⏱️General Chemistry II Unit 8 – Coordination Compounds: Structure & Theory

Key Concepts

• Coordination compounds consist of a central metal atom or ion surrounded by ligands, which are ions or molecules that donate electron pairs to the metal
• The coordination number represents the number of ligands bonded to the central metal atom or ion and typically ranges from 2 to 9, with 4 and 6 being the most common
  • Examples of coordination numbers: 4 (square planar), 6 (octahedral)
• Ligands can be classified as monodentate (one donor atom), bidentate (two donor atoms), or polydentate (multiple donor atoms) based on the number of atoms that form bonds with the central metal
• The denticity of a ligand refers to the number of donor atoms it possesses that can bond to the central metal atom or ion
• The chelate effect describes the enhanced stability of coordination compounds containing polydentate ligands compared to those with monodentate ligands due to entropic factors
• Crystal field theory explains the splitting of d-orbital energies in transition metal complexes based on the arrangement and nature of the ligands surrounding the central metal ion
• The spectrochemical series ranks ligands according to their ability to split d-orbital energies, with strong-field ligands causing a larger splitting than weak-field ligands

Historical Background

• Coordination compounds have been known since ancient times, with early examples including pigments such as Prussian blue and dyes like mauve
• In the late 19th century, Alfred Werner proposed the concept of coordination compounds and developed the first comprehensive theory to explain their structures and bonding
  • Werner's theory introduced the idea of a central metal atom surrounded by ligands, which laid the foundation for modern coordination chemistry
• The development of X-ray crystallography in the early 20th century provided experimental evidence supporting Werner's theory and allowed for the determination of the structures of coordination compounds
• The discovery of ferrocene in the 1950s by Kealy and Pauson led to the development of organometallic chemistry, which explores the bonding and reactivity of compounds containing metal-carbon bonds
• Coordination compounds have played a crucial role in the development of various fields, including catalysis, materials science, and bioinorganic chemistry

Types of Coordination Compounds

• Homoleptic complexes contain only one type of ligand coordinated to the central metal atom or ion, such as $[Co(NH_3)_6]^{3+}$
• Heteroleptic complexes contain two or more different types of ligands coordinated to the central metal atom or ion, such as $[Co(NH_3)_4Cl_2]^+$
• Cationic complexes have a positive overall charge, with the central metal ion having a higher positive charge than the total negative charge of the ligands, such as $[Cr(H_2O)_6]^{3+}$
• Anionic complexes have a negative overall charge, with the total negative charge of the ligands exceeding the positive charge of the central metal ion, such as $[Fe(CN)_6]^{4-}$
• Neutral complexes have no net charge, with the positive charge of the central metal ion balanced by the negative charge of the ligands, such as $[Ni(CO)_4]$
• Chelate complexes contain one or more polydentate ligands that form multiple bonds with the central metal atom or ion, creating a ring structure, such as $[Ni(en)_3]^{2+}$ (en = ethylenediamine)
• Macrocyclic complexes involve ligands that form a cyclic structure around the central metal atom or ion, such as porphyrins and crown ethers

Nomenclature

• The nomenclature of coordination compounds follows a systematic approach established by the International Union of Pure and Applied Chemistry (IUPAC)
• The name of a coordination compound consists of the name of the cation followed by the name of the anion, with the ligands named in alphabetical order before the central metal
• The oxidation state of the central metal atom or ion is indicated by a Roman numeral in parentheses immediately following the metal name
• Anionic ligands end in "-o," while neutral ligands are named as the molecule or ion they represent, such as chloro for $Cl^-$ and ammine for $NH_3$
• Prefixes are used to indicate the number of each type of ligand: di- (2), tri- (3), tetra- (4), penta- (5), and hexa- (6)
  • For example, $[Co(NH_3)_6]Cl_3$ is named hexaamminecobalt(III) chloride
• Bridging ligands, which connect two or more metal centers, are indicated by the prefix "μ-" followed by the ligand name
• Geometrical isomers are distinguished using prefixes such as "cis-" (same side) and "trans-" (opposite sides) for square planar and octahedral complexes
  • For example, $[Pt(NH_3)_2Cl_2]$ can exist as cis-diamminedichloroplatinum(II) or trans-diamminedichloroplatinum(II)

