Chemistry of Coordination Compounds (Chapter 24)

Chemistry of Coordination Compounds

(Chapter 24)

The Structure of ComplexesThe Structure of Complexes

• We know Lewis acids are electron pair acceptors.• Coordination compounds: metal compounds formed by

Lewis acid-base interactions.• Complexes: Have a metal ion (can be zero oxidation

state) bonded to a number of ligands. Complex ions are charged. Example, [Ag(NH3)2]+.

• Ligands are Lewis bases.• Square brackets enclose the metal ion and ligands.• Coordination Sphere: The area of space encompassing

the ligands and metal ion.

The Structure of ComplexesThe Structure of Complexes

• Ligands can alter properties of the metal:Ag+(aq) + e- Ag(s), E = +0.799 V

[Ag(CN)2]-(aq) + e- Ag(s) + 2CN-(aq), E = -0.031 V

• Most metal ions in water exist as [M(H2O)6]n+.

Charges, Coordination Numbers and GeometriesCharges, Coordination Numbers and Geometries• Charge on complex ion = charge on metal + charges on

ligands.• Donor atom: the atom bonded directly to the metal.• Coordination number: the number of ligands attached to

the metal.

The Structure of ComplexesThe Structure of ComplexesCharges, Coordination Numbers and Charges, Coordination Numbers and

GeometriesGeometries– Most common coordination numbers are 4 and 6.– Some metal ions have constant coordination number (e.g. Cr3+ and Co3+ have

coordination numbers of 6).– The size of the ligand affects the coordination number (e.g. [FeF6]3- forms but only

[FeCl4]- is stable).– The amount of charge transferred from ligand to metal affects coordination number

(e.g. [Ni(NH3)6]2+ is stable but only [Ni(CN)4]2- is stable).• Four coordinate complexes are either tetrahedral or square planar (commonly seen for

d8 metal ions).• Six coordinate complexes are octahedral.

ChelatesChelates

• Monodentate ligands bind through one donor atom only.– Therefore they occupy only one coordination site.

• Polydentate ligands (or chelating agents) bind through more than one donor atom per ligand.– Example, ethylenediamine (en), H2NCH2CH2NH2.

• The octahedral [Co(en)3]3+ is a typical en complex.• Chelate effect: More stable complexes are formed with

chelating agents than the equivalent number of monodentate ligands.

ChelatesChelates

ChelatesChelates[Ni(H2O)6]2+(aq) + 6NH3 [Ni(NH3)6]2+(aq) + 6H2O(l)

Kf = 4 108

[Ni(H2O)6]2+(aq) + 3en [Ni(en)3]2+(aq) + 6H2O(l)

Kf = 2 1018

• Sequestering agents are chelating agents that are used to remove unwanted metal ions.

• In medicine sequestering agents are used to selectively remove toxic metal ions (e.g. Hg2+ and Pb2+) while leaving biologically important metals.

• One very important chelating agent is ethylenediaminetetraacetate (EDTA4-).

ChelatesChelates

ChelatesChelates

• EDTA occupies 6 coordination sites, for example [CoEDTA]- is an octahedral Co3+ complex.

• Both N atoms (blue) and O atoms (red) coordinate to the metal.

• EDTA is used in consumer products to complex the metal ions which catalyze decomposition reactions.

ChelatesChelates

Metals and Chelates in Living SystemsMetals and Chelates in Living Systems• Many natural chelates are designed around the

porphyrin molecule.• After the two H atoms bound to N are lost, porphyrin is a

tetradentate ligand.• Porphyrins: Metal complexes derived from porphyrin.• Two important porphyrins are heme (Fe2+) and

chlorophyll (Mg2+).• Myoglobin is protein containing a heme unit, which

stores oxygen in cells.

ChelatesChelatesMetals and Chelates in Living SystemsMetals and Chelates in Living Systems

ChelatesChelatesMetals and Chelates in Living SystemsMetals and Chelates in Living Systems• A five membered nitrogen containing ring binds

the heme unit to the protein.• When oxygen is attached to the iron(II) in heme,

oxymyoglobin is formed. • The protein has a molecular weight of about

18,000 amu.• The Fe2+ ion in oxyhemoglobin or oxymyoglobin is

octahedral.• Four N atoms from the porphyrin ring (red disk)

are attached to the Fe2+ center.


