Preparation of Bis Acetylacetonao Copper 2

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  • Topic: Ligand, Ammonia, Lone pair
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  • Published : February 17, 2013
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Experiment 6

Title: Preparation of bis(acetylacetonato)copper(II) complex

Objective: To synthesize the bis(acetyacetonato) copper(II) complex

Theory/Background:

All metal ions in solution are 'associated' with water. The water molecules can also be weakly bonded or more strongly as a ligand to form a complex ion, and these can also present in solid 'hydrated' salts of crystallization. E.g Copper(II) nitrate (Cu(NO3) 2.3H2O).

A complex ion has a metal ion at its centre with a number of other molecules or ions surrounding it. These can be considered to be attached to the central ion by co-ordinate (dative covalent) bonds. The molecules or ions surrounding the central metal ion are called ligands. Simple ligands include water, ammonia, acetyl acetone and chloride ions. Ligand has active lone pairs of electrons in the outer energy level. These are used to form co-ordinate bonds with the metal ion. All ligands are lone pair donors. In other words, all ligands function as Lewis bases.

Lewis acid-base theory reminders:
A base is an electron pair donor and an acid is an electron pair acceptor. Ligands like water, can donate a pair of non-bonding electrons (lone pair) into a vacant orbital of a central metal ion and so dative covalent (co-ordinate) bonds hold a complex together. The central metal ion with vacant bonding orbitals can act as a Lewis acid. Ligands act as Lewis bases by electron pair donation to form the metal-ligand bond.

Bronsted-Lowry acid-base theory reminders (essentially a sub-set of Lewis Theory) A base is a proton acceptor.
This is via an electron lone pair on the base (a Lewis base is a lone pair donor). e.g. NH3, HCO3-, OH- etc.
An acid is proton donor.
This involves a heterolytic breakage of an X-H bond (a Lewis acid is an electron pair acceptor). e.g. HCl, HCO3-, H2SO4, CH3COOH etc.
• Many hexa-aqa complex ions can undergo acid-base reactions with water to produce solutions of pH less than 7. o Usually group 2, 3 and transition metal ions.
o The positive central metal ion polarises a water molecule, releasing a proton, H+. o In the deprotonation reaction the proton transfers to water and the overall charge on the complex falls by 1 unit since the H2O - H+ = OH-, i.e. one of the ligands is now a hydroxide ion.

• e.g. for hexaaquametal(II) ions ...
• [M(H2O)6]2+(aq) + H2O(l) [pic][M(H2O)5(OH)]+(aq) + H3O+(aq)  •
o e.g. when M = Mn, Fe, Co, Ni, Cu, Mg etc. gives a very weak acid solutions with pH's just less than 7 • A complex is formed by the combination of a central metal ion surrounded by, and bonded to, neutral molecules or ions acting as 'ligands' (bits stuck on or appendages). • A ligand is an atom, ion or molecule which can act as an electron pair donor (Lewis base) and usually forms a dative covalent or 'co-ordinate' bond with the central metal ion. o The lone pair donation is usually from an O, N or halogen atom of the ligand in this covalent co-ordinate bonding. o The central metal ion acts as a Lewis Acid, that is, an electron pair acceptor from the ligand by way of vacant 3d, 4s, 4p  and even 4d orbitals for the 3d-block transition elements. o The ligand acts as a Lewis Base, that is, an electron pair donor e.g. neutral ligands like H2O: (water, aqua in complex name) or :NH3 (ammonia, ammine in complex name) and negatively charged ligands like :OH- (hydroxide, hydroxo in complex name), Cl- (chloride ion, chloro in complex name) and :CN- (cyanide ion, cyano in complex name).

Materials: copper(II) nitrate, water, ammonia solution, acetylacetone

Apparatus : 100ml beaker, glass rod, dropper, vacuum suction

Procedures :

1) 6g of copper(II) nitrate is dissolved on 60cm³ of water. 2) The solution of 1:1 ammonia solution is added slowly by stirring until the precipitate first formed is dissolved. 3) The solution is stirred continuously and 6cm³ of acetyl...
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