Lecture 10: coordination compounds

coordination compounds aka complexes

a central metal atom is surrounded by nonmetal atoms or groups of atoms, called ligands

complex ions

the combination of a transition metal cation with a ligand molecule or ion

Lewis acid and base

acid: electron acceptor

base: electron donor

reaction of ammonia with water

NH3 + H2O → NH4+ + OH-

• NH₃ donates its electron pair → Lewis base

• H⁺ accepts it → Lewis acid NH3​+H+→NH4+​

The new N–H bond is a:

coordinate covalent bond

the coordinate covalent bond

a covalent bond, in which the two electrons forming the bond derive from the same atom.

(unlike e.g. H• + •H → H–H, H2)

Once the bond is formed, it cannot be distinguished from a «conventional» covalent bond

protonated water

The atom, molecule or ion donating the electron pair is called the ____, often abbreviated _____

ligand, L

The atom or ion accepting the electron pair is called the __________

The atom or ion accepting the electron pair is called the coordination centre

Since the coordination centre in a complex is in almost all cases a metal atom or ion, it is generally abbreviated ___

Thus L| -) M -)

M
[M-L]

An atom, molecule, or ion can only function as a ligand (and as a LEWIS base) if it has at least one ______

lone pair

NC: All anionic complexes (i.e. complex anions) get the ending

-ate

Central particle of anions is named based on the origin of the element's symbol:

with iron:

copper:

tin :

Mercury:

Ag:

iron: ferrate (Fe-ferrum)

copper: cuprate (Cu-cuprum)

tin; stannate (Sn-stannum)

Exception: HG= hydragyrum, but an anionic complex containing mercury is called mercurate.

Silver: argentate

Naming anionic complexes

1.1 determine charge

1) counter cation

2) number of ligands (greek numerical prefixes)

3) name of ligand

4) name of central particle ending with -ate

5) overall charge of the complex ion

Ex: [Cu(OH)4]2-

Tetrahydroxidocuprate(2-) ion

K3[Fe(CN)6]

Potassium hexacyanidoferrate(3-)

Na3[Ag(S2O3)2]

Sodium dithiosulfatoargentate(3-) ion

naming cationic complexes

1.1 determine charge

1) number of ligands

2) name of the ligands

3)name of central particle

4) charge of the complex ion

5) counter ion

[Cr(H₂O)₆]³⁺

Hexaaquachromium(3+)

[Co(NH₃)₆]³⁺

Hexaaminecobalt(3+)

[Pt(NH₃)₂Cl₂]NO₃)

diammine-dichloroplatinum(+1) nitrate

[Co(NH₃)₄Cl₂]Br

terraammine-dichlorocobalt(+1) bromide

polydentate ligands

Polydentate ligands are molecules or ions that can bind to a central metal atom or ion through multiple donor atoms

A polydentate ligand binds to a metal at multiple points at the same time

chelate complexes

complexes that contain polydentate ligands are called chelate complexes.

Chelate complexes are coordination compounds where a multidentate ligand binds to a central atom through multiple coordination sites

what do chelate complexes have to do with entropy and statistics?

Statistics (probability argument)

A mono-dentate ligand binds with one point → easy to detach.

A polydentate ligand binds at multiple points → if one bond breaks, the rest are still attached.

=)So it’s more likely to stay bound → more stable.

Entropy

[M(L)6] +3 x→[M(x)3]+6L

=) More particles = higher entropy DeltaS = negative DeltaG = reaction is favored = stabler products

M(x)3 is more stable it has lower energy than M(L)6

uncoordinated

The molecule is free: H₂N–CH₂–CH₂–NH₂

The two nitrogen atoms each have a lone pair (blue marks)

coordinated

The same molecule now binds to a metal

Both nitrogen atoms attach to the metal

It forms a ring (chelate ring)

en → binds through 2 N atoms

oxalate → binds through 2 O atoms

“Dentate” only applies when a ligand is _____

“Dentate” only applies when a ligand is coordinated

name the spatial arrangement

differnce between octahedrally coordinated and octa-dentate

Octahedral = geometry (6 positions)

Octadentate = One ligand binding through 8 atoms

nomenclature for polylingands like en

1) modified greek prefixes:

di- becomes bis-

tri- becomes tris-

tetra- becomes tetrakis- etc.

(meaning “two times, three times, four times, respectively)–

2) The name of the ligand is then set in parenthesis: (name)

Ex:[Cu(en)2]2+

bis(ethylendiamine)copper(2+) cation

Upon adding chloride anions (Cl-) to an aqueous solution of silver ions (Ag+), solid silver chloride (AgCl) precipitates due to the low solubility product (10-10 mol2/L2).

If now an aqueous solution of ammonia (NH3(aq)) is added, the precipitate will dissolve again

why?

since the silver ions (in solution!) react with ammonia to form the complex cation [Ag(NH3)2]+ (diammine silver(I)-ion), thus lowering the amount of silver ions present in the solution

→ the precipitate slowly dissolves. The compound [Ag(NH3)2]Cl has a much higher solubility product

cis-trans-isomerism

which has higher solubility?

Cis isomers are usually more soluble because they are more polar, since their dipols dont cancel out

Polar molecules interact better with polar solvents (like water)

chirality

Chirality is the property of an object not being identical to its mirror image

chirality needs four different groups

or lingands

complexes formed with ethylenediamines (en) in an octahedral geometry are always chiral.

Cis arrangement:

Trans arrangement:

what is chiral what is achiral?

Cis arrangement: No inversion center → can form non-superimposable mirror images → chiral.

Trans arrangement: Inversion center exists → mirror image is superimposable → achiral.

facial (fac)

The three identical ligands occupy one face of the octahedron.

All three A ligands form a triangle on one side of the octahedron.

meridional (mer)

The three identical ligands occupy a plane passing through the metal center.

One ligand is opposite another, and the third is perpendicular

chiral, achiral, chiral

chiral or achiral

meridional is achiral

facial is also achiral