Few ligands equal water with respect to the number and variety of metal ions with which they form complexes. Nearly all metallic elements form aqua complexes, frequently in more than one oxidation state. Such aqua complexes include hydrated ions in aqueous solution as well as hydrated salts such as hexaaquachromium(3+) chloride, [Cr(H₂O)₆]Cl₃. For metal ions with partially filled d subshells (i.e., transition metals), the coordination numbers and geometries of the hydrated ions in solution can be inferred from their light-absorption spectra, which are generally consistent with octahedral coordination by six water molecules. Higher coordination numbers probably occur for the hydrated rare-earth ions such as lanthanum(3+).

When other ligands are added to an aqueous solution of a metal ion, replacement of water molecules in the coordination sphere may occur, with the resultant formation of other complexes. Such replacement is generally a stepwise process, as illustrated by the following series of reactions that results from the progressive addition of ammonia to an aqueous solution of a nickel(2+) salt:

[Ni(H₂O)₆]²⁺+ NH₃⇌ [Ni(NH₃)(H₂O)₅]²⁺+ H₂O

With increasing additions of ammonia, the equilibria are shifted toward the higher ammine complexes (those with more ammonia and less water) until ultimately the hexaamminenickel(2+) ion predominates:

[Ni(NH₃)₅(H₂O)]²⁺+ NH₃⇌ [Ni(NH₃)₆]²⁺+ H₂O

The tendency of metal ions in aqueous solution to form complexes with ammonia as well as with organic amines (derivatives of ammonia, with chains of carbon atoms attached to the nitrogen atom) is widespread. The stabilities of such complexes exhibit a considerable range of dependence on the nature of the metal ion as well as on that of the amine. The marked enhancement of stability that results from chelation is reflected in the equilibrium constants of the reactions—values that indicate the relative proportions of the starting materials and the products at equilibrium. Complexes of hexaaquanickel(2+) ions can be formed with a series of polyamines—i.e.,

[Ni(H₂O)₆]²⁺+ nL ⇌ [NiL_n(H₂O)_{6 −n}]²⁺+ nH₂O,

in which L is the ligand and n the number of water molecules displaced from the complex. In this series the equilibrium constants, K_L, increase dramatically as the possibilities for chelation increase (that is, as the number of nitrogen atoms available for bonding to the metal atom increases).

It should be noted that, in the particular examples cited above, the coordination number of the metal ion is invariant throughout the substitution process, but this is not always the case. Thus, the ultimate products of the addition of the cyanide ion to an aqueous solution of hexaaquanickel(2+) ion are tetracyanonickelate(2−) and pentacyanonickelate(3−), both containing nickel in the +2 oxidation state. Similarly, addition of the chloride ion to a solution of hexaaquairon(3+) yields tetrachloroferrate(3−). Both complexes contain iron in the same oxidation state of +3.

Halo complexes

Probably the most widespread class of complexes involving anionic ligands is that of the complexes of the halide ions—i.e., the fluoride, chloride, bromide, and iodide ions. In addition to forming simple halide salts, such as sodium chloride and nickel difluoride (in which the metal ions are surrounded by halide ions, these in a sense being regarded as coordinated to them), many metals form complex halide salts—such as potassium tetrachloroplatinate(2−), K₂[PtCl₄]—that contain discrete complex ions. Most metal ions also form halide complexes in aqueous solution. The stabilities of such complexes span an enormous range—from the alkali-metal ions (lithium, sodium, potassium, and so on), whose formation of halide complexes in aqueous solution can barely be detected, to extremely stable halide complexes, such as the tetraiodomercurate(2−), tetrachlorothallate(1−), and tetrachloropalladate(2−) ions, the extent of whose dissociation is extremely small.

The stabilities of halide complexes reflect a pattern by which metal ions can be divided into two general classes, designated as A and B or as hard and soft, respectively. (Generally, the electrons in the atoms of the hard elements are considered to form a compact and not easily deformable group, whereas those in the atoms of the soft elements form a looser group—that is, one more easily deformed.) For the former class—which includes Be, Mg, Sc, Cr, Fe, Ni, Cu, In, and Sn—the order of increasing stability of the halide complexes in aqueous solution is iodides < bromides < chlorides < fluorides. Conversely, for the class B (or soft) ions—such as Pt, Ag, Cd, Hg, Tl, and Pb—the order of increasing stability of the halide complexes is fluorides < chlorides < bromides < iodides. In contrast to class-A metals, those of class B also tend to form more stable complexes with sulfur-containing ligands than with oxygen-containing ligands and more stable complexes with phosphorus ligands than with nitrogen ligands.