ChelatesChelatesMetals and Chelates in Living SystemsMetals and Chelates in Living Systems• The fifth coordination site is occupied by O2 (or H2O in

deoxyhemoglobin or CO in carboxyhemoglobin).• The sixth coordination site is occupied by a base, which

attaches the structure to the protein.• Photosynthesis is the conversion of CO2 and water to

glucose and oxygen in plants in the presence of light.• One mole of sugar requires 48 moles of photons.• Chlorophyll absorbs red and blue light and is green in

color.• The pigments that absorb this light are chlorophylls.


ChelatesChelatesMetals and Chelates in Living SystemsMetals and Chelates in Living Systems• Chlorophyll a is the most abundant chlorophyll. • The other chlorophylls differ in the structure of

the side chains.• Mg2+ is in the center of the porphyrin-like ring.• The alternating double bonds give chlorophyll its

green color (it absorbs red light).• Chlorophyll absorbs red light (655 nm) and blue

light (430 nm).


ChelatesChelates

Metals and Chelates in Living SystemsMetals and Chelates in Living Systems

• The reaction6CO2 + 6H2O C6H12O6 + 6O2

is highly endothermic. • Plant photosynthesis sustains life on Earth.

NomenclatureNomenclature• Rules:

– For salts, name the cation before the anion. Example in [Co(NH3)5Cl]Cl2 we name [Co(NH3)5Cl]2+ before Cl-.

– Within a complex ion, the ligands are named (in alphabetical order) before the metal. Example [Co(NH3)5Cl]2+ is tetraamminechlorocobalt(II). Note the tetra portion is an indication of the number of NH3 groups and is therefore not considered in the alphabetizing of the ligands.

– Anionic ligands end in o and neutral ligands are simply the name of the molecule. Exceptions: H2O (aqua) and NH3 (ammine).

NomenclatureNomenclature

NomenclatureNomenclature• Rules:

– Greek prefixes are used to indicate number of ligands (di-, tri-, tetra-, penta-, and hexa-). Exception: if the ligand name has a Greek prefix already. Then enclose the ligand name in parentheses and use bis-, tris-, tetrakis-, pentakis-, and hexakis.

• Example [Co(en)3]Cl3 is tris(ethylenediamine)cobalt(III) chloride.

– If the complex is an anion, the name ends in -ate.– Oxidation state of the metal is given in Roman

numerals in parenthesis at the end of the complex name.

IsomerismIsomerism

• Isomers: two compounds with the same formulas but different arrangements of atoms.

• Coordination-sphere isomers and linkage isomers: have different structures (i.e. different bonds).

• Geometrical isomers and optical isomers are stereoisomers (i.e. have the same bonds, but different spatial arrangements of atoms).

• Structural isomers have different connectivity of atoms.• Stereoisomers have the same connectivity but different

spatial arrangements of atoms.

IsomerismIsomerism

IsomerismIsomerismStructural IsomerismStructural Isomerism• Some ligands can coordinate in different ways.• That is, the ligand can link to the metal in different ways.• These ligands give rise to linkage isomerism.• Example: NO2

- can coordinate through N or O (e.g. in two possible [Co(NH3)5(NO2)]2+ complexes).

• When nitrate coordinates through N it is called nitro.– Pentaamminenitrocobalt(III) is yellow.

• When ONO- coordinates through O it is called nitrito.– Pentaamminenitritocobalt(III) is red.

IsomerismIsomerismStructural IsomerismStructural Isomerism

IsomerismIsomerismStructural IsomerismStructural Isomerism• Similarly, SCN- can coordinate through S or N.

– Coordination sphere isomerism occurs when ligands from outside the coordination sphere move inside.

– Example: CrCl3(H2O)6 has three coordination sphere isomers: [Cr(H2O)6]Cl3 (violet), [Cr(H2O)5Cl]Cl2

.H2O (green), and [Cr(H2O)4Cl2]Cl.2H2O (green).

StereoisomerismStereoisomerism• Consider square planar [Pt(NH3)2Cl2].

• The two NH3 ligands can either be 90 apart or 180 apart.• Therefore, the spatial arrangement of the atoms is different.

IsomerismIsomerismStereoisomerismStereoisomerism

IsomerismIsomerismStereoisomerismStereoisomerism• This is an example of geometrical isomerism.• In the cis isomer, the two NH3 groups are adjacent. The

cis isomer (cisplatin) is used in chemotherapy.• The trans isomer has the two NH3 groups across from

each other.• It is possible to find cis and trans isomers in octahedral

complexes. • For example, cis-[Co(NH3)4Cl2]+ is violet and trans-

[Co(NH3)4Cl2]+ is green.• The two isomers have different solubilities.


