Transition Metals and Coordination Complexes

Transition Metals & Coordination Compounds

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Coordination CompoundsCoordination CompoundsA 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. An example of a coordinate covalent bond is

the one formed between ammonia and boron trifluoride.

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

Ammine ComplexesAmmine Complexes

The ammine complexes contain NH3 molecules 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+.

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Ammine ComplexesAmmine ComplexesCu and Fe both react with aqueous

ammonia to form hydroxides.

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• Metal hydroxides dissolve in excess aqueous NH3 to form ammine complexes.

Properties of Coordination Properties of Coordination CompoundsCompoundsThe 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

Important TermsImportant 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.

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Ion/Molecule NameName as a

Ligand

NH3 ammonia ammine

COcarbon

monoxidecarbonyl

NOnitrogen

monoxidenitrosyl

PH3 phosphine phosphine

Typical Neutral Unidentate LigandsTypical Neutral Unidentate Ligands

Ion/Molecule NameName as a

Ligand

CN- cyanide cyano

F- fluoride fluoro

NO2- nitrite nitro

OH- hydroxide hydroxo

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Typical Anionic Unidentate LigandsTypical Anionic Unidentate Ligands


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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The coordination number is the number of donor atoms coordinated to a metal atom or ion.

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

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NomenclatureNomenclatureRules for Naming Complex Species

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

2. 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.

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NomenclatureNomenclature

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

◦ bis = 2 ◦ tris = 3◦ tetrakis = 4◦ pentakis = 5◦ hexakis = 6

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[Co(en)3]3+ - tris(ethylenediamine) NOT tri(ethylenediamine)[Co(en)2 Cl2 ]+ - dichlorobis(ethylenediamine) NOT dichlorobi(ethylenediamine)


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 SO4

2- sulfato S2O3

2- thiosulfato CNS- thiocyanato

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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

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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


Name the following compounds:Na3[Fe(Cl)6]

sodium hexachloroferrate (III)

[Ni(NH3)4(OH2)2](NO3)2

tetraamminediaquanickel(II) nitrate

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Write formulas for the following compounds:potassium hexacyanochromate(III)

K3[Cr(CN)6]

tris(ethylenediammine) cobalt(III) nitrate

[Co(en)3] (NO3)3

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StructuresStructures

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.

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StructuresStructuresCoordination

Number GeometryMetal

Hybridization Example

2 linear sp Ag[NH3)2]+

4 tetrahedral sp3 [Zn(CN)4]2-

4 square planar dsp2 or sp2d [Ni(CN)4]2-

5trigonal

bipyramiddsp3

Fe(CO)5

5Square

pyramidald2sp2

[Ni(CN)5]3-

6 octahedral d2sp3 or sp3d2 [Fe(CN)6]4-

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Structure and FunctionStructure and Function

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Isomerism in Coordination Isomerism in Coordination CompoundsCompounds

Structural (Constitutional) IsomersStructural 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 Cl’s and Br’s are in the

structures, that is what makes these two species isomers.

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Structural (Constitutional) Structural (Constitutional) IsomersIsomers

[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.

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[Cr(OH2)6]Cl3 [Cr(OH2)5Cl]Cl2. H2O [Cr(OH2)4Cl2]Cl2

. 2H2O

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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.

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[Pt(NH3)4][PtCl6]

[Pt(NH3)4Cl2][PtCl4]


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

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[Co(NH3)5NO2]Cl2

StereoisomersStereoisomers

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.

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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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cis- [Pt(NH3)2Cl2]

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trans-[Pt(NH3)2Cl2]


Isomerism due to changes in spatial orientation is known as sterioisomerism

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-

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

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

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

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

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Mer-triamminetrichlorocobalt (III)

Fac-triamminetrichlorocobalt (III)


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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These are the optical isomers of:cis-diammine-cis-diaqua-cis-dichloroplatinum (IV) ion

StereoisomersStereoisomersSeparate 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.

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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)

Compounds of Transition metal complexes solution.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 Black & & WhiteWhite

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

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

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

Absorption and ReflectionAbsorption and ReflectionIf the sample absorbsIf the sample absorbsall but all but orangeorange, the, thesample appears sample appears orange.orange.

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

Bonding in Coordination Bonding in Coordination CompoundsCompounds

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 TheoryCrystal Field TheoryCrystal field theory treats the ligands as point charges

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

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• 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.

Crystal Field TheoryCrystal 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 t2g orbitals (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 TheoryCrystal Field TheoryThe 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 Color and the Spectrochemical SeriesSeriesThe 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.

Violet

Magnetic Properties and Magnetic Properties and Spectrochemical SeriesSpectrochemical 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 Octahedral, Tetrahedral & Square PlanarPlanar

CF Splitting pattern for CF Splitting pattern for various molecular geometryvarious molecular geometry

M

dz2dx2-y2

dxzdxydyz

M

dx2-y2 dz2

dxzdxy dyz

M

dx

z

dz2

dx2-y2

dxy

dyz

OctahedralOctahedralTetrahedralTetrahedral Square Square

planarplanar

Pairing energy Vs. Weak field < Pe

Strong field > Pe

Small High Spin

Mostly d8

(Majority Low spin)Strong field ligandsi.e., Pd2+, Pt2+, Ir+, Au3+

Color and the Color and the Spectrochemical SeriesSpectrochemical Series

Name the compound K4[MnF6]. potassium hexafluoromanganate (II)

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

The 6 fluoride ions are in an octahedral geometry.

There will be 5 unpaired electrons in a high spin complex.

The Mn atom has sp3d2 hybrid orbitals.

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Color and the Color and the Spectrochemical SeriesSpectrochemical Series

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

manganese (II) hexaamminechlorideWhat are its geometry, magnetic properties, and

hybridization at Mn?

The 6 ammonia molecules are in an octahedral geometry.

There will be 1 unpaired electron in a low spin complex

The Mn atom is d2sp3 hybridized.

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Transition Metals and Coordination Complexes