Metal Complexes -- Chapter 24

Metal Complex -- consists of a set of ligands that are bonded to a central metal ion by coordinate covalent bonds.

e.g. Cu2+ + 4 L --> [CuL4]2+ ( L = NH3, H2O, etc)

Ligands are Lewis Bases and can be:– monodentate -- one donor atom

e.g. H2O, NH3, Cl–, OH–, etc.

– bidentate -- two donor atomse.g. ethylenediamine, NH2–CH2–CH2–NH2

“en”

M

NH2

CH2H2C

H2N

= H2N

M

NH2

Polydentate ligands– polydentate -- more than two donor atoms

e.g. EDTA -- ethylenediaminetetraacetic acid

(6 donor atoms)

N

H2C CH2

N

CH2 C

O

O

H2C C

O

O

H2CC

O

O

CH2C

O

O

= EDTA4–

Chelate EffectChelate effect -- complexes with bi- or polydentate ligands are

more stable than those with similar monodentate ligands

e.g. [Ni(en)3]3+ is more stable than [Ni(NH3)6]

3+

Ni

NH2

NH2

NH2

NH2

H2N

H2N

3+

Know Table 24.2!

Writing Formulas of Complex Ions

Metal ion first, then ligands. Charge outside brackets.total charge = sum of metal ion + ligands

e.g. metal ion ligand complex

Cu2+ H2O [Cu(H2O)]2+

Co3+ NH3 [Co(NH3)6]3+

Fe3+ CN– [Fe(CN)6]3–

Nomenclature• 1. Name the ligands, put in alphabetical order.• 2. Add prefixes for ligands that appear more than once.• 3. Name the metal

– Cationic complex; metal name– Anionic complex; metal prefix + ate

• 4. Add oxidation state in roman numerals• 5. As separate word, write “ion” if not neutral, or list

counterions (positive first, negative second).Examples:• Complex Name

[Ni(CN)4]2– tetracyanonickelate(II) ion

[CoCl6]3– hexachlorocobaltate(III) ion

[CoCl2(NH3)4]+ tetraaminedichlorocobalt(III) ion

Na3[Co(NO2)6] sodium hexanitrocobaltate(III)

[CrCl2(en)2]2SO4

dichlorobis(ethylenediamine)chromium(III) sulfate

Coordination Number and StructureCoordination # -- number of donor atoms attached to the

metal center(a) Two-Coordinate Complexes -- linear structures

Rare except for Ag+

e.g. [Ag(NH3)2]+ and [Ag(CN)2]

–

(b) Four-Coordinate Complexes -- two structural typesTetrahedral structures -- common for ions with filled

d subshells, e.g. Zn2+ as in [Zn(OH)4]2–

Zn

OH

HO

OH

OH

2

More Coordination Number and Structure

(b) Four-Coordinate Complexes -- two structural typessquare planar structures -- common for d8 metal ions (Ni2+,

Pd2+, Pt2+) and for Cu2+ e.g.

(c) Six-Coordinate Complexes -- the most common!“always” octahedral structures, e.g.

PtClCl

ClCl

2

CuNH3H3N

NH3H3N

2+

Ni

NH2

NH2

NH2

NH2

H2N

H2N

3+

Cr

Cl

Cl

NH3

ClNH3

NH3

Sample Problem1. Give the name or formula, as required. Draw the

structure of each complex.a) [AgI2]–

b) [Co(en)3]3+

c) cis-dichlorobis(ethylenediamine)cobalt(III) chlorided) sodium tetracyanonickelate(II)

Answer1. a) diiodoargentate(I) ion

[I—Ag—I]–

b) tris(ethylenediamine)cobalt(III) ion

c) cis-[CoCl2(en)2]Cl

c) Na2[Ni(CN)4]

Co

NH2

NH2

NH2

NH2NH2

NH2

3+

Co

Cl

NH2

Cl

NH2NH2

NH2

+

Cl

NiCNNC

CNNC

2

2 Na+

Structural Isomers of Coordination Complexes

• Linkage isomers

• Coordination isomers

Co

NH3

N

NH3

H3N

H3N

NH3

O

O

2+

Co

NH3

O

NH3

H3N

H3N

NH3

N O

2+

pentaamminenitrocobalt(II) ion pentaamminenitritocobalt(II) ion

Co

NH3

Cl

NH3

H3N

H3N

NH3

Br Co

NH3

Br

NH3

H3N

H3N

NH3

Cl

pentaamminechlorocobalt(II) bromide pentaamminebromocobalt(II) chloride

11

Isomerism; Overview

Stereoisomers of Coordination Complexes

(a) Geometrical Isomers

Cr

Cl

NH3

NH3

ClNH3

NH3

Cr

Cl

Cl

NH3

NH3NH3

NH3

cis trans

Cr

Cl

Cl

Cl

H3N

H3N

NH3

Cr

NH3

Cl

Cl

Cl

H3N

NH3

fac mer

(b) Enantiomers

Cr

NH2

Cl

NH2

ClNH2

NH2

Cr

NH2

Cl

NH2

NH2Cl

NH2

Three common ways to get enantiomers: --chiral ligand--tetrahedral complex with 4 different groups--octahedral complexes with chelating ligands (and no mirror plane)

