COMPLEX IONS: PART 1

From left to right, aqueous solutions of: Co(NO3)2 (red); K2Cr2O7 (orange);

K2CrO4 (yellow); NiCl2 (green);

CuSO4 (blue); KMnO4 (purple).

Coordination Complexes

• CuSO4· 5H2O, [FeCl4] - ion and CrCl3·6 NH3,

are called coordination complexes because they contain ions or molecules linked (or coordinated) to a transition metal.

• They are also known as complex ions because they are Lewis acid-base complexes.

• The ions or molecules that bind to transition-metal ions to form these complexes are called ligands (from Latin, "to tie or bind").

• The number of ligands bound to the transition metal ion is called the coordination number

WRITING THE FORMULA

• Use brackets to enclose the ion [not always required]

• The metal is written first.

• The ligands follow, written in alphabetical order.

• The charge of the ion is written outside of the bracket.

METALS THAT FORM COMPLEXES

• mostly transition metals

• some main group elements also form complexes

• Aluminum, tin, and lead, for example, form complexes such as the [AlF6]3-, [SnCl4]2- and [PbI4]2- ions.

THE SHAPES OF COMPLEX METAL IONS

• These shapes are for complex ions formed using monodentate ligands - ligands which only form one bond to the central metal ion.

• Octahedral – coordination # = 6

• Linear - coordination # = 2

• Tetrahedral – coordination # = 4

• Square planar – coordination # = 4

6-co-ordinated complex ions

• These are complex ions in which the central metal ion is forming six bonds to six ligands.

• These ions have an octahedral shape.

• Four of the ligands are in one plane, with the fifth one above the plane, and the sixth one below the plane.

4-co-ordinated complex ions

• These are far less common, and they can take up one of two different shapes.

• Tetrahedral ions• [CuCl4]2- and [CoCl4]2-.• The copper(II) and cobalt(II) ions have four

chloride ions bonded to them rather than six, because the chloride ions are too big to fit any more around the central metal ion.

Tetrahedral ions

4-co-ordinated complex ions

• A square planar complex

• Occasionally a 4-co-ordinated complex turns out to be square planar.

• There's no easy way of predicting that this is going to happen.

• The only one you might possibly come across at this level is cisplatin which is used as an anti-cancer drug.

Cisplatin is a neutral complex, Pt(NH3)2Cl2. It is neutral because the 2+ charge of the

original platinum(II) ion is exactly cancelled by the two negative charges supplied by the

chloride ions.

Geometric isomerism

Geometric isomerism• The two structures are isomers because there is no

way that you can just twist one to turn it into the other. The complexes are both locked into their current forms.

• The terms cis and trans are used in the same way as they are in organic chemistry.

• Trans implies "opposite" or “across” - notice that the ammonias are arranged opposite each other in that version, and so are the chlorines.

• Cis implies "on the same side" - in this instance, that just means that the ammonias and the chlorines are adjacent to each other.

• Cis-trans isomerism occurs in octahedral and square planar complexes (but not tetrahedral).

Monodentate and Bidentate Ligands

WRITING EQUATIONS

• Excess ammonia is added to a solution of zinc nitrate.

• Zn2+ + NH3 [Zn(NH3)6]2+

• 2005 AP Exam

WRITING EQUATIONS

• Excess hydrochloric acid is added to a solution of nickel(II) nitrate.

• NI2+ + Cl- [NiCl4]2-

• 2006 AP test

WRITING EQUATIONS

• Aqueous sodium hydroxide is added to a saturated solution of aluminum hydroxide, forming a complex ion.

• Al3+ + OH- [Al(OH)6]3-

• 2008 AP Exam

WRITING EQUATIONS

• Excess hydrochloric acid is added to a solution of cobalt (II) nitrate to produce a coordination complex.

• Co+2 + Cl- [CoCl4]2-

• 2011 AP test