Transition Metal Coordination Compounds

Transition Metals Valence electrons in a d subshell. Form cations not anions. Generally solids, except for Mercury (Hg) Form coloured compounds

: : : : M : : L L L L L L Coordination Complexes M = metal L = Ligand A metal cation acts as a Lewis acid. A Lewis acid is a substance that can accept a pair of electrons from another atom to form a new bond. The electron pairs which are received come from surrounding groups which are called ligands. Ligands are typically either neutral or anionic atoms or molecules. Ligands act as Lewis bases. A Lewis base is a substance that can donate a pair of electrons to another atom to form a new bond. The combination of a metal cation and all its ligands is called a coordination complex L : : M = metal L Accepts electrons ie. Lewis acid : : L M L : L = Ligand L : donates electrons ie. Lewis base L

Examples of coordination complexes Consider the Lewis diagrams of the ligands H2O, Cl-, and CO One lone pair per donor atom can be donated to a “naked” metal ion, for example Fe2+ in water Copper(II) ions in the presence of a high concentration of chloride ion forms a chloro complex: Nickel when finely divided reacts readily with an atmosphere of gaseous carbon monoxide to form the liquid nickel tetracarbonyl: Note that coordination complexes may be overall neutral, cationic or anionic! Ionic complexes require counter ions to form salts of complex ions Fe2+ + 6 H2O  [Fe(OH2)6]2+ Cu2+ + 4 Cl-  [CuCl4]2- Ni + 4 CO  [Ni(CO)4]

Hemoglobin The Heme (the porphyrin in hemoglogin) molecule has chains branching off the porphyrin ring.

Variable Oxidation States Transitions elements are known to have many oxidation states COMMON occurrences are in burgundy The elements in the middle are capable of existing in many oxidation states, while the ones at either end have fewer possibilities The flasks hold examples of Cr(III) (violet and green) and Cr(VI) (yellow and orange) d electrons can be added or removed at relatively little cost

Chelating ligands A molecule such as the oxallate anion: Two of its four oxygen atoms (red lone pairs in diagram) to donate to a metal, forming a ring This is called chelation, from the word for a crab’s claw Whereas iron(III) has room in the primary coordination sphere for six water molecules in the complex ion [Fe(OH2)6]3+, it only has room for three oxalate ions in the complex ion [Fe(C2O4)3]3-.

The Trioxallatoferrate(II) complex The picture below gives three examples of six-coordinate metal ions complexed by three bidentate (= two toothed) ligands The one on the left is the [Fe(C2O4)3]3- ion that you will prepare and then crystallize as the K3[Fe(C2O4)3]·3H2O salt – the salt crystals have the beautiful luminescent green colour The crystals are light sensitive, and must be stored in the dark The orange is an ethylenediamine complex of Co3+ that is overal cationic, while the burgandy solid is the overall neutral triacetylacetonatochromium(III) complex

Electronic Structure and Colour of Transition Metal Coordination Compounds Transition metal complexes are very often coloured, whereas the metals and metalloids of the s and p blocks form colourless complexes. Consider the aqueous solutions of nitrate salts of Fe3+, Co2+, Ni2+, Cu2+ and Zn2+ shown in the following photograph. Why are four of them coloured, while the last is colourless? Consider the electron configurations of the ions Fe3+ Co2+ Ni2+ Cu2+ Zn2+

D orbitals in an octahedral ligand field In the geometry octahedral the dx2-y2 and dz2 point directly at the ligands, the others do not While all the d electrons are repulsed by the ligand lone pairs, those that point directly at the ligands are repulsed more, leading to the octahedral ligand field splitting as follows:

Origin of colour Consider our Fe(III) complex, with electron configuration There are thus five d electrons, and there are two choices on how to distribute them, as follows: high spin low spin When the donor atom is oxygen, the result is always high spin for reasons beyond this course Photons can be absorbed that promote electrons from the lower to the upper levels of this electron configuration When that happens, light is absorbed, and in fact more than one way of absorbing a photon is possible, leading to absorption at several different wavelengths The green colour of the complex is the net result from absorbing the other wavelengths of light

Spectrophotometer A spectrophotometer (such as the Spectronic 20) measures absorption of light energy as a function of altered wavelength (dial on the instrument) and the results are graphed For the iron complex, this leads to: