Coordination Compounds

• Transition metals form complex compounds where metal atoms are bound to anions or neutral molecules by sharing electrons, known as coordination compounds
• Coordination compounds are an important area of modern inorganic chemistry, providing insights into biological systems
• Examples of coordination compounds include chlorophyll (magnesium), haemoglobin (iron), and vitamin B12 (cobalt)
• Coordination compounds are used in metallurgical processes, industrial catalysts, analytical reagents, electroplating, textile dyeing, and medicinal chemistry
• Alfred Werner was the first to formulate ideas about coordination compounds, introducing primary and secondary valence for metal ions
• Werner's theory of coordination compounds includes postulates about primary and secondary valences, spatial arrangements, and coordination polyhedra
• Different geometrical shapes like octahedral, tetrahedral, and square planar are common in coordination compounds of transition metals
• Double salts like carnallite dissociate into simple ions in water, while complex ions like [Fe(CN)6]4– do not dissociate
• Coordination entities consist of a central metal atom/ion bonded to a fixed number of ions or molecules, with ligands bound to the central atom/ion
• Ligands in coordination entities can be simple ions like Cl–, small molecules like H2O or NH3, larger molecules like H2NCH2CH2NH2, or even macromolecules like proteins
• When a ligand is bound to a metal ion through a single donor atom, it is unidentate (e.g., Cl–, H2O, NH3)
• A ligand is didentate when it can bind through two donor atoms (e.g., H2NCH2CH2NH2, C2O4^2–), and polydentate when several donor atoms are present in a single ligand (e.g., N(CH2CH2NH2)3)
• Ethylenediaminetetraacetate ion (EDTA^4–) is a hexadentate ligand that can bind through two nitrogen and four oxygen atoms to a central metal ion
• A chelate ligand is a di- or polydentate ligand that uses its two or more donor atoms simultaneously to bind a single metal ion, resulting in more stable complexes
• An ambidentate ligand, like NO2– and SCN– ions, has two different donor atoms and can coordinate through either of them to a central metal atom/ion
• The coordination number of a metal ion in a complex is the number of ligand donor atoms directly bonded to the metal (e.g., [PtCl6]^2– has a coordination number of 6)
• The coordination sphere includes the central atom/ion and attached ligands, enclosed in square brackets, while counter ions are written outside the bracket
• The coordination polyhedron is defined by the spatial arrangement of ligand atoms directly attached to the central atom/ion, with common shapes being octahedral, square planar, and tetrahedral
• The oxidation number of the central atom in a complex is the charge it would carry if all ligands and shared electron pairs were removed, represented by a Roman numeral in parenthesis after the name of the coordination entity
• Homoleptic complexes have a metal bound to only one kind of donor groups, while heteroleptic complexes have a metal bound to more than one kind of donor groups
• Nomenclature in Coordination Chemistry follows IUPAC recommendations for writing systematic names based on the groups surrounding the central atom
• Coordination compounds involve nomenclature rules for naming complex ions
• Geometric isomerism in coordination compounds:

  • Stereoisomerism includes geometrical isomerism and optical isomerism
  • Geometrical isomerism occurs in heteroleptic complexes due to different geometric arrangements of ligands
  • Examples include square planar complexes like [MX2L2] and octahedral complexes like [MX2L4]
• Hydrate isomerism occurs when water is involved as a solvent in coordination compounds
• Solvate isomers differ by whether a solvent molecule is directly bonded to the metal ion or present as free solvent molecules in the crystal lattice
• Example: Aqua complex [Cr(H2O)6]Cl3 (violet) and its solvate isomer [Cr(H2O)5Cl]Cl2.H2O (grey-green)
• Werner was the first to describe bonding features in coordination compounds
• Approaches to explain bonding in coordination compounds: Valence Bond Theory (VBT), Crystal Field Theory (CFT), Ligand Field Theory (LFT), and Molecular Orbital Theory (MOT)
• According to Valence Bond Theory (VBT) and Crystal Field Theory (CFT), metal atoms or ions under the influence of ligands use specific orbitals for hybridization to yield a set of equivalent orbitals of definite geometry
• Types of hybridizations and their corresponding geometries:

  • sp: Tetrahedral
  • dsp: Square planar
  • sp3d: Trigonal bipyramidal
  • sp3d2: Octahedral
• Magnetic properties of coordination compounds can be used to determine the number of unpaired electrons and structures adopted by metal complexes
• Magnetic data of coordination compounds of metals of the first transition series reveal complications based on the number of d electrons present
• Limitations of Valence Bond Theory:

  • Involves assumptions
  • Does not quantitatively interpret magnetic data
  • Does not explain color exhibited by coordination compounds
  • Does not give quantitative interpretation of thermodynamic or kinetic stabilities
  • Does not make exact predictions regarding tetrahedral and square planar structures
• Crystal Field Theory (CFT) provides a better explanation for the formation, structures, and magnetic behavior of coordination compounds compared to Valence Bond Theory
• Crystal field theory (CFT) is an electrostatic model that considers the metal-ligand bond to be ionic, arising purely from electrostatic interactions between the metal ion and the ligand
• In CFT, ligands are treated as point charges in the case of anions or point dipoles in the case of neutral molecules
• In an isolated gaseous metal atom/ion, the five d orbitals have the same energy, i.e., they are degenerate
• Degeneracy of the d orbitals is maintained in a spherically symmetrical field of negative charges surrounding the metal atom/ion
• When the negative field is due to ligands in a complex, the degeneracy of the d orbitals is lifted, resulting in splitting of the d orbitals
• Crystal field splitting in octahedral coordination entities:

  • Repulsion between metal d orbitals and ligands leads to splitting of d orbitals
  • d orbitals directed towards ligands experience more repulsion and are raised in energy (eg set)
  • d orbitals directed between the axes are lowered in energy (t2g set)
  • Crystal field splitting in octahedral complexes results in ∆o energy separation