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-) Jahn Teller CN=6 also

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 Cu2+ 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 distorted 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 Small Δoct large 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