Metal-ligand complexes: coordination number, ligand types, crystal-field and ligand-field theories, chelate effect, and the spectrochemical series. Bridge to organometallic and bioinorganic chemistry.
coordination-chemistry
Ligand
An ion or molecule (monodentate or polydentate) that donates a lone pair of electrons to a central metal atom to form a coordination…
Coordination number
Number of donor-atom contacts a central metal atom has with its ligands. Common values are 4 (tetrahedral, square planar) and 6…
Crystal-field theory
Electrostatic model that treats ligands as point charges splitting the degeneracy of the d-orbitals of a central transition-metal ion.…
Ligand-field theory
Molecular-orbital extension of crystal-field theory that accounts for covalent metal-ligand bonding and π-donation/acceptance. Explains the…
Chelate effect
A complex with one polydentate (chelating) ligand is thermodynamically more stable than an otherwise-equivalent complex with multiple…
Spectrochemical series
Empirical ordering of ligands by the magnitude of the crystal-field splitting Δ_o they induce at a given metal centre: I⁻ < Br⁻ < Cl⁻ < F⁻…
Werner's theory of coordination compounds
Central metal ion exhibits both a primary valence (oxidation state) and a secondary valence (coordination number) that is independent of…
Jahn–Teller distortion
Any non-linear molecule with a spatially-degenerate electronic ground state will undergo a geometric distortion that lifts the degeneracy…
Spin crossover complexes
d⁴-d⁷ complexes near HS/LS crossover switch by T, P, hν (LIESST). Fe(II) N-donor complexes; applications in memory, switching.
Tanabe–Sugano diagrams
Energy levels of d^n vs Δ_o/B; interpret UV-vis spectra; extract Δ_o, Racah parameters B, C; high/low-spin crossover.
Racah parameters A, B, C
Parametrize d-d repulsion terms; B and C only needed for term energies; nephelauxetic effect lowers B from free-ion value.
Spin crossover (SCO) complexes
Thermal/light-induced high-↔low-spin transition in Fe(II) d⁶ systems; LIESST; magnetic bistability for molecular memory.
trans effect & trans influence
Kinetic labilization of trans ligand in Pt(II) square-planar; CO, CN⁻ strong; basis of cis/trans-platin synthesis.
Chelate effect (thermodynamic)
Multidentate ligands give more stable complexes than monodentate analogues; entropic origin; macrocyclic effect even larger.
σ-donor / π-acceptor backbonding
CO, CN⁻, NO⁺ donate σ, accept into π* from filled d_π; lowers CO stretch; stabilizes low oxidation states.
Magnetic susceptibility of complexes
Evans, SQUID, Gouy; μ_eff = √(n(n+2)) spin-only; orbital contribution in heavy metals; Curie-Weiss fitting.
Vaska complex & oxidative addition
IrCl(CO)(PPh₃)₂ d⁸ 16e⁻; reversibly binds H₂, O₂; prototype for OA/RE catalytic cycle.
18-electron rule
Transition-metal complexes with 18 valence e⁻ around metal are stable (d-block noble-gas config); carbonyls, ferrocene.
Lanthanide/actinide coordination
f orbitals contracted; coordination ionic, CN 8-12 typical; Ln³⁺ used as NMR shift reagents, luminescent probes; An redox flexibility.
Werner (1893, Nobel 1913)
A Werner 1893 (Nobel 1913) coordination-theory; modern primary-secondary valences + Δ-Λ chirality + 18-electron rule.
CFT (Bethe 1929)
H Bethe 1929 crystal-field-theory; modern d-orbital-splitting Δ_oct + Δ_tet + LFSE + spectrochemical-series.
Ligand field (Griffith-Orgel 1957)
J Griffith-L Orgel 1957 ligand-field-theory; modern Tanabe-Sugano + AOM angular-overlap + JT-distortion theory.
Jahn-Teller (1937)
H Jahn-E Teller 1937 + Renner 1934 Jahn-Teller distortion; modern Cu(II) octahedral + manganite colossal-magnetoresistance.
MLCT/LMCT (Tsuchida 1938)
R Tsuchida 1938 spectrochemical series + later Marcus 1993 MLCT/LMCT; modern Ru(bpy)3 + photocatalysis + OLED phosphors.
M-M bond (Cotton 1964)
F A Cotton 1964 quadruple Re-Re bond Re2Cl8 2-; modern σπδ-bonding + multiple-bond-orders + Pd / Ru / Mo dimers.