Coordination Complexes and Transition Metals in Action

Al2O3 crystal with traces of Cr3+ (ruby)

Spring and summer –

chlorophyll and xanthophyll

Fall –

xanthophyll colors dominate

Plants and Animals

chlorophyll heme

Colors of Chromium

Cr3+ Cr6+

Cr(NO3)3 CrCl3 K2CrO4 K2Cr2O7

Coordination Compound and Complex

[Co(NH3)6]Cl3 [Co(NH3)6]3+ + 3 Cl-

Coordination Compound is [Co(NH3)6]Cl3

Coordination Complex is [Co(NH3)6]3+

Components of Complex (Coordination Sphere)

• Metal ion – usually transition metals with empty valence orbitals– Specifically empty d

orbitals

– Act as Lewis acid (electron pair acceptor)

• Ligand – complexing agent bound to (surrounding) the metal ion (Lewis base)– Normally ligands are anions

or polar molecules

– Anion (CN-)

– Polar molecule (NH3)

– Donor atom

[Co(NH3)6]3+

Characteristics of Complex (Coordination Sphere)

• Metal ion– Oxidation number

Co = ?

• Ligand– Charge on ligand

NH3 = ?

Charge of Complex – sum of charges on the central metal ion and the surrounding ligands

[Co(NH3)6]Cl3

What is the charge of the complex?

Coordination number – The number of donor atoms attached to the metal.

Example Problem

• Indicate the coordination number of the metal and the oxidation number of the metal in each of the following complexes:

Na2[CdCl4] [Co(NH3)4Cl2]Cl

K2[MoOCl4] [Zn(en)2]Br2

Types of Ligands

• Monodentate ligand NH3, H2O, Cl-

• Bidentate ligand ethylenediamine (en)

• Polydentate ligand ethylenediaminetetraacetate ion (EDTA)4-

Chelating Agent

• Polydentate ligands (including bidentate) are called chelating agents because they appear to grasp the metal between donor atoms

Example Ligands

What kind of ligands are these examples?

Chelate Effect

• Chelating agents form more stable complexes with metal ions than monodentate ligands

Ni2+ (aq) + 6NH3 (aq) [Ni(NH3)6]2+ (aq) Kf = 4 x 108

Ni2+ (aq) + 3 en (aq) [Ni(en)3]2+ (aq) Kf = 2 x 1018

Sequestering agents – because the chelating agents can be used to remove or separate ions

removal of ions from hard waterremoval of trace metals from food

removal of heavy metal ions from blood

Geometry of Complex

• Common coordination # of 4

Tetrahedral

Most common

square planar

Transition metal ions with 8 d electrons

Geometry of Complex

• Coordination # 6

Octahedral

Effect of Ligand on Coordination Number

• The larger the size of the ligand, the fewer ligands that can get close enough to bind to the central metal.

[FeF6]3- [FeCl4]-

• Ligands which provide negative charge to the complex reduce the coordination number.

[Ni(NH3)6]2+ [Ni(CN)4]2-

Metal Complexes

• Distinct chemical properties different from the metals and ligands from which they were formed.– Different colors– Different electrochemical properties– Different solubility properties

Nomenclature

1. In naming salts, the name of the cation is given before the name of the anion.

[Mo(NH3)3Br3]NO3

Cation Anion = nitrate

Nomenclature

2. Within a complex ion or molecule the ligands are named before the metal. Ligands are listed in alphabetical order, regardless of charge on the ligand. Prefixes that give the number of ligands are not considered part of the ligand name in determining alphabetical order.

[Mo(NH3)3Br3]NO3

Ammonia, bromide, molybdenum

Nomenclature

3. The names of the anionic ligands end in the letter o, whereas neutral ones ordinarily bear the name of the molecules.

[Mo(NH3)3Br3]NO3

Ammine, bromo, molybdenum

Example ligand names

NH3 – ammine

CO – carbonyl

NO - nitrosyl

H2O – aqua

CN- – cyano

en - ethylenediammine

Nomenclature

4. Greek prefixes (di, tri, tetra, penta, hexa) are used to indicate the number of each kind of ligand when more than one is present.

[Mo(NH3)3Br3]NO3

If the ligand itself contains a prefix, then these prefixes are used for the ligand name (bis-, tris-, tetrakis-, pentakis-, etc.).

[Ru(bipy)3]Cl3

triamminetribromomolybdenum

tris-bipyridineruthenium

Nomenclature

5. If the complex is an anion, its name ends in –ate.

K3[V(C2O4)3]

cation = potassium anion = trioxalatevanadate

Nomenclature

6. The oxidation number of the metal is given in parentheses in Roman numerals following the name of the metal.

[Mo(NH3)3Br3]NO3

[Ru(bipy)3]Cl3

K3[V(C2O4)3]

triamminetribromomolybdenum(IV) nitrate

tris-bipyridineruthenium(III) chloride

Potassium trioxalatevanadate(III)

Isomers

Several categories of isomers are possible in coordination complexes

Ionization isomer

Hydrate isomer

Structural Isomerism have the same numbers and kinds of atoms, but differ

in the bonds that are present. Ionization isomer – exchange of ligand with an anion or neutral

molecule [CoBr(NH3)5]SO4 and [CoSO4(NH3)5] Br

Hydrate isomer – the exchange of H2O molecule with another ligand [CrBr(H2O)6]Cl3 and [CrClBr(H2O)5]Cl2*H2O

Coordination isomers- differ due to the exchange of one or more ligands between a cationic complex and an anionic complex [Cr(NH3)6][Fe(CN)6]

[Fe(NH3)6][Cr(CN)6]

Linkage isomers- contain the same ligand coordinated to the metal through different donor atoms[Pd(NH3)3SCN]+ and [Pd(NH3)3NCS]+

Linkage Isomerism

Nitro Nitrito

Stereoisomerism

Geometric isomers have the same number and kinds of bonds, but differ in the relative positions of the ligands.

