Coordination Compounds - Vocabulary Flashcards

A vocabulary set covering essential terms and concepts introduced in the Coordination Compounds lecture notes.

Werner’s theory

First systematic theory of coordination compounds proposing primary (ionic) and secondary (non‑ionic) valences, the concept of coordination number, and characteristic geometries (octahedral, tetrahedral, square planar).

Coordination entity

A central metal atom/ion bound to a fixed number of ligands, forming a discrete complex within a coordination compound.

Central atom/ion

The metal atom or ion at the heart of a coordination entity to which ligands are bonded; acts as a Lewis acid.

Ligand

Ions or molecules bound to the central metal, donating electron pairs to form coordinate bonds; can be unidentate, didentate, or polydentate.

Denticity

The number of donor atoms in a single ligand that bind to a central metal ion.

Unidentate ligand

A ligand that donates electrons through a single donor atom.

Didentate ligand

A ligand that binds through two donor atoms.

Polydentate (multidentate) ligand

A ligand that binds through three or more donor atoms to the same metal.

Chelate

A polydentate ligand that forms rings with the metal ion; chelate complexes are generally more stable than similar non‑chelate complexes.

Ambidentate ligand

A ligand that can bind to a metal via two different donor atoms (e.g., NO2– through N or O).

Coordination number

The number of ligand donor atoms directly bonded to the central metal; determined by sigma bonds (pi bonds are not counted).

Coordination sphere

The central atom/ion and its attached ligands inside brackets; counter ions lie outside the brackets.

Coordination polyhedron

The spatial arrangement of ligands around the central atom; common shapes include octahedral, tetrahedral, and square planar.

Oxidation number (of the central atom)

Formal charge of the central atom in a complex; indicated by Roman numerals in the name.

Homoleptic complex

A complex in which the metal is bound to only one kind of donor group (e.g., [Co(NH3)6]3+).

Heteroleptic complex

A complex in which the metal is bound to more than one kind of donor group (e.g., [Co(NH3)4Cl2]+).

Mononuclear coordination entity

A coordination entity with a single central metal atom/ion.

Monodentate ligand

A ligand that binds through a single donor atom (synonym of unidentate).

Bidentate ligand

A ligand that binds through two donor atoms (example of a didentate ligand).

Tridentate ligand

A ligand that binds through three donor atoms.

Tetradentate ligand

A ligand that binds through four donor atoms.

Hexadentate ligand

A ligand that binds through six donor atoms (e.g., EDTA4–).

Chelation effect

Increased stability of chelate complexes due to ring formation and entropic factors.

Mononuclear vs polynuclear

Mononuclear contains a single metal center; polynuclear contains two or more metal centers connected in the complex.

Ligand denticity in naming

Monodentate, bidentate, etc., describe how many donor atoms a ligand uses to bind.

IUPAC nomenclature (prefixes)

Prefixes such as mono, di, tri (and bis, tris, tetrakis) indicate ligand numbers; ligands are named in alphabetical order and oxidation state is given in parentheses.

Geometrical isomerism

Stereoisomerism arising from different spatial arrangements of ligands (e.g., cis/trans in square planar or octahedral complexes).

Fac/mer isomerism

In [MA3B3] type, fac (facial) has all ligands on one face; mer (meridional) has ligands spread around the meridian.

Optical isomerism

Chiral, non‑superimposable mirror images (enantiomers); common in octahedral complexes with didentate ligands (e.g., [PtCl2(en)2]2+).

Linkage isomerism

Ambidentate ligands bind through different atoms (e.g., NCS– can bind via N or S).

Coordination isomerism

Isomerism due to exchange of ligands between cationic and anionic parts of a complex salt.

Ionisation isomerism

Different ions produced upon dissolution due to different distribution of ligands between cation and anion.

Solvate isomerism

Isomers differing in whether a solvent molecule is bound to the metal or present as free solvent in the lattice.

Valence Bond Theory (VBT)

A bonding model using hybridized orbitals (e.g., sp3, sp3d2) to describe geometry, magnetism, and bonding in coordination compounds.

Crystal Field Theory (CFT)

An electrostatic model treating ligands as point charges; explains d‑orbital splitting (t2g vs eg) and color/magnetism trends. In octahedral fields: Δo separates t2g and eg.

Spectrochemical series

Experimental order of ligand field strengths from weak to strong: I– < Br– < SCN– < Cl– < S2– < F– < OH– < C2O4 2– < H2O < NCS– < EDTA4– < NH3 < en < CN– < CO.

Δo (crystal field splitting energy)

Energy difference between the eg and t2g sets in an octahedral crystal field; larger Δo with stronger field ligands.

P (pairing energy)

Energy required to pair two electrons in a single orbital; contest with Δo determines high‑spin vs low‑spin.

High-spin vs low-spin

High-spin occurs when Δo < P (weak field); low-spin when Δo > P (strong field).

t2g and eg orbitals

In octahedral fields, t2g (lower energy) consists of dxy, dyz, dxz; eg (higher energy) consists of dx2−y2, dz2.

d–d transitions

Electronic transitions between split d‑orbitals that give rise to color in many transition metal complexes.

Color in coordination compounds

Observed color is complementary to absorbed light; explained by d–d transitions in CFT/VBT frameworks.

Inner orbital complex

A complex using (n−1)d orbitals for bonding (low spin; often with strong field ligands).

Outer orbital complex

A complex using outer-shell d orbitals (e.g., nd) for bonding (high spin; weaker field ligands).

Magnetic moment (spin‑only)

μeff ≈ sqrt(n(n+2)) Bohr Magneton, where n is the number of unpaired electrons; used to infer spin state.

M–CO synergic bonding

In metal carbonyls, σ donation from CO to metal and π back‑donation from metal to CO antibonding orbitals strengthen the M–CO bond.

Wilkinson catalyst

RhCl(PPh3)3, a well‑known complex used as a hydrogenation catalyst (alkene hydrogenation).

EDTA

Hexadentate chelating ligand used to bind metal ions; called in water hardness tests and chelation therapy.

Chelate therapy

Medical use of chelating ligands to remove excess metals from biological systems (e.g., D‑penicillamine, desferrioxamine B).