The IUPAC name "tetraammineaquachloridocobalt(III) chloride" corresponds to the formula:

The IUPAC (International Union of Pure and Applied Chemistry) system for naming coordination compounds provides a systematic and unambiguous method for assigning names to any coordination compound, however complex. The complete set of rules is as follows: First, the cation (or the complete complex if neutral) is named before the anion. Within the complex formula, the central metal is preceded by its ligands when writing the name. Ligands are named before the metal: anionic ligands take the suffix -o (or -ido for IUPAC 2005 recommendations): F- = fluorido, Cl- = chlorido, Br- = bromido, I- = iodido, O2- = oxido, OH- = hydroxido, CN- = cyanido, NO2- = nitrito, S2- = sulfido, N3- = azido, CO3^2- = carbonato, SO4^2- = sulfato, C2O4^2- = oxalato, CH3COO- = acetato/acetato. Neutral ligands: NH3 = ammine, H2O = aqua, CO = carbonyl, NO = nitrosyl, NO2 = dinitrogen dioxide (when N-bonded neutral), N2 = dinitrogen, O2 = dioxygen, H2 = dihydrogen; all other neutral ligands use their systematic chemical names (e.g., ethane-1,2-diamine for en, pyridine for py, triphenylphosphine for PPh3). Number of ligands: use Greek prefixes di, tri, tetra, penta, hexa for simple ligands, and bis, tris, tetrakis, pentakis, hexakis for complex ligands or when the ligand name itself contains a numerical prefix (e.g., bis(ethane-1,2-diamine) instead of diethane-1,2-diamine, to avoid ambiguity). Alphabetical order: ligands are named in alphabetical order (ignoring numerical prefixes when alphabetising — "diammine" is alphabetised under A for ammine, "dichloro" under C for chlorido). Metal name and oxidation state: for a complex cation or neutral complex, the English name of the metal is used followed by its oxidation state in Roman numerals in parentheses (cobalt(III), chromium(III)). For a complex anion, the Latin/trivial root of the metal name + -ate suffix is used (cobalt → cobaltate, iron → ferrate, copper → cuprate, gold → aurate, silver → argentate, platinum → platinate; all other metals simply add -ate to the English name). Finally, the counter-ion (outer sphere) is named as a separate word: ionic counter-ions are named as for simple salts.

One of the remarkable features of coordination chemistry is that a given molecular formula can correspond to multiple isomers of quite different structures, properties, and even biological activities. For the formula [CoCl2(NH3)4]+, there are exactly two geometric isomers: cis (both Cl on adjacent positions, 90° apart, sometimes called "violet cobalt") and trans (Cl diametrically opposite, 180° apart, sometimes called "green cobalt"). Both are Co(III) complexes with the same composition, charge, and coordination number, but they differ in colour, solubility, reactivity, and most dramatically in biological activity (cisplatin is a major anticancer drug while transplatin is inactive — the same principle applies here). For more complex formulas, the number of isomers can be much larger: [Co(en)(NH3)2Cl2] can have geometric isomers (en can be in equatorial-equatorial or axial-equatorial positions, and the Cl can be in various arrangements relative to en and NH3) as well as optical isomers. The systematic method for counting isomers uses group theory: the number of distinct isomers of a coordination complex with a given formula and geometry equals the number of distinct arrangements of substituents on the polyhedron, counting as distinct only those arrangements that cannot be superimposed (they must be non-congruent; mirror images count as 2 only if they are non-superimposable). For octahedral MA2B2C2, there are 6 distinct geometric isomers, while for MA3B3, only 2 (fac and mer).

Cobalt coordination chemistry has occupied a central place in the development of inorganic chemistry from the earliest days of the field. Cobalt(III) ammine complexes were the primary systems studied by Alfred Werner in developing his coordination theory (1893-1913), and the systematic variation of cobalt(III)-ammonia-chloride complexes (the Werner series) provided the crucial evidence that established the concepts of primary and secondary valence and the octahedral geometry of six-coordinate cobalt. Werner was awarded the Nobel Prize in Chemistry in 1913 specifically for this work. Cobalt(III) complexes have several features that make them particularly amenable to study: Co3+ (d6) complexes are almost always low-spin (because the crystal field splitting for 3d6 is large enough to overcome the pairing energy for most ligands), making them diamagnetic and easier to characterise magnetically. Low-spin Co3+ complexes are also kinetically inert — the ligand substitution rate is extremely slow (half-life of hours to years for many reactions at room temperature), in contrast to kinetically labile d4-d7 high-spin complexes. This kinetic inertness means that Co3+ complexes, once formed, do not rapidly exchange ligands and can be isolated and characterised as pure compounds — a crucial advantage in Werner's era before modern analytical instruments. The kinetic inertness also underlies the mechanism of cisplatin (a Pt2+ complex): the drug must hydrolyse slowly (replace Cl ligands with H2O) and then react with DNA at the appropriate rate, not too fast (would not reach DNA) and not too slow (would be excreted before reacting).

