Learning Objectives To learn the basis for complex ion and compound nomenclature Coordination complexes have their own classes of isomers , different magnetic properties and colors , and various applications (photography, cancer treatment, etc), so it makes sense that they would have a naming system as well. Consisting of a metal and ligands, their formulas follow the pattern [Metal ligands] ±Charge , while names are written Prefix Ligands Metal (Oxidation State). Introduction According to the Lewis Acid-Base theory , ligands are Lewis bases since they can donate electrons to the central metal atom. The metals, in turn, are Lewis acids since they accept electrons. Coordination complexes consist of a ligand and a metal center cation and their charge can be positive, negative, or neutral. Coordination compounds are neutral compounds that contain at least one complex ions. Example of coordination complexes and compounds are: Complex Cation: \(\ce{[Co(NH3)6]^{3+}}\) Complex Anion: \(\ce{[CoCl4(NH3)2]^{-}}\) Neutral Complex: \(\ce{[CoCl3(NH3)3]}\) Coordination Compound: \(\ce{K4[Fe(CN)6]}\) A ligand can be an anion or a neutral molecule that donates an electron pair to the complex (\(\ce{NH3}\), \(\ce{H2O}\), \(\ce{Cl^{-}}\)). The number of ligands that attach to a metal depends on whether the ligand is monodentate or polydentate . Rules to Complex Ion Nomenclature To begin naming coordination complexes, here are some things to keep in mind. Ligands are named first in alphabetical order. The name of the metal comes next. The oxidation state of the metal follows, noted by a Roman numeral in parentheses (II, IV). Rule 1: Anionic Ligands Ligands that act as anions which end in "-ide" are replaced with an ending "-o" (e.g., Chloride → Chloro). Anions ending with "-ite" and "-ate" are replaced with endings "-ito" and "-ato" respectively (e.g., Nitrite → Nitrito, Nitrate → Nitrato). Molecular Formula Ligand Name Molecular Formula.1 Ligand Name.1 F- Fluoro OH- Hydroxo Cl- Chloro SO42- Sulfato Br- Bromo S2O32- Thiosulfato I- Iodo NO2- Nitrito-N-; Nitro O2- Oxo ONO- Nitrito-O-; Nitrito CN- Cyano SCN- Thiocyanato-S-; Thiocyanato NC- Isocyano NCS- Thiocyanato-N-; Isothiocyanato Rule 2: Neutral Ligands Most neutral molecules that are ligands carry their normal name. The few exceptions are the first four on the chart: ammine, aqua, carbonyl, and nitrosyl. Molecular Formula of Ligand Ligand Name NH3 Ammine H2O Aqua CO Carbonyl NO Nitrosyl CH3NH2 Methylamine C5H5N Pyridine Polydentate ligands follow the same rules for anions and neutral molecules. Short name Extended name en Ethylenediamine ox2- Oxalato EDTA4- Ethylenediaminetetraacetato Rule 3: Ligand Multiplicity The number of ligands present in the complex is indicated with the prefixes di, tri, etc. The exceptions are polydentates that have a prefix already in their name (en and EDTA 4 - are the most common). When indicating how many of these are present in a coordination complex, put the ligand's name in parentheses and use bis (for two ligands), tris (for three ligands), and tetrakis (for four ligands). Number of Ligands Monodentate Ligands Polydentate Ligands 1 mono - 2 di bis 3 tri tris 4 tetra tetrakis 5 penta pentakis 6 hexa hexakis Prefixes always go before the ligand name; they are not taken into account when putting ligands in alphabetical order. Note that "mono" often is not used. For example, \(\ce{[FeCl(CO)2(NH3)3]^{2+}}\) would be called triamminedicarbonylchloroiron(III) ion. Remember that ligands are always named first , before the metal is. Example \(\PageIndex{1}\) What is the name of this complex ion: \(\ce{[CrCl2(H2O)4]^{+}}\)? Solution Let's start by identifying the ligands. The ligands here are Cl and H 2 O. Therefore, we will use the monodentate ligand names of "chloro" and "aqua". Alphabetically, aqua comes before chloro, so this will be their order in the complex's name. There are 4 aqua's and 2 chloro's, so we will add the number prefixes before the names. Since both are monodentate ligands, we will say "tetra[aqua]di[chloro]". Now that the ligands are named, we will name the metal itself. The metal is Cr, which is chromium. Therefore, this coordination complex is called tetraaquadichlorochromium(III) ion. See the next section for an explanation of the (III). Exercise \(\PageIndex{1}\) What is the name of this complex ion: \(\ce{[CoCl_2(en)_2]^{+}}\)? Answer We take the same approach. There are two chloro and ethylenediamine ligands. The metal is Co, cobalt. We follow the same steps, except that \(en\) is a polydentate ligand with a prefix in its name (ethylene di amine), so "bis" is used instead of "di", and parentheses are added. Therefore, this coordination complex is called dichlorobis(ethylenediamine)cobalt(III) ion. Rule 