The Chemistry of the Transition Metals | General Chemistry 3

The chemistry of the transition metals is studied in this chapter: d-block transition metal series, transition metal complexes and nomenclature, isomer cis and trans, d-orbital splittings, electronic configurations of transition metal ions, low-spin and high-spin configuration

d-Block Transition Metal Series

Transition metal: any element in the d-block of the periodic table (groups 3 to 12)

Much of the chemistry of the transition metals is determined by the shape of the d orbitals: dxy, dxz, dyz, dx²-y² and d orbitals. The energies of the five d orbitals are equal in the absence of electric or magnetic field. In all the chemistry of the transition metals, the 4s orbital behaves as the outermost, highest energy orbital compared to 3d orbitals

A d-block transition metal ion has generally a characteristic number of electrons. However, some of transition metal ions can have variable oxidation states
A transition-metal ion with x d electrons is generally called dx ion and can have variable oxidation states

Iron is an element of the group 8 and has 26 electrons: Fe: [Ar] 3d64s2
The most important oxidation states of iron are +2 and +3:
Fe2+: [Ar] 3d6 ⇒ Fe (II) is a d6 transition-metal ion
Fe3+: [Ar] 3d5 ⇒ Fe (III) is a d5 transition-metal ion

Transition Metal Complexes

Transition metal complex: central metal atom or ion bonded to ligands
Ligand: anion or neutral molecule attached directly to the metal atom or ion
Coordination number: number of ligands attached to the central metal atom or ion

[Fe(CN)6]4- is a transition metal complex
Fe2+ is the central ion and CN- are the ligands
The coordination number of this complex is equal to 6

Ligand-substitution reaction: reaction involving modification of ligands by substitution
It produces a modified electrical environment around the central atom or ion ⇒ the energies of the d-orbitals change, causing the electronic transition energies to change (the color of the complex is different)

Ligand-substitution reaction:

[Ni(H2O)6]2+ + 6 NH3 $⇌$ [Ni(NH3)6]2+ + 6 H2O
[Ni(H2O)6]2+ is green while [Ni(NH3)6]2+ is blue

Transition metal complexes can have various geometries depending on their coordination number
The most common are linear (2 ligands), tetrahedral, square planar (4 ligands) and octahedral (6 ligands)
Transition metal complexes are generally used as catalysts

Nomenclature of Transition Metal Complexes

1. State the name of the cation first and then the anion
2. For the anionic transition metal complexes, name the ligands first and then the metal
3. If the ligand is neutral: give the name of the ligand molecule
If the ligand is negative: end the name with the letter o
4. Use the prefix di-, tri-, tetra-, penta-, or hexa- to indicate the number of ligands
5. If the complex ion is positive or neutral: use the ordinary name for the metal
If the complex ion is negative: end the name of the metal in -ate
6. Use a Roman numeral to indicate the oxidation state of the metal

[AgBr2]-
K2[CuCl4]
[Pt(NH3)2Cl2]

dibromoargentate (I)
potassium tetrachlorocuprate (II)
diamine dichloroplatinum (II)

Isomers

Geometric isomers: molecules that differ only in the spatial arrangement of the constituent atoms
Cis isomer (“on the same side”): identical ligands are placed next to each other
Trans isomer (“opposite”): identical ligands are placed directly opposite each other

[Pt(NH3)2Cl2] has 2 geometric isomers:

d-Orbital Splittings

The 5 d-orbitals of a transition metal ion in an octahedral complex are split into 2 groups by ligands

t2g orbitals: lower-energy set of orbitals (dxy, dxz, dyz)
eg orbitals: higher-ernergy set of orbitals (dx²-y², d)
Δo: octahedral splitting energy energy = difference between the 2 sets of d orbitals
It depends on the central metal ion and ligands

The difference in the energies of the 2 sets of d orbitals means that the d electrons can make a transition by absorbing light (Δo = hν) ⇒ this explains the color of many coordination compounds

Electronic Configurations

d1, d2, d3 and d8, d9, d10 transition metal ions:
only one possible electronic ground state configuration

d4, d5, d6 and d7 transition metal ions:
low or high spin configuration ⇒ depending on the central metal ion and ligands

If splitting energy Δo > pairing energy:

the electrons first fill the t2g orbitals before occupying the higher-energy eg orbitals
⇒ low-spin configuration

If splitting energy Δo < pairing energy:

the d-electrons occupy the eg orbitals before they pair up in the t2g orbitals
⇒ high-spin configuration

Electronic configuration of transition metal complexes with 6 d-electrons:

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Fajans-Tsuchida spectrochemical series:
arrangement of ligands in order of increasing ability to split d orbitals