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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
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



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



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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