Optical isomers rotate the plane of polarized light in opposite directions.

Geometric Isomers

Geometric Isomers

Optical Isomers

Rotate the plane of polarized light in opposite directions.

Levrorotatory – rotate left

Dextrorotatory – rotate right

Optical Isomers

Chiral molecules have mirror-image structures that cannot be superimposed.

Only chiral molecules are optically active.

Enantiomers are chiral molecules of each other.

Racemic Mixture occurs when equal amounts of each enantiomer are mixed. When this happens, the optical activity of each is canceled by the other.

Enantiomers

Properties of Coordination Complexes

• Color – Many coordination complexes exhibit a wide variety of colors, that depend on the metal, its oxidation state, and the ligands present.

– The observed colors result from the absorption of light in the visible region by the complexes.

Color Exhibited ColorlessPartially filled d orbital Totally filled or empty

d orbital (d0 and d10)

Color Exhibited or Colorless

Color Exhibited

[Cr(NH3)6]3+

[Fe(SO4)(H2O)4]

Colorless

[Cd(NH3)4](NO3)2

Na[AlCl4]

Color of Complexes

Crystal Field Theory

Crystal field theory assumes electrostatic interactions between the negative or neutral ligands and the positive metal ion lower the energy of the system.– Anionic ligands – electrostatic attraction

– Neutral ligands – ion dipole

Lowered Energy of Metal/Ligand Complex

Crystal Field – repulsive interaction between electrons in the ligands and the d orbital electrons in the metal

Consequence:d electron repulsion

Crystal Field

• The negative ligands repel the electrons in the metal ion d orbitals.

• The repulsion energy of d electrons depends on the orientation of the orbital, relative to the location of the negative ligands.

d Orbitals

Transition Element Configurations

3d orbitals usedfor Sc - Zn

d Orbitals

Octahedral complex

d orbital splitting

= Energy gap – the energy necessary for an electron to move across the gap is similar to energy of a visible light photonExplains why d0 or d10

transition elements do not show color

Spectrochemical Series

Electron Configurations

Co3+ a d6 ion

Electron Configurations

• High spin

• Low spin

Weak Field ligand

Strong Field ligand

Spin pairing energy is the energy required to pair 2 electrons in orbital

Properties of Coordination Complexes

• Paramagnetism - a property due to unpaired electrons, is common among transition metal complexes.

– Different complexes of the same metal ion, may have different numbers of unpaired electrons.

Predict the magnetic properties of:

[Fe(H2O)6]2+ [Fe(CN)6]4-

Energy Calculation

• The complex [Ti(H2O)6]3+ absorbs light of wavelength 510 nm. What is the crystal field d orbital splitting energy () for the complex?

Metal and Oxidation #

• Color – Many coordination complexes exhibit a wide variety of colors, that depend on the metal, its oxidation state, and the ligands present.

[Cr(H2O)6]3+ [V(H2O)6]2+

The larger the charge on the metal ion involved in the complex, the more metal-ligand interaction. Therefore, will be larger when the oxidation state of the metal is larger.

Increased Interaction Between Metal and Ligand: Increased Crystal Field

Crystal Field – repulsive interaction between electrons in the ligands and the d orbital electrons in the metal

Crystal Field Theory Tetrahedral

Tetrahedral complex always have high spin because is small

Crystal Field Theory Square Planar

Square planar complexes always have high spin because is large

Tetrahedral and Square Planar

• Draw the crystal field splitting diagrams for [Ni(CN)4]2- and [NiCl4]2- and predict the magnetic properties of each.

Electron Configurations

• High spin

• Low spin

Weak Field ligand

Strong Field ligand

Spin pairing energy is the energy required to pair 2 electrons in orbital

Energy Calculation

• The complex [Ti(H2O)6]3+ absorbs light of wavelength 510 nm. What is the crystal field d orbital splitting energy () for the complex?

c = E = h = c/ = E = (6.63 x 10-34 Js) (5.88 x 1014 s-1) = 3.00 x 108 m/s______ E = 3.90 x 10-19 J

nm) ___1 m___ E = (3.90 x 10-19 J) _1 kJ_ x 109 nm 1000 J

= 5.88 x 1014 s-1 E = 3.90 x 10-22 kJ/photon

E = (3.90 x 10-22 __kJ__) (6.022 x 1023 photons) = 235 _kJ_

photon mole mole

Spectroscopy

Sources of Electromagnetic Radiation

Window Material

Wavelength Selector

Detectors