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Complex Ion Formation
An Introduction to Complex Metal Ions
Complex Ions and Ligands
A complex ion consists of a central metal ion surrounded by
some number of molecules or other ions (called LIGANDS)
Ex: simple ligands include water (aquo), ammonia (ammine),
chloride (chloro), cyanide (cyano), and hydroxide (hydroxo)
ions.
Ex: [Co(NH3)6]Cl3
COUNTER IONS can be found around the complex ion in
order to create a neutral compound. In above ex, the Cl- are
the counter ions.
Simple Ligands
What do each of these have in common?
They each have active lone electron pairs in their outer
energy levels. It is these pairs that form the coordinate
covalent bonds with the metal ion. They are LEWIS BASES!
T-metals as a Lewis Acid
All ligands are lone pair donors (Lewis Base).
The transition metal is accepting the lone pair in order to
fill its d orbital with e-. Accepting lone pair means it acts
as a Lewis acid.
Note: The resulting lewis structures indicate this donation by
replacing the single line (indicating a single bond) with an arrow
showing the direction of donation).
Co-ordinate Covalent Bonding
REVIEW: A covalent bond is formed by two atoms sharing a
pair of electrons. The atoms are held together because the
electron pair is attracted by both of the nuclei.
In the formation of a simple covalent bond, each atom
supplies one electron to the bond.
But that doesn't have to be the case! A co-ordinate bond is a
covalent bond (a shared pair of electrons) in which both
electrons come from the same atom (i.e. the lewis base ligand
in coordination compounds!)
Coordination Chemistry
The number of ligands attached to the central metal ion
determines the complex ion’s “coordination number” (cannot
exceed 6). {Often the coordination number is 2x the oxidation
number of the metal.}
The are mostly anions (only rarely do complex cations form).
The net charge on the coordination complex (the ion) is the
sum of the charges of the metal and its attached groups.
Practice…
Give the formula of the following coordination complex…
A Ni2+ ion is bound to
2 water molecules & 2 oxalate ions.
2[Ni(C2O4)2(H2O)2]
net charge = 1(2+)2(2-) + 4 (neutral) = 2-
Ex: Tetraamminediaquacopper(II)
Cu2+ w/ 4 NH3 and 2 H2O
Naming Complex Ions
1. To name any complex ion, list first the ligands, then the
central atom.
2. The ligand names are made to end in -O if they are negative.
Examples of ligand names are chloro, hydroxo, cyano, aqua (for
H2O), ammine (for NH3), and thiosulfato (for S2O32-).
3. Anions that end in -ate and, when a Latin symbol is used for the
element, the element takes the Latin name in anions but not in
cations. For example, Cu(NH3)42+ is called the
tetraamminecopper(II) ion but Cu(CN)64- is called the
hexacyanocuprate(II) ion. Likewise Al(NH3)63+ is called the
hexaamminealuminum(III) ion but Al(OH)4- is called the
tetrahydroxoaluminate(III) ion.
Naming Complex Ions
4. The number of each kind of ligand is specified by the usual
Greek prefix. The ligands are named in alphabetic order. For
example, Cu(Cl2Br2I2)4- is the dibromodichlorodiiodocuprate(II)
ion while Cu(Cl2Br2INH3)4- is the
amminedibromodichloroiodocuprate(I) ion.
THIS IS ACTUAL ALPHA ORDER, NOT LIKE ORGO CHEM!
5. The oxidation state of the central metal atom to which the
ligands are attached must be indicated.
Naming Complex Ions
Go for it… I dare you!
1. [Co(NH3)4Cl2]Cl
2. [Co(NH3)5Cl]Cl2
3. K3[Co(NO2)6]
4. [Pt(NH3)3Cl]2[PtCl4]
5. [Pt(NH3)4][PtCl4]
6. Reinecke's Salt, NH4[Cr(NH3)2(SCN)4]
7. Drechsel's Chloride, [Pt(NH3)6]Cl4
8. Cossa's First Salt, K[Pt(NH3)Cl3]
9. Cossa's Second Salt, K[Pt(NH3)Cl5]
Naming Complex Ions
1. [Co(NH3)4Cl2]Cl: dichlorotetramminecobalt(III) chloride.
2. [Co(NH3)5Cl]Cl2: monochloropentamminecobalt(III) chloride.
3. K3[Co(NO2)6], known as Fischer's Salt: potassium
hexanitritocobaltate(III).
4. [Pt(NH3)3Cl]2[PtCl4], known as Magnus' Pink Salt:
di(monochlorotriammineplatinum(II)) tetrachloroplatinate(II).
5. [Pt(NH3)4][PtCl4], known as Magnus' Green Salt:
tetrammineplatinum(II) tetrachloroplatinate(II).
6. Reinecke's Salt, NH4[Cr(NH3)2(SCN)4]: ammonium
tetrathiocyanatodiamminechromate(III).
7. Drechsel's Chloride, [Pt(NH3)6]Cl4: hexammineplatinum(IV)
chloride.
8. Cossa's First Salt, K[Pt(NH3)Cl3]: potassium
trichloroammineplatinate(II).
9. Cossa's Second Salt, K[Pt(NH3)Cl5]: potassium
pentachloroammineplatinate(IV)
When Will a Complex Ion Form?
Complex ions are usually formed from a transition metal
ion surrounded by ligands (polar molecules or negative
ions). As a “rule of thumb” you place twice the
number of ligands around an ion as the charge on
the ion.
Examples: the dark blue copper-ammonia complex,
Cu(NH3)42+ (ammonia is a test for Cu2+ ions) and AgCl2(solid AgCl dissolves when excess, concentrated HCl is
added).
Just so you know….
NOTE: Aluminum forms complex ions as do some of the post
transition metals.
NOTE: The names are very impressive, but easy… Just
follow the naming conventions closely.
+
NOTE: Acid-base reactions may change NH3 into NH4 (or
vice versa) which will alter its ability to act as a ligand.
NOTE: Visually, a precipitate may go back into solution as a
complex ion is formed. For example, Cu2+ plus a little NH4OH
will form the light blue precipitate, Cu(OH)2. With excess
ammonia, the complex, Cu(NH3)42+ forms. If a little HCl is
+
added, the NH3 becomes NH4 and no longer acts as a
ligand; theCu(OH)2 will re-form.
NOTE: Keywords such as “excess” and “concentrated” may
indicate complex ions!!!
When Will a Complex Ion Form?
Watch for complex ion formation when double
replacement does not form new compounds!!!
For example, “Excess concentrated sodium hydroxide solution
is added to a precipitate of zinc hydroxide”…
Zn(OH)2 + 2NaOH  Zn(OH)4 + 2 Na
2-
+
Last thing – Here are particles that generally
combine with excess ligand (often water, hydroxide
or ammonia) to form complex ions with a charge:
Cu2+ + 4 NH3  [Cu(NH3)4]2+
Al3+ + 4 OH-  [Al(OH)4]AgCl + 2 NH3  [Ag(NH3)2]+ + ClFe3+ + SCN-  [Fe(SCN)]2+