💏Intro to Chemistry Unit 19 – Transition Metals & Coordination Compounds

Transition metals, with their partially filled d orbitals, form coordination compounds by bonding with ligands. These compounds exhibit unique properties like variable oxidation states, vibrant colors, and paramagnetism, making them crucial in catalysis, pigments, and biological systems. Crystal field theory explains the bonding and properties of coordination compounds based on d orbital splitting. This splitting leads to characteristic colors and magnetic properties, influencing their applications in various fields from industrial catalysts to medical treatments.

Study Guides for Unit 19

19.1

Occurrence, Preparation, and Properties of Transition Metals and Their Compounds

3 min read

19.2

Coordination Chemistry of Transition Metals

3 min read

19.3

Spectroscopic and Magnetic Properties of Coordination Compounds

3 min read

Key Concepts

Transition metals are elements in the d-block of the periodic table that have partially filled d orbitals in their neutral or ionic states
Coordination compounds consist of a central metal atom or ion bonded to surrounding molecules or ions called ligands
Ligands donate electron pairs to the central metal atom to form coordinate covalent bonds
Crystal field theory explains the bonding, structure, and properties of coordination compounds based on the splitting of d orbitals
The splitting of d orbitals in coordination compounds leads to the absorption of specific wavelengths of light, resulting in the characteristic colors of transition metal complexes
Transition metals and their coordination compounds have important applications in various fields (catalysis, pigments, and biological systems)

Transition Metal Properties

Transition metals exhibit a wide range of oxidation states due to their ability to lose electrons from both the s and d subshells
They form stable coordination compounds with ligands, resulting in complex geometries (octahedral, tetrahedral, and square planar)
Transition metals often display paramagnetism, meaning they are attracted to magnetic fields due to the presence of unpaired electrons
Many transition metals and their compounds are colored due to d-d electronic transitions
- The specific color depends on the energy gap between the split d orbitals
Transition metals have high melting and boiling points compared to main group elements due to their strong metallic bonding
They exhibit good electrical and thermal conductivity, making them useful in various industrial applications (wiring and heat exchangers)
Transition metals are generally hard and dense, with high tensile strength and ductility

Electron Configuration

The electron configuration of transition metals involves the filling of the d subshell after the s subshell of the valence shell
The general electron configuration for transition metals is $[Noble Gas] (n-1)d^x ns^2$ $[N o b l e G a s] (n - 1) d^{x} n s^{2}$ , where n is the principal quantum number and x is the number of electrons in the d subshell
- For example, the electron configuration of iron (Fe) is $[Ar] 3d^6 4s^2$
Exceptions to the general electron configuration occur when the d subshell is half-filled or completely filled, as these configurations are more stable
- For example, chromium (Cr) has the electron configuration $[Ar] 3d^5 4s^1$ instead of $[Ar] 3d^4 4s^2$
The partially filled d subshell contributes to the unique properties of transition metals, such as variable oxidation states and the formation of coordination compounds

Coordination Compounds

Coordination compounds are composed of a central metal atom or ion surrounded by ligands
The central metal atom or ion is typically a transition metal, but can also be a main group element or a lanthanide/actinide
Ligands are molecules or ions that donate electron pairs to the central metal atom, forming coordinate covalent bonds
The number of ligands bonded to the central metal atom is called the coordination number, which typically ranges from 2 to 9
- Common coordination numbers include 4 (tetrahedral and square planar) and 6 (octahedral)
The geometry of a coordination compound depends on the coordination number and the nature of the ligands
Coordination compounds can be neutral or charged, depending on the charges of the central metal ion and the ligands
- For example, $[Co(NH_3)_6]Cl_3$ is a cationic complex with a +3 charge on the cobalt ion and three chloride counterions