4: The Metals When naming the metal center, you must know the formal metal name and the oxidation state. To show the oxidation state, we use Roman numerals inside parenthesis. For example, in the problems above, chromium and cobalt have the oxidation state of +3, so that is why they have (III) after them. Copper, with an oxidation state of +2, is denoted as copper(II). If the overall coordination complex is an anion, the ending "-ate" is attached to the metal center. Some metals also change to their Latin names in this situation. Copper +2 will change into cuprate(II). The following change to their Latin names when part of an anion complex: Transition Metal Latin Iron Ferrate Copper Cuprate Tin Stannate Silver Argentate Lead Plumbate Gold Aurate The rest of the metals simply have -ate added to the end (cobaltate, nickelate, zincate, osmate, cadmate, platinate, mercurate, etc. Note that the -ate tends to replace -um or -ium, if present). Finally, when a complex has an overall charge, "ion" is written after it. This is not necessary if it is neutral or part of a coordination compound (Example \(\PageIndex{3}\)). Here are some examples with determining oxidation states, naming a metal in an anion complex, and naming coordination compounds. Example \(\PageIndex{2}\) What is the name of \(\ce{[Cr(OH)4]^{-}}\)? Solution Immediately we know that this complex is an anion. There is only one monodentate ligand, hydroxide. There are four of them, so we will use the name "tetrahydroxo". The metal is chromium, but since the complex is an anion, we will have to use the "-ate" ending, yielding "chromate". The oxidation state of the metal is 3 (x+(-1)4=-1). Write this with Roman numerals and parentheses (III) and place it after the metal to get tetrahydroxochromate(III) ion. Exercise \(\PageIndex{2}\) What is the name of \(\ce{[CuCl4]^{2-}}\)? Answer tetrachlorocuprate(II) ion A last little side note: when naming a coordination compound, it is important that you name the cation first, then the anion. You base this on the charge of the ligand. Foe xample, for \(\ce{NaCl}\). The the positive \(\ce{Na}\) cation comes first and the negative \(\ce{Cl^{-}}\) anion follows. Example \(\PageIndex{3}\) What is the name of \([\ce{Pt(NH3)4}][\ce{Pt(Cl)4}]\)? Solution NH 3 is neutral, making the first complex positively charged overall. Cl has a -1 charge, making the second complex the anion. Therefore, you will write the complex with NH 3 first, followed by the one with Cl (the same order as the formula). This coordination compound is called tetraammineplatinum(II) tetrachloroplatinate(II). Exercise \(\PageIndex{3}\): The Nitro/Nitrito Ambidentate Ligand What is the name of \(\ce{[CoCl(NO2)(NH3)4]^{+}}\) ? Answer This coordination complex is called tetraamminechloronitrito-N-cobalt(III). N comes before the O in the symbol for the nitrite ligand, so it is called nitrito-N. If an O came first, as in [CoCl(ONO)(NH 3 ) 4 ] + , the ligand would be called nitrito-O, yielding the name tetraamminechloronitrito-O-cobalt(III). Nitro (for NO 2 ) and nitrito (for ONO) can also be used to describe the nitrite ligand, yielding the names tetraamminechloronitrocobalt(III) and tetraamminechloronitritocobalt(III). Writing Formulas of Coordination Complexes While chemistry typically follow the nomenclature rules for naming complexes and compounds, there is disagreement with the rules for constructing formulas of inorganic complexes. For example, the order of ligand names in their formula has been ambiguous with different conventions being used (charged vs neutral, number of each ligand, etc.). In 2005, IUPAC adopted the recommendation that all ligand names in formulas be listed alphabetically (in the same way as in the naming convention) irrespective of the charge or number of each ligand type. However, this rule is not adhered to in many chemistry laboratories. For practice, the order of the ligands in chemical formulas does not matter as long as you write the transition metal first, which is the stance taken here. Examples \(\PageIndex{4}\) Write the chemical formulas for: Amminetetraaquachromium(II) ion Amminesulfatochromium(II) Solution Amminetetraaquachromium(II) ion could be written as \(\ce{[Cr(H2O)4(NH3)]^{+2}}\) or \(\ce{[Cr(NH3)(H2O)4]^{+2}}\). Amminesulfatochromium (II) could be written as \(\ce{[Cr(SO4)(NH3)]}\) or \(\ce{[Cr(NH3)(SO4)]}\). Exercise \(\PageIndex{4}\) Write the chemical formulas for Amminetetraaquachromium (II) sulfate Potassium hexacyanoferrate (III) Answer Amminetetraaquachromium (II) sulfate can be written as \(\ce{[Cr(H2O)4(NH3)]SO4}\). Although \(\ce{[Cr(NH3)(H2O)4]SO4}\) is also acceptable. Potassium hexacyanoferrate (III) is be written as \(\ce{K3[Fe(CN)6]}\)