Mastering Coordination Compounds and Bonding Theories

Introduction to Coordination Compounds and Theories of Bonding in Transition Metal Complexes

Coordination compounds consist of a metal atom or ion and one or more ligands that donate electrons to the metal center.
Ligands are electron-rich species that have at least one pair of electrons and behave as Lewis bases or nucleophiles.
Metal cations are electron-deficient species that can accept a pair of electrons and behave as Lewis acids or electrophiles.
The coordination number of a complex depends on the size of the central atom or ion, steric and electronic interactions between the ligands and the central atom or ion.
Addition compounds can be classified into double salts or coordination compounds, with the latter retaining their identity in both crystalline and solution form.
Theories of bonding in transition metal complexes include Valence Bond Theory, Crystal Field Theory, and Molecular Orbital Theory.
Valence Bond Theory predicts the shapes of complexes efficiently and can determine magnetic moment, but does not explain the color or distortion of shapes.
Crystal Field Theory considers the electrostatic attraction between ligands and the metal, explains color, arranges ligands by strength, and explains distortion of complexes.
Molecular Orbital Theory forms bonding, non-bonding, and antibonding orbitals from both metal and ligand group orbitals, explaining color, magnetism, and energetics.
Crystal Field Theory explains physical properties such as color, magnetic properties, shape distortion, and temperature dependence of magnetic moments.
Crystal Field Stabilization Energy (CFSE) arises from splitting of d-orbitals in a ligand field, with some becoming lower in energy than before, resulting in a more stable complex.
Factors affecting the magnitude of crystal field splitting include the nature of the metal ion, its oxidation state, the number and strength of ligands, and the size of the metal ion. The Spectrochemical Series ranks ligands by strength.Crystal Field Theory: Octahedral, Tetrahedral, and Square Planar Complexes
Crystal Field Theory (CFT) explains the electronic structure of transition metal complexes based on the interaction between metal ions and ligands.
The energy difference between the d-orbitals in a free ion is called the crystal field splitting energy (Δo).
The magnitude of Δo depends on the nature of the ligands and the geometry of the complex.
In octahedral complexes, the ligands approach the metal ion along the x, y, and z axes, resulting in the splitting of the d-orbitals into two groups: the t2g and eg orbitals.
The magnitude of crystal field splitting energy (Δ) for octahedral complexes is around 10,000-20,000 cm-1.
In tetrahedral complexes, the ligands approach the metal ion along the four vertices of a tetrahedron, resulting in a smaller splitting of the d-orbitals compared to octahedral complexes.
The magnitude of crystal field splitting energy (Δt) for tetrahedral complexes is around 4/9 Δo.
Tetrahedral complexes are always high spin since the crystal field splitting energy is smaller than the pairing energy.
Square planar complexes are rare and only occur for d8 metal ions such as Ni(II), Pd(II), and Pt(II).
The crystal field splitting energy (Δsq. planar) for square planar complexes is around 1.3 Δo.
The number and geometry of ligands affect the magnitude of crystal field splitting energy in octahedral, tetrahedral, and square planar complexes.
CFSE can be calculated using the crystal field splitting energy and the electron configuration of the metal ion in the complex.

Properties of Transition Metal Ions in the Presence of Ligands

The expected decrease in ionic radii of M2+ ions from Ca2+ to Zn2+ due to the increase in nuclear charge is absent for weak field ligands, except for Ca2+, Mn2+, and Zn2+.
Strong field ligands like CN- cause a steady decrease in ionic radii until d6 (t2g6) +2 oxidation state +3 oxidation state is reached.
The repulsion between the metal electrons and ligand electrons is higher than normal when eg orbitals point directly towards the ligands, leading to an eventual increase in the ionic radius.
The lattice energy is the energy released when one mole of an ionic solid is formed from isolated gaseous ions, and it can be calculated theoretically using the Born-Lande Equation or experimentally determined using the Born-Haber cycle.
The discrepancy for a six-coordinate transition metal ion in lattice energy can be explained by crystal field stabilization energy (CFSE).
The Born-Lande equation suggests a smooth increase in lattice energies going from left to right, but instead, a double-hump shaped curve is obtained.
Enthalpy of hydration of transition metal ions is the heat exchange involved when one mole of gaseous ions become hydrated, and it is expected to increase smoothly on going from left to right of the transition metals.
The heats of hydration show two "humps" consistent with the expected CFSE for the metal ions.
The stabilization of different geometries (octahedral vs. tetrahedral) depends on the number of d electrons and the CFSE.
For d0, d5 (WFL), and d10, the octahedral site stabilization energy (OSSE) is zero, and such complexes exist in both tetrahedral and octahedral geometry.
If OSSE > -2.67 (Dq)o, then the CFSE for oct geometry is so high that octahedral geometry is preferred over tetrahedral geometry.
If OSSE < -2.67 (Dq)o, then octahedral geometry is preferred, but tetrahedral geometry can also exist depending on the type of ligand.