Chm2911 Topic 1: Properties of Transition Metal Complexes

TOPIC 1: PROPERTIES OF TRANSITION METAL COMPLXES

LIGANDS

In coordination compounds, ligands donate lone pairs of electrons to the central transition metal atom or ion, the number of ligands that bond in this way is called the coordination number. Most transition metals exhibit more than one coordination number, factors which determine this are steric effects, oxidation state, energetics, and the nature of the ligand.

The atoms, molecules, or ions attached to the metal ion are called ligands. These can exist as the following:

Monodentate: one lone pair of electrons.

Bidentate: take up two bonding sites exhibiting two lone pairs, these can form rings known as chelates. [pic 1]

Chelated complexes are more stable than unchelated complexes, due to the chelate effect and results from an increase in entropy – and thus stability – as there ae more species in solution when a chelated ligand is formed.
[pic 2]

Polydentate: ligands that bond to three, four, five, or six coordination sites. These species are

called tri-, tetra-, penta-, and hexadentate respectively.

ELECTRON CONFIGURATION

In the elements, the $s orbital lies lower in energy than the 3d orbital and thus is filled first.

Fe: [Ar]4s23d6

However, there are some anomalies to the rule which possess a half-filled 4s shell as half-filled or full
d-shells prove more stable.

Cr: [Ar]4s13d5

Cu: [Ar]4s13d10

In transition metal complexes the d orbitals lie lower in energy than the s orbitals and hence are removed from the s orbital prior to the d orbital.

Fe: [Ar]4s23d6

Fe3+: [Ar]3d5

In compounds the number of d electrons can be obtained simply from the group number, for example; Cr is in group 6, therefore Cr(0) has a configuration of [Ar]d6, and Cr(III) a configuration of [Ar]d3.

STRUCTURAL ISOMERISM

Structural isomerism refers to the way in which ligands are bonded and has four common types.

Hydration isomerism: identity of the ligand species is different for the isomers, here it is the proportion of coordinated water molecules that differ.

[Cr(H2O)6] Cl3                        - water can exist as a balancing cation when outside the brackets, or as

[Cr(H2O)5Cl] Cl2.H2O                a ligand when inside brackets.

[Cr(H2O)4Cl2] Cl.2H2O

Ionisation isomerism: give different ions when dissolved in solution.

[Co(NH3)5Br]SO4 ⇌ [Co(NH3)5Br]2+ + SO42-

[Co(NH3)5SO4]Br ⇌ [Co(NH3)5SO4]+ + Br-

Linkage isomerism: form bonds through more than one atom, in the example below, the metal can bond via the nitrogen or the sulfur on the nitrite ion.

M-NCS

M-SCN

Coordination isomerism: both the anion and cation are complex, here the isomers are formed by the interchange of ligands between the anion and cation.

[Co(NH3)6][Cr(CN)6]

[Co(CN)6][Cr(NH3)6]

STEREOISOMERISM

There are two types of inorganic stereoisomers – geometric and optical – which are parallel to those found in organic chemistry. However, the rule that optical isomerism is most common for a metal ion in an octahedral environment – as opposed to the tetrahedral environment – is excluded.

Geometric isomers: must have two different ligands attached to the same metal. For square planar compounds the following geometric isomers can exist:[pic 3]

[pic 4][pic 5]

Optical isomers: one isomer is a non-superimposable mirror image of the other. Here, optical isomers rotate the plane of polarised light with one isomer rotating the light in one direction and the other isomer in the opposite direction. These compounds exist as chiral compounds.

[pic 6]

NAMING COMPOUNDS

1. The cation is named first followed by the anion.
2. The ligands are listed in alphabetical order.
3. Some ligands have special names in a complex.
   NH3        ammine
   CO        carbonyl
   N2        dinitrogen
   O2        dioxygen
   - anionic ligands end in ‘o’.
   - organic groups are given their radical name.
   - in ambidentate ligands, the bonding atom is indicated by κ.
4. The oxidation state of the metal is given in Roman numerals in parenthesis.
5. If the anion is complex, the metal is designated by the word ending in ‘-ate’.
6. For multiple ligands the prefixes di-, tri-, tetra-, penta-, and hexa- are used for two, three, four, five, and six, respectively.
7. For multiple ligands already containing numerical prefixes, the prefixes bis-, tris-, and tetrakis-, are used for two, three, and four, respectively. This is not a rigid rule.

THE 18 ELECTRON RULE

The 18-electron rule (or effective atomic number rule) states that a complex is stable. If there are eighteen electrons around the metal. This model works for many metal compounds in which the metal is in a low oxidation state.