PHD → Inorganic chemistry → Coordination chemistry ↓
Electronic spectra of coordination compounds
Coordination compounds, also called complex compounds, have been a subject of great interest in the field of chemistry. The study of their electronic spectra is important as it provides information about their structural and electronic properties, which aids in understanding their behaviour in various chemical processes. The electronic spectra of these compounds arise from electronic transitions between different energy levels, mainly due to the presence of d-electrons in transition metal ions.
Basics of coordination compounds
Coordination compounds contain a central metal atom or ion that is bound to surrounding molecules or ions, called ligands. Metal-ligand bonding involves d-orbitals in transition metals, and these interactions lead to the formation of unique electronic structures.
Components of coordination compounds
- Central metal ion or atom: Usually a transition metal, characterized by d-orbitals.
- Ligands: Molecules or ions that donate electron pairs to the metal centre, forming coordination bonds.
- Coordination number: The number of ligand donor atoms bonded to the central metal.
Understanding electronic spectra
Electronic spectra arise from the absorption of light, causing transitions of electrons within d-orbitals. In the context of coordination compounds, the most relevant transitions are:
- D-D transition: Transition between d-orbitals of different energies due to crystal field splitting.
- Charge transfer transition: It involves the transfer of electron between metal and ligand.
Key concepts
To understand the electronic spectra of coordination compounds, it is necessary to understand some key concepts, such as crystal field theory, ligand field theory and spectrochemical series.
Crystal field theory
Crystal field theory (CFT) describes the effect of the approach of the ligand to the metal ion in terms of electrostatic interactions. In an octahedral field, the approach of the ligand along the axes leads to repulsion of the metal d-orbitals and splitting into two groups:
-t 2g:
three orbitals (d xy, d xz, d yz) -e g:
two orbitals (d z 2, d x 2 -y 2)
The difference in energy between these sets of orbitals corresponds to the crystal field splitting energy (Δ
). When these absorption energies are in the visible region, the compound exhibits colours.
Ligand field theory
Ligand field theory is based on CFT and considers covalent interactions by incorporating molecular orbital theory. Metal-ligand bonds can have partial covalent character, which further affects the energy levels and observed spectra.
Spectrochemical series
The spectrochemical series arranges ligands based on their ability to split d-orbitals. Strong field ligands, such as CN -
and CO
cause larger splitting than weak field ligands, such as I -
or Br -
As a result, the color and absorption characteristics of coordination compounds vary considerably depending on the ligands present.
The spectrochemical series is represented as follows:
I - < Br - < S 2- < SCN - < Cl - < NO 3 - < F - < OH - < C 2 O 4 2- < H 2 O < NCS - < EDTA 4- < NH 3 < en < bipy < phen < NO 2 - < PPh 3 < CN - < CO
Interpretation of electronic spectra: An example
Let us consider the octahedral complex [Co(NH 3) 6] 3+
. In this complex, we use electronic spectroscopy to determine the types of electronic transitions and the absorption wavelengths:
- dd transitions: In an octahedral field
Co 3+
ion will undergo dd transitions. These transitions are often forbidden by selection rules, making them weak but observable. - Charge Transfer: The charge transfer process may involve electron transfer from the ligand to the metal or vice versa, resulting in strong absorption.
Absorption of light causes excitation from lower energy t 2g
orbitals to higher energy e g
orbitals. This energy difference can be determined, giving information about the extent of orbital splitting and the nature of the ligands.
Applications and significance
The study of electronic spectra of coordination compounds has various applications in fields such as analytical chemistry, materials science and even medicine.
Analytical chemistry
The characteristic colors of coordination compounds can be used for analytical purposes, such as identifying transition metals or determining their concentrations in a sample using spectrophotometry.
Physics
Coordination compounds are used to develop new materials with unique electronic, magnetic, and optical properties. For example, they are important in the development of dyes and pigments as well as catalysts.
Medicine
The ability of coordination compounds to interact strongly with biological molecules makes them useful in pharmaceuticals. For example, some platinum complexes are used in the treatment of cancer.
Conclusion
Electronic spectra of coordination compounds provide in-depth information about their electronic structure and chemical behavior. By understanding the changes that occur, scientists can elucidate structural properties, predict reactivity, and apply these compounds in a variety of scientific and industrial contexts.
This exploration of electronic spectra not only advances our knowledge of coordination chemistry but also paves the way for advances in many scientific fields by taking advantage of the unique properties of these compounds.