Cobalt coordination compounds are prepared by a variety of methods depending on the target oxidation state and ligands desired. Co(II) → Co(III) oxidation is often required: Co2+ salts are starting materials, and oxidation to Co3+ is achieved by air oxidation (in the presence of NH3), H2O2, or other oxidants, typically in the presence of the desired ligands. A classic example: [CoCl2(NH3)4]Cl preparation: CoCl2 + NH3(aq) + NH4Cl + H2O2 → [Co(NH3)4Cl2]Cl. [Co(en)3]Cl3 preparation: CoCl2 + en + oxidant → [Co(en)3]Cl3. Properties of Co(III) complexes: always octahedral (coordination number 6), almost always low-spin (diamagnetic, 0 unpaired electrons for strong field ligands). Colours vary considerably depending on the ligands: strong-field ligands give large delta_o → absorption at high energy (short wavelength) → appear in the yellow-orange-red visible range. Weak-field ligands give small delta_o → absorption at low energy (long wavelength) → appear in the purple-blue visible range. Stability: Co3+ complexes are thermodynamically and kinetically stable — Co3+ is a strong oxidising agent in acid medium (Co3+/Co2+ E° = +1.81 V) but is stabilised in the low-spin d6 configuration by large crystal field stabilisation energy.

The application of coordination chemistry to medicine (bioinorganic chemistry and medicinal inorganic chemistry) has generated several clinically important drugs and diagnostic agents and represents one of the fastest-growing areas of modern inorganic chemistry. The story began with the serendipitous discovery of cisplatin's anticancer activity by Barnett Rosenberg in 1965: he was studying the effect of electric fields on bacterial growth using platinum electrodes, and noticed that bacteria near the Pt electrodes stopped dividing (but did not die). Investigation revealed that the electrolysis of the platinum-containing saline medium had produced cis-[Pt(NH3)2Cl2] (cisplatin), which was responsible for inhibiting bacterial cell division. Subsequent testing showed that cisplatin was highly active against a wide range of cancers in animal models, and it entered clinical use in 1978. Cisplatin works by crosslinking adjacent guanine bases in DNA: the drug undergoes aquation (Cl- replaced by H2O in the cell, where Cl- concentration is lower), and the bis-aqua species then forms a bifunctional adduct with two adjacent guanines on the same DNA strand (1,2-intrastrand crosslink), creating a bend of ~40° in the DNA double helix that blocks replication and triggers apoptosis. Limitations of cisplatin: nephrotoxicity (kidney damage), neurotoxicity, ototoxicity, nausea, and acquired resistance. Second-generation platinum drugs: carboplatin (replacing the two Cl ligands with a cyclobutanedicarboxylate leaving group — slower aquation gives reduced side effects but maintained anticancer activity) and oxaliplatin (using trans-1,2-diaminocyclohexane as the diamine ligand instead of two NH3 — active against cisplatin-resistant tumours) are now widely used. Third-generation: satraplatin (oral administration, absorbed from gut). Non-platinum metal drugs: arsenic trioxide (As4O6, As2O3) for acute promyelocytic leukaemia; auranofin (gold phosphine complex, [Au(C6H11O5)(Et3P)]) for rheumatoid arthritis and showing promise in cancer; ruthenium complexes (NAMI-A, KP1019) for solid tumours; titanocene dichloride [Ti(Cp)2Cl2] for ovarian and colon cancer.

Beyond simple Werner-type mononuclear coordination compounds, the principles of coordination chemistry have been extended to create increasingly complex and architecturally sophisticated supramolecular assemblies that exhibit emergent properties not found in simple complexes. Polynuclear coordination clusters: many transition metals form oligomeric or polymeric structures with bridging ligands connecting multiple metal centres. Iron-sulfur clusters [Fe2S2]2+ and [Fe4S4]n+ are found in numerous enzymes involved in electron transfer (ferredoxins, Rieske proteins, nitrogenase) and substrate activation, performing essential biochemical functions that require fine-tuned electrochemistry controlled by the protein environment. Metal-organic frameworks (MOFs): coordination polymers formed by connecting metal nodes (single ions or polynuclear clusters) with organic ligand linkers (typically carboxylates or nitrogen donors) to create extended 3D porous crystalline networks with enormous surface areas (up to 7,000 m2/g — far exceeding activated carbon at 3,000 m2/g). MOFs have potential applications in gas storage (hydrogen for fuel cell vehicles, CO2 capture for climate change mitigation, natural gas storage), drug delivery (controlled release from porous framework), catalysis, and chemical sensing. Supramolecular cage compounds: metal-directed self-assembly of multitopic organic ligands around metal ions of complementary geometry can produce discrete cage structures with a hollow interior capable of encapsulating guest molecules. Examples: Fujita's [M12L24] sphere (12 cis-blocked Pd2+ at the vertices of a cuboctahedron, 24 bent ditopic pyridyl ligands on the edges — self-assembles in water from mixing the components, encloses a ~5 nm diameter void that can selectively encapsulate hydrophobic guest molecules and catalyse reactions inside the confined space). These "flasks" confine reactive intermediates, change selectivity of reactions, stabilise otherwise unstable species, and model enzyme active sites.