Bonding Theories

• Valence bond theory (VBT) describes the bonding in coordination compounds using hybrid orbitals formed by the mixing of atomic orbitals on the central metal atom or ion
  • VBT explains the geometry and magnetic properties of coordination compounds but has limitations in describing electronic spectra and the color of complexes
• Crystal field theory (CFT) focuses on the electrostatic interactions between the ligands and the d-orbitals of the central metal ion, explaining the splitting of d-orbital energies and the resulting properties of coordination compounds
  • CFT successfully accounts for the color, magnetic properties, and electronic spectra of many coordination compounds
• Ligand field theory (LFT) is an extension of CFT that incorporates covalent bonding interactions between the ligands and the central metal ion, providing a more comprehensive description of bonding in coordination compounds
• Molecular orbital theory (MOT) treats the coordination compound as a single entity, considering the overlap of atomic orbitals from both the metal and the ligands to form molecular orbitals
  • MOT offers a more complete understanding of the electronic structure and properties of coordination compounds but can be computationally demanding

Structural Isomerism

• Structural isomers are compounds with the same chemical formula but different bonding arrangements or spatial orientations of the ligands
• Ionization isomers differ in the distribution of counter ions between the inner and outer coordination spheres, such as $[Co(NH_3)_5Br]SO_4$ and $[Co(NH_3)_5SO_4]Br$
• Coordination isomers have the same ligands but differ in which ligands are coordinated to the central metal atom or ion, such as $[Co(NH_3)_6][Cr(CN)_6]$ and $[Cr(NH_3)_6][Co(CN)_6]$
• Linkage isomers have the same ligands but differ in the atom of the ligand that is bonded to the central metal, such as $[Co(NH_3)_5(NO_2)]^{2+}$ and $[Co(NH_3)_5(ONO)]^{2+}$
• Geometrical isomers, such as cis and trans isomers, have the same ligands and bonds but differ in the spatial arrangement of the ligands around the central metal atom or ion
• Optical isomers, or enantiomers, are non-superimposable mirror images of each other and exhibit optical activity, such as $[Co(en)_3]^{3+}$

Properties and Applications

• Coordination compounds exhibit a wide range of colors due to d-d transitions, which involve the excitation of electrons between split d-orbitals of the central metal ion
  • The color of a coordination compound depends on the nature of the metal, its oxidation state, and the ligands present
• Many coordination compounds are paramagnetic, meaning they have unpaired electrons and are attracted to magnetic fields, while others are diamagnetic, with no unpaired electrons and weak repulsion by magnetic fields
• Coordination compounds play essential roles in biological systems, such as hemoglobin (oxygen transport), chlorophyll (photosynthesis), and vitamin B12 (cofactor in enzymatic reactions)
• Coordination compounds are used as catalysts in various industrial processes, including the Wacker process (oxidation of ethylene to acetaldehyde) and the Monsanto process (production of acetic acid from methanol and carbon monoxide)
• Coordination compounds are used in the development of new materials, such as metal-organic frameworks (MOFs) for gas storage and separation, and in the design of sensors and molecular switches
• Medicinal applications of coordination compounds include their use as anticancer agents (cisplatin), diagnostic imaging agents (gadolinium complexes for MRI), and radiopharmaceuticals for targeted therapy

Analytical Techniques

• X-ray crystallography is used to determine the solid-state structures of coordination compounds, providing information about bond lengths, angles, and the arrangement of ligands around the central metal atom or ion
• Infrared (IR) spectroscopy helps identify the presence of specific functional groups in coordination compounds, such as carbonyl (C≡O) and cyano (C≡N) ligands, based on their characteristic vibrational frequencies
• UV-visible spectroscopy is used to study the electronic transitions in coordination compounds, providing information about the d-orbital splitting, ligand field strength, and color of the complexes
  • The position and intensity of absorption bands in the UV-visible spectrum can help determine the geometry and electronic configuration of the central metal ion
• Nuclear magnetic resonance (NMR) spectroscopy is used to study the local environment of the central metal atom or ion and the ligands in solution, providing information about the symmetry and bonding in coordination compounds
• Electron paramagnetic resonance (EPR) spectroscopy is used to study coordination compounds with unpaired electrons, providing information about the oxidation state, coordination geometry, and electronic structure of the central metal ion
• Mössbauer spectroscopy is a technique specific to certain isotopes, such as $^{57}Fe$, and provides information about the oxidation state, coordination number, and bonding environment of the metal atom in coordination compounds
• Cyclic voltammetry is an electrochemical technique used to study the redox properties of coordination compounds, providing information about the stability of different oxidation states and the influence of ligands on the redox potential of the metal center