1

Ch. 23: Transition metals and Coordination Chemistry

Learning goals and key skills:

Determine the oxidation number and number of d

electrons for metal ions in complexes

Name coordination compounds given their formula

and write the formula given their name

Recognize and draw geometric isomers of a complex

Recognize and draw isomers of a complex

Use crystal-field theory to explain the colors and to

determine the number of unpaired electrons in a

complex.

2

3

Co2+ Ni2+ Cu2+ Zn2+

4

5

Coordination compounds• A central metal atom bonded to a group of

molecules or ions is a metal complex.• If the complex bears a charge, it is a complex

ion.• Compounds containing complexes are

coordination compounds.• The molecule or ions that bond to the metal ion

are known as ligands.

• The central metal and its ligands constitute the coordination sphere.

• The donor atom is the atom directly bound to the metal.

• The coordination number is the number of atoms directly bonded to the central atom.

Metal-Ligand Bond• Metal: Lewis acid (has empty orbitals)• Ligand: Lewis base (has nonbonding electrons)

• The metal complex has distinct physical and chemical properties (color, reduction potentials, etc.) from the metal ion and ligand ions.

6

Oxidation NumberKnowing the charge on a complex ion and the charge on each ligand, one can determine the oxidation number for the metal.

Knowing the oxidation number on the metal and the charges on the ligands, one can calculate the charge on the complex ion.

Find the charge on a complex ion with Cr, four water molecules and two chloride ions.

[Cr(H2O)4Cl2]+

7

Geometry and Coordination Number

Coordination numberGeneral trends:

Lower coordination numbers are found with

• Larger ligands[FeCl4]– vs [FeF6]3–

• Higher charged ligands[Ni(CN)4]2– vs [Ni(NH3)6]2+

8

Polydentate Ligands• Monodentate: “one tooth”• Bidentate: “two teeth”

• Polydentate: “many-toothed”• Chelating agents: “claw”

EDTA4-

9

ethylenediamine

ethylenediamine (en) is a bidentate ligand.

ethylenediaminetetraacetate ion

EDTA is a polydentate

ligand with six donor atoms.

10

Porphyrins: Important biomolecules like chlorophyll and heme are porphyrins.

Nomenclature of Coord. Comp.• Cation is 1st; anion is 2nd

• Prefixes: di-, tri-, tetra-, penta-, hexa-[or bis-, tris-, tetrakis-, pentakis-, for complex

ligands in parenthesis].• Within the complex ion:

– ligands 1st then metal– list ligands (not prefixes) alphabetically

• anions end in “o”; neutral ones use normal names– exceptions: aqua, ammine, carbonyl

• complex anions end in –ate• oxidation number of the metal is in Roman numerals

Pentaaquachlorochromium (III) chloridePotassium hexacyanoferrate (III)

11

Nomenclature

Example:

Isomers:same composition, different arrangement of atoms

12

Structural isomers: different bonds

Coordination sphere isomer

CrCl3·6H2O chromium (III) chloride hexahydrate

[Cr(H2O)6]Cl3 hexaaquachromium(III) chloride

[Cr(H2O)5Cl]Cl2·H2O

pentaaquachlorochromium(III) chloride monohydrate

[Cr(H2O)4Cl2]Cl·2H2O

tetraaquadichlorochromium(III) chloride dihydrate

Linkage isomers[Co(NH3)5(NO2)]2+

If a ligand (like the NO2 group at the bottom of the complex) can bind to the metal with one or another atom as the donor atom, linkage isomers are formed.

13

Common Ligands in Linkage Isomers

nitrito nitro

cyano isocyano

isocyanato cyanato

isothiocyanato thiocyanato

Stereoisomers:same bonds, different spatial arrangements

geometric isomers:cis-Isomers have like groups on the same side.trans-Isomers have like groups on opposite sides.

14

Stereoisomers: optical isomers/entaniomers: mirror images

Two enantiomers cannot be superimposed on each other. These are chiral molecules.

Enantiomers are optically active• The physical properties of chiral molecules are

the same except in instances where the spatial placement of atoms matters.

• One example is the interaction of a chiral molecule with plane-polarized light.

15

Enantiomers: D and L

D – dextrorotatory (to the right)L – levorotatory (to the left)racemic: a mixture of the two

Amino acids in proteins are L-isomers.

D-Amino acids have been found in small peptides of bacterial cell walls and some peptide antibiotics.

Complexes and color

The color arises from the fact that the complex absorbssome wavelengths of visible light and reflects others.

16

Complexes and color•A substance appears black if it absorbs all the visible lightthat strikes it, whereas if a substance absorbs no visible light it is white or colorless. •An object appears green if it reflects all colors except red, the complementary color of green.•Similarly, this concept applies to transmitted light that pass through a solution.

Complexes and color

[Ti(H2O)6]3+ appears red-violet in color because it absorbs light at the center (yellow and green)parts of the spectrum.

17

Metal–ligand bond formation, the ligand acts as a Lewis base and the metal acts as a Lewis acid.

Crystal field splittingWhen negative charges are brought up to the ion, the average energy of the d orbitals increases. • Because the repulsion felt by the dz2 and dx2–y2

orbitals is greater than that felt by the dxy, dxz, and dyz orbitals, the five d orbitals split into a lower-energy set of three (the t2 set) and a higher-energy set of two (the e set).

18

The d-d transition in [Ti(H2O)6]3+ is produced by the absorption of 495-nm light.

Complexes and colorInteractions between electrons on a ligand and the orbitals on the metal cause differences in energies between orbitals in the complex.

19

Electron Configurations in Octahedral Complexes

The magnitude of the energy gap, D, increases by a factor of two from the left to the right of the spectrochemical series below.

increasing D →

Cl- < F- < H2O < NH3 < en < NO2- < CN-

weak-field ligands

strong-field ligands

20

Tetrahedral complexes

Square planar complexes

d8

21

Charge-transfer transitions