IsomerismIsomerismStereoisomerismStereoisomerism• In general, geometrical isomers have different physical

and chemical properties.• It is not possible to form geometrical isomers with

tetrahedra. (All corners of a tetrahedron are identical.)• Optical isomers are mirror images which cannot be

superimposed on each other.• Optical isomers are called enantiomers.• Complexes which can form enantiomers are chiral.• Most of the human body is chiral (the hands, for

example).


IsomerismIsomerismStereoisomerismStereoisomerism• Enzymes are the most highly chiral substances known.• Most physical and chemical properties of enantiomers

are identical.• Therefore, enantiomers are very difficult to separate.• Enzymes do a very good job of catalyzing the reaction of

only one enantiomer.• Therefore, one enantiomer can produce a specific

physiological effect whereas its mirror image produces a different effect.


IsomerismIsomerism

StereoisomerismStereoisomerism• Enantiomers are capable of rotating the plane of

polarized light.• Hence, they are called optical isomers.• When horizontally polarized light enters an optically

active solution.• As the light emerges from the solution, the plane of

polarity has changed.• The mirror image of an enantiomer will rotate the plane

of polarized light by the same amount in the opposite direction.

IsomerismIsomerism

StereoisomerismStereoisomerism• Dextrorotatory solutions rotate the plane of polarized

light to the right. This isomer is called the d-isomer.• Levorotatory solutions rotate the plane of polarized light

to the left. This isomer is called the l-isomer. • Chiral molecules are optically active because of their

effect on light.• Racemic mixtures contain equal amounts of l- and d-

isomers. They have no overall effect on the plane of polarized light.

IsomerismIsomerism

StereoisomerismStereoisomerism• Pasteur was the first to separate racemic

ammonium tartarate (NaNH4C4H9O6) by crystallizing the solution and physically picking out the “right-handed” crystals from the mixture using a microscope.

• Optically pure tartarate can be used to separate a racemic mixture of [Co(en)3]Cl3: if d-tartarate is used, d-[Co(en)3]Cl3 precipitates leaving l-[Co(en)3]Cl3 in solution.

Color and MagnetismColor and Magnetism

ColorColor• Color of a complex depends on: (i) the metal and (ii) its

oxidation state.• Pale blue [Cu(H2O)6]2+ can be converted into dark blue

[Cu(NH3)6]2+ by adding NH3(aq).• A partially filled d orbital is usually required for a

complex to be colored.• So, d0 metal ions are usually colorless. Exceptions:

MnO4- and CrO4

2-.• Colored compounds absorb visible light.• The color perceived is the sum of the light not absorbed

by the complex.

Color and MagnetismColor and MagnetismColorColor

Color and MagnetismColor and MagnetismColorColor• The amount of absorbed light versus wavelength is an

absorption spectrum for a complex.• To determine the absorption spectrum of a complex:

– a narrow beam of light is passed through a prism (which separates the light into different wavelengths),

– the prism is rotated so that different wavelengths of light are produced as a function of time,

– the monochromatic light (i.e. a single wavelength) is passed through the sample,

– the unabsorbed light is detected.


Color and MagnetismColor and Magnetism

ColorColor• The plot of absorbance versus wavelength is the

absorption spectrum.• For example, the absorption spectrum for

[Ti(H2O)6]3+ has a maximum absorption occurs at 510 nm (green and yellow).

• So, the complex transmits all light except green and yellow.

• Therefore, the complex is purple.


Color and MagnetismColor and MagnetismMagnetismMagnetism• Many transition metal complexes are

paramagnetic (i.e. they have unpaired electrons).• There are some interesting observations.

Consider a d6 metal ion:– [Co(NH3)6]3+ has no unpaired electrons, but [CoF6]3-

has four unpaired electrons per ion.• We need to develop a bonding theory to account

for both color and magnetism in transition metal complexes.

Crystal-Field TheoryCrystal-Field Theory• Crystal field theory describes bonding in transition metal

complexes.• The formation of a complex is a Lewis acid-base reaction.• Both electrons in the bond come from the ligand and are

donated into an empty, hybridized orbital on the metal.• Charge is donated from the ligand to the metal.• Assumption in crystal field theory: the interaction

between ligand and metal is electrostatic.• The more directly the ligand attacks the metal orbital,

the higher the energy of the d orbital.

