if they have coordination numbers equal to or greater than four.

Stereoisomers

Geometrical or positional isomers are

stereoisomers that are not optical isomers.

Cis-trans isomers have the same kind of ligand

either adjacent to each other (cis) or on the

opposite side of the central metal atom from

each other (trans).

Note where the ligands are positioned relative

to the central atom.

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Stereoisomers

cis- [Pt(NH3)2Cl2] trans-[Pt(NH3)2Cl2]

Stereoisomers

Other types of isomerism can occur in

octahedral complexes.

Complexes of the type [MA2B2C2] can occur in

several geometric isomeric forms:

trans- trans- trans-

cis- cis- cis-

cis- cis- trans-

Stereoisomers

trans-diammine-trans-diaqua-trans-dichloroplatinum(IV) ion

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Stereoisomers

cis-diammine-cis-diaqua-cis-dichloroplatinum(IV) ion

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Stereoisomers

trans-diammine-cis-diaqua-cis-dichloroplatinum(IV) ion

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Stereoisomers

cis-diammine-trans-diaqua-cis-dichloroplatinum(IV) ion

Mer-riamminetrichlorocobalt (III) Fac-riamminetrichlorocobalt (III)

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Stereoisomers

Optical isomers are mirror images of each other

that are not superimposable.

The cis-diammine-cis-diaqua-cis-

dichlorocobalt(III) ion has two different forms

called optical isomers or enantiomers.

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Stereoisomers

These are the optical isomers of:

cis-diammine-cis-diaqua-cis-dichloroplatinum (IV) ion

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Stereoisomers

Separate equimolar solutions of the two

isomers rotate plane polarized light by equal

angles but in opposite directions.

The phenomenon of rotation of polarized light is

called optical activity.

Delta cis-

dichlorobis(ethylenediamine)cobalt (III)

Lambda cis-

dichlorobis(ethylenediamine)cobalt (III)

Show all four isomers of [Co(NH3)3(NO2)3].

Determine the number and the types of isomers

in

(i) dichlorobis(ethylenediamine)platinum (IV) chloride

(ii) tris(ethylenediamine) chromium (III)

Draw the correct structure of

Triaqua-cis-dibromochlorochromium (III)

Optical isomers of cis-

diamminebis(ethylenediamine)cobalt (III)

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Compounds of Transition metal complexes solution.

[Fe(H2O)6]3+

[Co(H2O)6]2+

[Ni(H2O)6]2+

[Cu(H2O)6]2+

[Zn(H2O)6]2+

Black & White

If a sample absorbs all wavelength of visible light, none reaches our eyes from that sample. Consequently, it appears black.

When a sample absorbs light, what we see is the sum of the remaining colors that strikes our eyes.

If the sample absorbs novisible light, it is white or colorless.

Absorption and ReflectionIf the sample absorbsall but orange, thesample appears orange.

Further, we also perceive orange color when visible light of all colors except blue strikes our eyes. In a complementary fashion, if the sample absorbed only orange, it would appear blue; blue and orange are said to be complementary colors.

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Bonding in Coordination Compounds

Crystal Field Theory

Crystal field theory provides a satisfactory

explanation of the color and magnetic

properties of coordination compounds.

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Crystal Field Theory Crystal field theory treats the ligands as point charges and considers the

effect of these point charges on the relative energies of the d orbitals.

The five d orbitals can be divided into two subsets.

The dz2 and dx2-y2 orbitals called the eg orbitals

They are directed along the x, y, and z axes.

The dxy, dxz, and dyz orbitals called the t2g orbitals

These orbitals are directed between the x, y, and z axes.

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Crystal Field Theory

In an octahedral coordination complex, the ligands

approach the central metal along the x, y, and z axes.

There is a more repulsive environment for electrons in the

eg orbitals (on x, y, and z axes) than for electrons in the t2gorbitals (in between the axes).

An electric field (provided by the crystal field) splits

the degeneracy of the five d orbitals into:

1. Two higher energy orbitals (eg)

2. And three lower energy orbitals (t2g).

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Crystal Field Theory The energy separation between the two sets of d orbitals

is the crystal field splitting energy - oct.

octahedral is proportional to the crystal field strength of the

ligands.

Color and the Spectrochemical Series The spectrochemical series reflects the ligand arrangement

given on the previous slide.

Ligand field strength is proportional to the crystal field splitting.

Strong field ligands cause large crystal field splitting

Consequently, more energetic radiation needed to excite electron

Weak field ligands cause small crystal field splitting

Less energetic radiation needed to excite electron

Weak field,

absorbs low

energy radiation

eg. RED

Strong filed,

high energy

radiation eg.

Violet56

Magnetic Properties and

Spectrochemical Series Strong field ligands cause large crystal field splitting, which lead to low

spin complexes (more diamagnetic)

Weak field ligands cause small crystal field splitting, which lead to high

spin complexes (more paramagnetic)

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Octahedral, Tetrahedral & Square

PlanarCF Splitting pattern for various molecular geometry

M

dz2dx2-y2

dxzdxy dyz

M

dx2-y2 dz2

dxzdxy dyz

M

dxz

dz2

dx2-y2

dxy

dyz

Octahedral

TetrahedralSquare planar

Pairing energy Vs. Pairing energy Vs.

Weak field < Pe

Strong field > Pe

Small High SpinSmall High SpinMostly d8Mostly d8

(Majority Low spin)

Strong field ligands

i.e., Pd2+, Pt2+, Ir+, Au3+

Color and the

Spectrochemical Series

Name the compound K4[MnF6].

potassium hexafluoromanaganate (II)

What are its geometry, magnetic properties, and hybridization at Mn?

Name the compound [Mn(NH3)6]Cl2.

What are its geometry, magnetic properties, and hybridization at

Mn?

Transition Metals and Coordination Complexes

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