Crystal Field theory (CFT)

Crystal Field Theory (CFT) rests on treating each ligand as nothing more than a simple POINT NEGATIVE CHARGE, with the entire metal-ligand interaction assumed to be purely electrostatic (ionic-type) -- explicitly, CFT assumes there is NO orbital overlap or covalent-bonding character between metal and ligand at all, in direct contrast to VBT's orbital-overlap picture. In an isolated, gaseous metal ion (with no ligands nearby), all five d orbitals -- dx2-y2, dz2, dxy, dyz and dzx -- are degenerate, meaning they all have exactly the same energy. If an approaching ligand field were perfectly symmetrical with respect to all five d orbitals, this degeneracy would simply persist; but in a real octahedral complex, the six ligands approach along the +/-x, +/-y and +/-z axes specifically, and this is NOT symmetrical with respect to the five different d-orbital shapes, so the degeneracy is destroyed (Fig. 9.2). The two orbitals whose lobes point DIRECTLY along those axes -- dx2-y2 and dz2 -- experience noticeably STRONGER electrostatic repulsion from the approaching ligands and are pushed UP in energy, forming the higher-energy eg set. The three orbitals whose lobes instead point diagonally, BETWEEN the axes -- dxy, dyz and dzx -- experience comparatively WEAKER repulsion and are pushed DOWN in energy, forming the lower-energy t2g set. The energy gap between these two sets is the crystal field splitting parameter, written Delta-o (the 'o' for octahedral, sometimes also written as 10 Dq); measured from the original degenerate baseline, the eg set sits 0.6 Delta-o (3/5 Delta-o) higher, and the t2g set sits 0.4 Delta-o (2/5 Delta-o) lower. Ligands are classified by how large a Delta-o they produce: strong-field ligands (those whose donor atom is carbon, nitrogen or phosphorus -- e.g. CN-, CO, NH3, en, EDTA) create a large Delta-o and favour PAIRING electrons in the lower t2g set before any electron occupies the upper eg set, giving a low-spin complex; weak-field ligands (donor atom a halogen, oxygen or sulfur -- e.g. F-, Cl-, Br-, I-, SCN-, C2O4(2-)) create a Delta-o smaller than the electron-pairing energy, so electrons instead spread out singly across BOTH sets first (Hund's rule) before any pairing occurs, giving a high-spin complex. Table …