Coordination complexes

COORDINATION COMPLEXES

* The d block metal form coordination complexes with molecules and ions

* 19.1 Coordination complexes

What is the electronic basis of the color of metal complexes?

Color and Magnetism

e- in partially filled d sublevel absorbs visible light moves to slightly higher energy d orbital
Magnetic properties due to unpaired electrons


* Coordination complex: A structure containing a metal (usually a metal ion) bonded (coordinated) to a group of surrounding molecules or ions.
Ligand (ligare is Latin, to bind): A ligand is a molecule or ion that is directly bonded to a metal ion in a coordination complex
Coordination sphere: A metal and its surrounding ligands
A ligand uses a lone pair of electrons (Lewis base) to bond to the metal ion (Lewis acid)

* Complex ions: Three common structural types

Octahedral: Most important
Tetrahedral
Square planar
What determines why a metal takes one of these shapes?

* Lewis acids and bases

A Lewis base is a molecule or ion that donates a lone pair of electrons to make a bond
A Lewis acid is a molecule of ion that accepts a lone pair of electrons to make a bond
Examples:
Examples:

* Lewis acid: Co3+

Lewis base:
Coordination complex: Lewis base (electron pair donor) coordinated to a Lewis acid (electron pair acceptor)
Coordination complex: Ligand (electron donor) coordinated to a metal (electron acceptor)
The formation of a coordinate complex is a Lewis acid-base reaction
The number of ligand bonds to the central metal atom is termed the coordination number



* The basic idea is that the ligand (Lewis base) is providing electron density to the metal (Lewis acid)
In terms of theory we visualize the coordination as the transfer of electrons from the orbital of the Lewis base to the lowest unoccupied orbital of the Lewis acid
Co3+
The bond from ligand to metal is covalent (shared pair), but both electrons come from the ligand (coordinate covalent bond)
Lewis base
Lewis acid

* Types of Ligands (electron pair donors: Monodentate (one tooth) Ligands

Latin: “mono” meaning one and “dens” meaning tooth Anions
Molecules with lone pairs

* Types of Ligands: Bidentate (two tooth) Ligands

Some common bidentate (chelates):
(en)

* Types of Ligands: Ethylenediaminetetraacetate ion (EDTA): a polydentate chelating ligand

Chelate from Greek chela, “claw” EDTA wraps around the metal ion at all 6 coordination sites producing an exceedingly tight binding to the metal



* Alfred Werner Switzerland University of Zurich Zurich, Switzerland b. 1866 (in Mulhouse, then Germany) d. 1919
The Nobel Prize in Chemistry 1913 "in recognition of his work on the linkage of atoms in molecules by which he has thrown new light on earlier investigations and opened up new fields of research especially in inorganic chemistry"
Alfred Werner: the father of the structure of coordination complexes


* (2) Brackets [] are used to indicate all of the atomic composition of the coordinate complex: the central metal atom and the ligands. The symbol for the central metal atom of the complex is first within the brackets
(3) Species outside of the [] are not coordinated to the metal but are require to maintain a charge balance
(1) A coordination compounds is a neutral species consisting of a coordinate complex and uncoordinated ions required to maintain the charge balance
[Co(NH3)6]Cl3
Composition of complex
Free species
[Co(NH3)6]3+
3 Cl-


* Werner’s explanation of coordination complexes Metal ions exhibit two kinds of valence: primary and secondary valences
The primary valence is the oxidation number (positive charge) of the metal (usually 2+ or 3+)
The secondary valence is the number of atoms that are directly bonded (coordinated) to the metal
The secondary valence is also termed the “coordination number” of the metal in a coordination complex


* Exemplar of primary and secondary valence: [Co(NH3)6]Cl3
[Co(NH3)6]3+
What is the atomic composition of the complex?
What is the net charge of the complex?
[Co(NH3)6]Cl3
How do we know the charge is 3+ on the metal?
3+ is required to balance the three Cl- ions
The secondary valence of [Co(NH3)6]Cl3 is
The primary valence of [Co(NH3)6]Cl3 is
3 (charge on Co3+)
6 (six ligands)

