Preparation of Bis Acetylacetonao Copper

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 acetone added drop wise. 4) The crude product is filtered off by using suction filtration. 5) The product is washed well with water and drained thoroughly. 6) The product is dried in the oven.

7) Weight of the product is recorded and the percentage yield % is calculated.

Data/Results :

Mass of copper(II) nitrate = 6.013g
Mass of beaker=52.9072g

Mass of beaker + product after
drying in oven = 54.2723g

Mass of crude product after = 54.2723-52.9072
drying in oven =1.3651g

Calculations :

Mass of copper(II) nitrate used = 6.013g
Mass of crude product = 1.3651g

Theoretical yield

Given molar mass of copper(II) nitrate = 241

Num of moles of copper(II) nitrate = 6.013
241
= 0.0249 moles
Assume number of moles is equals number of moles of crude product = 0.0249
moles

Given that molar mass of crude product= 261.

Theoretical yield mass = 0.0249 moles × 261
= 6.4989g

Percentage yield of product = Mass of crude product × 100%
Theoretical mass
= 1.3651g × 100%
6.4989
= 21.0050 %

Discussion :

Copper(II) Nitrate, (Cu(NO3) 2 is an inorganic compound that forms a blue crystalline solid. Anhydrous copper nitrate forms deep blue-green crystals and sublimes in a vacuum at 150-200 °C. Copper nitrate also occurs as five different hydrates, the most common ones being the trihydrate and hexahydrate. .Hydrated forms of copper nitrate, also blue.

Copper(II) nitrate (Cu(NO3) 2.3H2O) is dissolved in water and Cu(H2O)6]2+( aq) ions are formed.

Small amount of ammonia is added to Cu(H2O)6]2+( aq) to form Cu(H2O)4 (OH) 2] ( aq) ions.. Reactions of the hexaaqua ions with ammonia solution are complicated by the fact that the ammonia can have two quite different functions. Small amount of ammonaia can act as a base (in the Bronsted-Lowry sense). But it is also a possible ligand which can replace water molecules around the central metal ion if excess of ammonia is added. When it acts as a ligand, it is acting as a Lewis base.

In our experiment, we are using acetyl acetone as the ligand. The acetylacetonate anion, acac-, forms complexes with many transition metal ions. A general method of synthesis is to react the metal ion with acetylacetone in the presence of a base (B):

Mz + z (acacH) M(acac)z +z BH+

which assists the removal of a proton from acetylacetone and shifts the equilibrium in favour of the complex. Both oxygen atoms bind to the metal to form a six-membered chelate ring. In some cases the chelate effect is so strong that no added base is needed to form the complex. Since the metal complex carries no electrical charge, it is soluble in non-polar organic solvents.

Questions

1) Draw the structure of the complex.

2) Describe the structure by mentioning the ligating atoms of the ligand and its geometry. Discuss the synthesis of the complex.

Acetyl acetonate is the ligand. It is a colourless liquid that is a precursor to acetylacetonate (acac), a common bidentate ligand. The acetylacetonate anion, acac-, forms complexes with many transition metal ions. Bidentate ligands have two lone pairs, both of which can bond to the central metal ion. CH3-C(O)-CH2-C(O)-CH3

Acetylacetonate bonds through both lone paor of oxygen.
Cu(acac)2 is unique among the metal complexes of acetylacetanato. It has square planar monomeric copper(II) complex shape. The acetylacetonate anion, acac-, forms complexes with many transition metal ions. A general method of synthesis is to react the metal ion with acetylacetone in the presence of a base (B), which assists the removal of a proton from acetylacetone and shifts the equilibrium in favour of the complex. Both oxygen atoms bind to the metal to form a six-membered chelate ring. In some cases the chelate effect is so strong that no added base is needed to form the complex. Since the metal complex carries no electrical charge, it is soluble in non-polar organic solvents.