Sample QuestionsDraw a clear, 3-dimensional structure of the geometrical

isomer of Co(en)(NH3)2Cl2 that is optically active. (define any abbreviations)

Excess silver nitrate is added to a solution containing 0.0522 mol of [Co(NH3)4Cl2]Cl. How many g of AgCl (FW = 143.3) will precipitate?

Sample QuestionsDraw a clear, 3-dimensional structure of the geometrical

isomer of Co(en)(NH3)2Cl2 that is optically active. (define any abbreviations)

Excess silver nitrate is added to a solution containing 0.0522 mol of [Co(NH3)4Cl2]Cl. How many g of AgCl (FW = 143.3) will precipitate?

Co

NH2

Cl

NH2

NH3Cl

NH3

Cr

NH2

Cl

NH2

ClNH3

NH3

7.48 g

Crystal Field Theory (Bonding in Transition Metal Complexes)

Metal complexes are usually highly colored and are often paramagnetic -- such facts can be explained by a “d-orbital splitting diagram”

energy

d orbitals of the free metal ion

dz2 dx2-y2

eg

= crystal field splitting energy

dxy dxz dyz

t2g

d orbitals of the metal ion in an octahedral field of ligands

Shape and Directionality of the d Orbitals

eg

t2g

Crystal Field StrengthThe size of depends on…• The nature of the ligand (ligand-metal bond strength!)

“spectrochemical series” -- decreases: CN– > NO2

– > en > NH3 > H2O > OH– > F– > Cl– > Br–

“strong field ligands” “weak field ligands”

• The oxidation state of the metal (charge!) is greater for M3+ than for M2+

• The row of the metal in the periodic table (size!)for a given ligand and oxidation state of the metal,

increases going down in a groupe.g. is greater in Ru(NH3)6

3+ than in Fe(NH3)63+

Colors of metal complexes are due to electronic transition between the t2g and eg energy levels

d Orbital Splitting Diagrams for Octahedral Complexes

eg

t2g t2g

eg

dx2 dx2–y2

dxy dxzdyz

dx2 dx2–y2

dxy dxz dyz

small “high spin”

large “low spin”

Fe(H2O)62+ Fe(CN)6

4–

High Spin vs. Low SpinCN– is a stronger field ligand than is H2O which leads to a

greater value (i.e. a greater d orbital splitting)As a result,

• Fe(H2O)62+ is a “high spin” complex and is

paramagnetic (4 unpaired electrons)while,

• Fe(CN)64– is a “low spin” complex and is diamagnetic

(no unpaired electrons)

The CN– complex with the larger value absorbs light of higher energy (i.e. higher frequency but shorter wavelength)

OMIT … d orbital splitting diagrams for other geometries (i.e. tetrahedral and square planar)

Sample QuestionsA complex [CoA6]3+ is red. The complex [CoB6]3+ is green.

a) Which ligand, A or B, produces the larger crystal field splitting ? b) If the two ligands are ammonia and water, which is A and which is B?c) Draw the d orbital diagram of either complex, assuming it is “high spin.” How many unpaired electrons will it have?

Give the order of increase of in the following sets:a) Cr(NH3)6

3+, CrCl63–, Cr(CN)63–

b) Co(H2O)62+, Co(H2O)6

3+, Rh(H2O)63+

Sample QuestionsA complex [CoA6]3+ is red. The complex [CoB6]3+ is green.

a) Which ligand, A or B, produces the larger crystal field splitting ? b) If the two ligands are ammonia and water, which is A and which is B?c) Draw the d orbital diagram of either complex, assuming it is “high spin.” How many unpaired electrons will it have?

a) Ab) A = NH3, B = H2O

c) 4 unpaired electrons

Give the order of increase of in the following sets:a) Cr(NH3)6

3+, CrCl63–, Cr(CN)6

3–

b) Co(H2O)62+, Co(H2O)6

3+, Rh(H2O)63+

a) Cr(CN)63– > Cr(NH3)6

3+ > CrCl63–

b) Rh(H2O)63+ > Co(H2O)6

3+ > Co(H2O)62+

dz2 dx2-y2

eg

dxy dxz dyz

t2g