In coordination chemistry, research on the main-group metal elements and their complexes is an important topic, but the subject has often been overshadowed by the intense interest in transition metal complexes. There are a variety of reasons for this, not the least of which is the perception that, in general, main-group complexes are less interesting in their chemical behavior because of their closed-shell propensity, more restricted electron counts, and fewer available oxidation states. However, over the past few decades, there has been a growing interest in main-group metal complexes, extending across all members of the s and p blocks, which has resulted in the discovery of numerous new classes of complexes with previously unknown structures and bonding. These findings have important scientific research and application values, and provide reliable clues for further exploration of the main-group metal complexes.
Differences in properties between main-group metal complexes and transition metal complexes
In the last two decades of the 20th century, the differences between main-group metal complexes and transition metal complexes were often masked by the similarities in stoichiometry, structure, and chemical behavior of the known compounds. However, the two complexes have fundamentally different properties from each other. These differences in properties are as follows:
- Orbital status: The valence s or p orbitals of the main-group metal complexes are either completely occupied or empty and are far apart energetically. Transition metal complexes, on the other hand, usually have partially occupied valence d orbitals, which tend to be closer in energy.
- Stereochemical electron pair: The main-group metal complexes have stereochemically active electron pairs which form the basis of valence shell electron pair repulsion theory. In contrast, the stereochemical electron pair characteristics of the transition metal complexes are less pronounced.
- Reactivity: The main-group metal complexes do not interact strongly with small molecules. In contrast, transition metal complexes frequently interact with small molecules.
- Appearance: Normally, the main-group metal complexes are colorless. However, transition metal complexes are often coloured as a consequence of small orbital energy separations.
- Magnetic: The main-group metal complexes are usually diamagnetic and the transition metal complexes are often paramagnetic.
The main-group metal complexes possess some excellent properties and are widely used in a variety of fields. Some simple examples are given below. It has been demonstrated that the main-group metal complexes could activate organic small molecules such as H2O, CO2, NO, CO, amines, olefins, alkynes, aldehydes, and ketones. In addition, the activation of NH3 by the main-group metal complexes has also received much attention. Some main metal ions in the p region of the periodic table of elements, such as bismuth, lead, tin, antimony, etc., whose valence electrons in the s and p sublayers are in the outermost layer of the electron layer, and have strong interaction with the coordination field. They are easy to form broadband luminescence like transition metal ions. Its luminescence wavelength can be extended to the near-infrared region. Therefore, it also has good application prospects and is expected to open new directions for the development of laser materials.
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