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: 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 5 d orbitals are equal in the absence of electric or magnetic field



Each d-block transition metal ion has a characteristic number of electrons
A transition-metal ion with x d electrons is generally called dx ion

Iron is an element of the group 8 and has 56 electrons: Fe: [Ar] 3d64s2
Fe2+: [Ar] 3d6 ⇒ Fe (II) is a d6 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 a change in the ligands
It produces a modified electrical environment around the central atom or ion ⇒ 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)

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 in 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



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



Geometric isomers: molecules that differ only in the spatial arrangements of the constituent atoms
Isomer cis (“on the same side”): identical ligands are placed next to each other
Isomer trans (“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 the 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 the ligands



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

Electronic Configurations

d1, d2, and d3 and d8, d9 and 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 the 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:











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