Metal complexes chapter 24
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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. Cu 2+ + 4 L --> [CuL 4 ] 2+ ( L = NH 3 , H 2 O, etc) Ligands are Lewis Bases and can be:

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Metal Complexes -- Chapter 24

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Metal complexes chapter 24

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 atoms

    e.g. ethylenediamine, NH2–CH2–CH2–NH2 “en”


Polydentate ligands

Polydentate ligands

  • polydentate -- more than two donor atoms

    e.g. EDTA -- ethylenediaminetetraacetic acid

    (6 donor atoms)

= EDTA4–


Chelate effect

Chelate Effect

Chelate 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+

Know Table 24.2!


Writing formulas of complex ions

Writing Formulas of Complex Ions

Metal ion first, then ligands. Charge outside brackets.

total charge = sum of metal ion + ligands

e.g. metal ionligandcomplex

Cu2+ H2O [Cu(H2O)]2+

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

Fe3+ CN– [Fe(CN)6]3–


Nomenclature

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:

  • ComplexName

    [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]2SO4dichlorobis(ethylenediamine)chromium(III) sulfate


Coordination number and structure

Coordination Number and Structure

Coordination # -- 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 types

Tetrahedral structures -- common for ions with filled d subshells, e.g. Zn2+ as in [Zn(OH)4]2–


More coordination number and structure

More Coordination Number and Structure

(b) Four-Coordinate Complexes -- two structural types

square 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.


Sample problem

Sample Problem

1. 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) chloride

d) sodium tetracyanonickelate(II)


Structural isomers of coordination complexes

Structural Isomers of Coordination Complexes

  • Linkage isomers

  • Coordination isomers

pentaamminenitrocobalt(II) ion

pentaamminenitritocobalt(II) ion

pentaamminechlorocobalt(II) bromide

pentaamminebromocobalt(II) chloride


Metal complexes chapter 24

Isomerism; Overview


Stereoisomers of coordination complexes

Stereoisomers of Coordination Complexes

(a) Geometrical Isomers

cis

trans

mer

fac

(b) Enantiomers

Three common ways to get enantiomers:

--chiral ligand

--tetrahedral complex with 4 different groups

--octahedral complexes with chelating ligands (and no mirror plane)


Sample questions

Sample Questions

Draw 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 questions1

Sample Questions

Draw 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?

7.48 g


Crystal field theory

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”

eg

dz2

dx2-y2

energy

D = crystal field splitting energy

t2g

dxy

dxz

dyz

d orbitals of the metal ion in

an octahedral field of ligands

d orbitals of the free metal ion


Shape and directionality of the d orbitals

Shape and Directionality of the d Orbitals

eg

D

t2g


Crystal field strength

Crystal Field Strength

The size of D depends on…

  • The nature of the ligand (ligand-metal bond strength!)

    “spectrochemical series” -- D decreases:

    CN– > NO2– > en > NH3 > H2O > OH– > F– > Cl– > Br–

    “strong field ligands” “weak field ligands”

  • The oxidation state of the metal (charge!)

    D 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, D increases going down in a group

    e.g. D is greater in Ru(NH3)63+ than in Fe(NH3)63+

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


D orbital splitting diagrams for octahedral complexes

d Orbital Splitting Diagrams for Octahedral Complexes

eg

dx2

dx2–y2

eg

dx2

dx2–y2

large D

“low spin”

small D

“high spin”

t2g

t2g

dyz

dxy

dxz

dxy

dxz

dyz

Fe(H2O)62+

Fe(CN)64–


High spin vs low spin

High Spin vs. Low Spin

CN– is a stronger field ligand than is H2O which leads to a greater D 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 D 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 questions2

Sample Questions

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

a) Which ligand, A or B, produces the larger crystal field splitting D?

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 D in the following sets:

a) Cr(NH3)63+, CrCl63–, Cr(CN)63–

b) Co(H2O)62+, Co(H2O)63+, Rh(H2O)63+


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