Crystal-Field TheoryCrystal-Field Theory

Crystal-Field TheoryCrystal-Field Theory• The complex metal ion has a lower energy than the

separated metal and ligands.• However, there are some ligand-d-electron repulsions

which occur since the metal has partially filled d-orbitals.• In an octahedral field, the degeneracy of the five d

orbitals is lifted.• In an octahedral field, the five d orbitals do not have the

same energy: three degenerate orbitals are higher energy than two degenerate orbitals.

• The energy gap between them is called , the crystal field splitting energy.



• We assume an octahedral array of negative charges placed around the metal ion (which is positive).

• The dz2 and dx2-y2 orbitals lie on the same axes as negative charges. – Therefore, there is a large, unfavorable interaction between ligand (-) and these

orbitals.– These orbitals form the degenerate high energy pair of energy levels.

• The dxy, dyz, and dxz orbitals bisect the negative charges.– Therefore, there is a smaller repulsion between ligand and metal for these orbitals.– These orbitals form the degenerate low energy set of energy levels.



• The energy gap is the crystal field splitting energy .• Ti3+ is a d1 metal ion.• Therefore, the one electron is in a low energy orbital.• For Ti3+, the gap between energy levels, is of the order of the wavelength of visible

light.• As the [Ti(H2O)6]3+ complex absorbs visible light, the electron is promoted to a higher

energy level.• Since there is only one d electron there is only one possible absorption line for this

molecule.• Color of a complex depends on the magnitude of which, in turn, depends on the metal

and the types of ligands.




Crystal-Field TheoryCrystal-Field Theory• Spectrochemical series is a listing of ligands in order of

increasing :Cl- < F- < H2O < NH3 < en < NO2

- (N-bonded) < CN-

• Weak field ligands lie on the low end of the spectrochemical series.

• Strong field ligands lie on the high end of the spectrochemical series.

• As Cr3+ goes from being attached to a weak field ligand to a strong field ligand, increases and the color of the complex changes from green to yellow.


Crystal-Field TheoryCrystal-Field TheoryElectron Configurations in Octahedral Electron Configurations in Octahedral

ComplexesComplexes• We still apply Hund’s rule to the d-orbitals.• The first three electrons go into different d orbitals with

their spins parallel.• Recall: the s electrons are lost first.• So, Ti3+ is a d1 ion, V3+ is a d2 ion and Cr3+ is a d3 ion.• We have a choice for the placement of the fourth

electron:– if it goes into a higher energy orbital, then there is an

energy cost ();



Crystal-Field TheoryCrystal-Field TheoryElectron Configurations in Octahedral Electron Configurations in Octahedral

ComplexesComplexes– if it goes into a lower energy orbital, there is a

different energy cost (called the spin-pairing energy due to pairing an electron).

• Weak field ligands tend to favor adding electrons to the higher energy orbitals (high spin complexes) because < pairing energy.

• Strong field ligands tend to favor adding electrons to lower energy orbitals (low spin complexes) because > pairing energy.

Crystal-Field TheoryCrystal-Field TheoryTetrahedral and Square-Planar ComplexesTetrahedral and Square-Planar Complexes• By using the same arguments as for the octahedral case,

we can derive the relative orbital energies for d orbitals in a tetrahedral field.

• The exact opposite of an octahedral field orbital arrangement is found (i.e. the dxy, dyz, and dxz orbitals are of lower energy than the and orbitals).

• Because there are only 4 ligands, for a tetrahedral field is smaller than for an octahedral field (four ninths).

• This causes all tetrahedral complexes to be high spin.


Crystal-Field TheoryCrystal-Field TheoryTetrahedral and Square-Planar ComplexesTetrahedral and Square-Planar Complexes• Square planar complexes can be thought of as follows:

start with an octahedral complex and remove two ligands along the z-axis.

• As a consequence the four planar ligands are drawn in towards the metal.

• Relative to the octahedral field, the orbital is greatly lowered in energy, the dyz, and dxz orbitals lowered in energy, the dxy, and orbitals are raised in energy.

• Most d8 metal ions for square planar complexes.– the majority of complexes are low spin (i.e.

diamagnetic).– Examples: Pd2+, Pt2+, Ir+, and Au3+.


Documents

Chemistry of Coordination Compounds (Chapter 24)