* Structures of Coordination Complexes: The ammonia complexes of Co(III) = Co3+

CoCl3.6NH3
CoCl3.5NH3
CoCl3.4NH3
CoCl3.3NH3
In all of these complexes there are no free NH3 molecules (No reaction with acid)
3 “free” Cl- ions Orange-Yellow
2 “free” Cl- ions Purple
1 “free” Cl- ions Green
0 “free” Cl- ions Green
How did Werner deduce the structure of coordination complexes?
Ions released
Composition
Color



* Compound 1: CoCl3.6NH3 = [Co(NH3)6]3+(Cl-)3 = [Co(NH3)6](Cl)3 Conclude: 3 free Cl- ions, complex = [Co(NH3)6]3+
Compound 2: CoCl3.5NH3 = [Co(NH3)5Cl]2+(Cl-)2 = [Co(NH3)5Cl](Cl)2 Conclude: 2 free Cl- ions, complex = [Co(NH3)5Cl]2+
Compound 3: CoCl3.4NH3 = [Co(NH3)4Cl2]1+(Cl-) = [Co(NH3)4Cl2](Cl) Conclude: 1 free Cl- ion, complex = [Co(NH3)4Cl2]1+
Compound 4: CoCl3.3NH3 = [Co(NH3)3Cl3] = complex No free Cl- ions, both Cl- and NH3 in sphere
“free” Cl- is not in sphere; all NH3 molecules are is in sphere

* CoCl3.6NH3

CoCl3.5NH3
CoCl3.4NH3
Isomers!
Coordination complexes: Three dimensional structures
Cl-
Cl-
Cl-
Cl-
Cl-
Cl-
Cl-
Bond toward you
Bond away from you

Coordination Compound

Consist of a complex ion and necessary counter ions [Co(NH3)5Cl]Cl2 Complex ion: [Co(NH3)5Cl]2+ Co3+ + 5 NH3 + Cl- = 1(3+) + 5 (0) + 1(1-) = 2+ Counter ions: 2 Cl-


K3[Fe(CN)6] K2[PtCl4] Na2[Fe(CO)4] [Co(H2O)2(NH3)4]Cl3 [Ni(H2O)(NH3)4]SO4 Na2[OsCl5N] [CoCl(NO2)(NH3)4]Cl [CoCl(NH2)(en)2]NO3 [FeH(CO)3(NO)] [PtCl(NH2CH3)(NH3)]Cl
NOW some for you to try!!!
Potassium hexacyanoferrate(III) Potassium tetrachloroplatinate(II) Sodium Tetracarbonylferrate(II) Tetraammindiaquacobalt(III) chloride Tetraaminediaquanickel(II) sulfate Sodium pentachloronitridoosmate(VI) Tetraaminechloronitritocobalt(III) chloride Amidochlorobis(ethylenediamine)cobalt(III) chloride Tricarbonylhydridonitrosyliron(I) ?(II)?? Amminchloro(methylamine) platinum(II) chloride

[V(H2O)6]2+

[V(H2O)6]3+
[Cr(NH3)6]3+
[Cr(NH3)5Cl]2+s

Linkage Isomers

[Co(NH3)5(NO2)]Cl2 Pentaamminenitrocobalt(III) chloride
[Co(NH3)5(ONO)]Cl2 Pentaamminenitritocobalt(III) chloride



Chromium
Chemical properties reflect oxidation state

* Water in outer sphere (water that is part of solvent)

Water in the inner sphere water (water is a ligand in the coordination sphere of the metal)
Hydrate isomers:

* Linkage isomers

Bonding to metal may occur at the S or the N atom
Example:
Bonding occurs from N atom to metal
Bonding occurs from S atom to metal

* Stereoisomers: geometric isomers (cis and trans)

Cl-
Cl-