Ligands and Bonding

Ligands are classified as monodentate (one donor atom), bidentate (two donor atoms), or polydentate (multiple donor atoms) based on the number of atoms that bond to the central metal
Common monodentate ligands include water ( $H_2O$ ), ammonia ( $NH_3$ ), and halides ( $Cl^-$ , $Br^-$ , $I^-$ )
Bidentate ligands, such as ethylenediamine ( $H_2NCH_2CH_2NH_2$ $H_{2} NC H_{2} C H_{2} N H_{2}$ ) and acetylacetonate ( $CH_3COCHCOCH_3^-$ $C H_{3} COC H COC H_{3}^{-}$ ), form chelate rings with the central metal atom
- Chelate rings enhance the stability of coordination compounds due to the chelate effect
Polydentate ligands, like EDTA (ethylenediaminetetraacetic acid), can form multiple chelate rings and lead to very stable complexes
The bonding in coordination compounds involves a combination of electrostatic interactions and covalent bonding between the ligands and the central metal atom
- The coordinate covalent bond is formed by the donation of an electron pair from the ligand to the metal, creating a shared electron pair

Crystal Field Theory

Crystal field theory (CFT) describes the bonding, structure, and properties of coordination compounds based on the splitting of d orbitals
In a coordination compound, the ligands create an electrostatic field that interacts with the d orbitals of the central metal atom
The d orbitals split into two energy levels: the lower energy $t_{2g}$ $t_{2 g}$ orbitals ( $d_{xy}$ $d_{x y}$ , $d_{xz}$ $d_{x z}$ , $d_{yz}$ $d_{yz}$ ) and the higher energy $e_g$ $e_{g}$ orbitals ( $d_{z^2}$ $d_{z^{2}}$ , $d_{x^2-y^2}$ $d_{x^{2} - y^{2}}$ )
- The energy difference between the $t_{2g}$ and $e_g$ orbitals is denoted as $\Delta_o$ (octahedral crystal field splitting energy)
The magnitude of the crystal field splitting depends on the nature of the ligands and the central metal atom
- Strong-field ligands (CN-, CO) cause a larger splitting, while weak-field ligands (I-, Br-) result in a smaller splitting
The electron configuration of the central metal atom in a coordination compound is determined by the relative energies of the $t_{2g}$ $t_{2 g}$ and $e_g$ $e_{g}$ orbitals and the pairing energy
- High-spin complexes have a smaller $\Delta_o$ and electrons occupy both $t_{2g}$ and $e_g$ orbitals before pairing
- Low-spin complexes have a larger $\Delta_o$ and electrons pair in the $t_{2g}$ orbitals before occupying the $e_g$ orbitals

Colors and Magnetism

The colors of transition metal coordination compounds arise from d-d electronic transitions between the split d orbitals
When light is absorbed, an electron is promoted from a lower energy d orbital to a higher energy d orbital
- The wavelength of light absorbed corresponds to the energy difference between the two orbitals
The complementary color to the absorbed wavelength is observed as the color of the complex
- For example, a complex that absorbs orange light will appear blue
The intensity of the color depends on the probability of the electronic transition, which is governed by selection rules
The magnetic properties of coordination compounds are determined by the number of unpaired electrons in the d orbitals
- High-spin complexes have more unpaired electrons and are paramagnetic, while low-spin complexes have fewer unpaired electrons and may be diamagnetic
The magnetic moment of a coordination compound can be measured experimentally and compared to the predicted values based on the spin-only formula
- $\mu = \sqrt{n(n+2)}$ $\mu_B$ , where n is the number of unpaired electrons and $\mu_B$ is the Bohr magneton

Applications and Real-World Examples

Transition metal coordination compounds have numerous applications in various fields, including catalysis, pigments, and biological systems
In catalysis, coordination compounds are used as homogeneous and heterogeneous catalysts for industrial processes (Wilkinson's catalyst for hydrogenation)
Pigments based on transition metal coordination compounds are used in paints, inks, and plastics (cobalt blue, chromium oxide green)
Biological systems employ coordination compounds for essential functions, such as oxygen transport (hemoglobin) and photosynthesis (chlorophyll)
Transition metal complexes are used in medicine for diagnostic imaging (contrast agents) and therapy (cisplatin for cancer treatment)
Coordination compounds are also used in analytical chemistry for the detection and quantification of metal ions (complexometric titrations with EDTA)
Transition metal complexes are employed in the fabrication of advanced materials, such as light-emitting diodes (LEDs) and solar cells (ruthenium-based dye-sensitized solar cells)
The study of transition metal coordination compounds has led to the development of new technologies and materials, such as superconductors and molecular magnets