Coordination Compounds

Peculiar compounds of transition metals

Coordination Compounds

• Transition metals have s, d and p orbitals all available for bonding

• Don’t obey the octet rule • They are most stable with filled d, s and p orbitals– s2d10p6 (18 e-)

• Transition metals act like a Lewis acid (electron pair acceptor) so as to fill valence orbitals

• Transition metals will bond with Lewis bases (e- pair donors) – species with lone pairs, these are called ligands

Transition Metal Complexes

Most often these complexes are octahedral or tetrahedral in shape with the metal at the center

Here we see • Cl- • F- • H2O • NH3

are behaving as ligands

Ligands• Lewis bases that bind with transition metals are also called ligands

• Some ligands bind once to the metal (monodentate)– NH3, H2O, CO, Cl-, Br-, I-, CN-, SCN-

• Some bind twice (bidentate)– oxalate, ethylenediamine, salicylate

• Some three times (tridentate)– diethylenetriamine

• Some are even hexadentate– Ethylenediaminetetraacetic acid

Coordination Number of TM ions

Transition Metals will have a coordination number (CN) that helps get them as close to 18 valence electrons as possible (can go higher up to 20 or lower)

• Cu2+ [Ar] 3d9 *CN = 4 (total 17 e-)• Cr3+ [Ar] 3d3 CN = 6 (total 15 e-)• Fe3+ [Ar] 3d5 CN = 6 (total 17 e-)• Fe2+ [Ar] 3d6 CN = 6 (total 18 e-)

• Ni2+ [Ar] 4s23d8 CN = 4 (total 18 e-)• Co2+ [Ar] 3d7 CN = 6 (total 15 e-)

• Mn2+ [Ar] 3d5 CN = 6 (total 17 e-)• Zn2+ [Ar] 3d10 CN = 4 (total 18 e-)

MO Diagram Octahedral Complex (CN = 6)

d-orbitals split, and the gap is responsible for

the color of many TM complexes

Jahn-Teller Distortion in Cu2+

• In some cases like [Cu(NH3).2H2O]2+ an distorted octahedron is more stable than CN=4

more stable

Color of TM Complexes• In transition metal complexes the d-orbitals (essentially non-

bonding) split in energy• Electrons in the lower d-orbitals can absorb visible light and go

into an unoccupied d-orbital

free ion

octahedron

distortedoctahedron

Cu2+

• Lowest energy transition is determined by Δoct

Color of TM Complexes• In transition metal complexes the d-orbitals (essentially non-

bonding) split in energy• Electrons in the lower d-orbitals can absorb visible light and go

into an unoccupied d-orbital

Weak and Strong Ligands

• The size of the splitting Δoct depends on the type of ligand

• The stronger the ligand metal bond the larger Δoct is

large Δoct

Small Δoct

The Experiment

• Part A: To make [Cu(NH3)4]SO4.H2O(s)

• Part B: To determine y in CuLy where L = ethylenediamine, diethylenetriamine, salicylate, and Ethylenediaminetetraacetic acid

• To determine the spectrochemical series for these ligands L

Part A: Synthesis of [Cu(NH3)4]SO4.H2O(s)

• 3g Copper (II) sulfate pentahydrate • Add 15 mL H2O (dissolve solid)

• Add 2.5x calculated volume of conc. NH3 (fume hood)

• Add 25 mL ethanol to reduce solubility• Place in ice water for 10 mins• Filter out solid using a Buchner funnel• Wash in ammonia/ethanol• Allow to dry till next lab

Part B: Determining y in CuLy• One of the objectives of this experiment is to determine y for different ligands L

that complex with Cu2+

• We will do this using Job’s method• First find a strong absorption wavelength λmax for the CuLy • The mole fraction x=[L]/[Cu2+] is varied from small to large while the intensity of

the color of the solution is measured at that wavelength λmax

• when y = x the color will have the largest absorbance

Complex Ligand

[Cu(dien)y]2+ dien=ethylenediamine

[Cu(trien)y]2+ trien = diethylenetriamine

[Cu(EDTA)y]2-4y EDTA = Ethylenediaminetetraacetic acid

[Cu(sali)y]2-y sali = salicylate