Carbonyl complexes

Following the discovery of the first metal carbonyl complex, tetracarbonylnickel, Ni(CO)₄, in 1890, many compounds containing carbon monoxide coordinated to transition metals have been prepared and characterized. For reasons already discussed, such compounds generally contain metal atoms or ions in low oxidation states. The following are some of the more common types of metal carbonyl compounds: (1) simple mononuclear carbonyls of metals in the zero oxidation state, such as tetracarbonylnickel, pentacarbonyliron, and hexacarbonylchromium—highly toxic volatile compounds, the most stable of which have filled valence shells of 18 electrons, (2) salts of anionic and cationic carbonyls, such as tetracarbonylcobaltate(−1) and hexacarbonylmanganese(+1), (3) dinuclear and polynuclear carbonyls, such as bis(tetracarbonylcobalt), the structural formula of which was shown earlier (see above Polynuclear), and (4) mixed complexes containing other ligands in addition to CO: pentacarbonylchloromanganese, tetracarbonylhydridocobalt, and tricarbonylnitrosylcobalt (see organometallic compound).

Although molecular nitrogen, N₂, is isoelectronic with carbon monoxide (that is, it has the same number and arrangement of electrons), its tendency to form complexes with metals is much smaller. The first complex containing molecular nitrogen as a ligand—i.e., pentaamminenitrogenruthenium(2+), [Ru(NH₃)₅(N₂)]²⁺—was prepared in 1965, and many others have been discovered subsequently. Such complexes have attracted considerable interest because of their possible roles in the chemical and biological fixation of nitrogen.

Nitrosyl complexes

Nitrosyl complexes can be formed by the reaction of nitric oxide (NO) with many transition metal compounds or by reactions involving species containing nitrogen and oxygen. Some of these complexes have been known for many years—e.g., pentaaquanitrosyliron(2+) ion, [Fe(H₂O)₅NO]²⁺, which formed in the classical brown-ring test for the qualitative detection of nitrate ion; Roussin’s red (K₂[Fe₂S₂(NO)₄]) and black (K[Fe₄S₃(NO)₇]) salts; and sodium pentacyanonitrosylferrate(3−) dihydrate (sodium nitroprusside), Na₂[Fe(CN)₅NO]∙2H₂O. Such complexes, which can be cationic, neutral, or anionic and which are usually deeply coloured (red, brown, purple, or black), have been extensively studied because they pose unique problems of structure and bonding and because they have potential uses as homogeneous catalysts for a variety of reactions. More recently, the research field has been expanded to include organometallic species (see organometallic compound).

Because the nitrosonium ion (NO⁺) is isoelectronic with carbon monoxide and because its mode of coordination to transition metals is potentially similar to that of carbon monoxide, metal nitrosyls have been recognized as similar to carbonyls and are sometimes formulated as NO⁺ complexes. Carbonyl ligands can be replaced by nitric oxide in substitution reactions. Such similarities may be deceptive, however, for the additional electron in neutral nitric oxide requires a more complicated treatment of M-NO bond formation. The NO ligand exhibits several geometries of coordination—linear (e.g., [IrH(NO){P(C₆H₅)₃}₃]⁺, [Mn(CO)₂(NO){P(C₆H₅)₃}₃], and Na₂[Ru(OH)(NO₂)₄(NO)]^.2H₂O); bent (e.g., [CoNO(NH₃)₅]²⁺and [IrCl₂(NO){P(C₆H₅)₃}₂]); or both (e.g., [RuCl(NO)₂{P(C₆H₅)₃}₂]⁺). Like CO, NO also can act as a bridging ligand between two (e.g., [{Cr(η⁵−C₅H₅)(NO)}₂(μ₂−NH₂)(μ₂−NO)]) or three (e.g., [Mn₃(η⁵−C₅H₅)₃(μ₂−NO)₃(μ₃−NO)]) metal atoms. (The η⁵ indicates that five carbon atoms of the C₅H₅⁻ group are bonded to the chromium atom.)

Cyano and isocyano complexes

Cyano complexes, such as Prussian blue, mentioned above, are among the oldest coordination compounds. In addition to being a pseudohalide, the CN⁻ ion is isoelectronic with CO, RCN, RNC, N₂, and NO⁺ (R is an alkyl group), and metal carbonyls and cyanide complexes are structurally similar. Also, like CO, CN⁻ enters into π as well as σ bonding with transition metal atoms or ions. Cyano complexes are among the most stable transition metal complexes; the extreme toxicity of CN⁻ (like that of CO) is due to its irreversible formation of a strong complex with hemoglobin, which prevents oxygen from binding reversibly to hemoglobin, thereby prohibiting the transport and release of oxygen in the body. Similarly, the ability of CN⁻ to form very stable complexes with silver (Ag(CN)₂⁻) and gold (Au(CN)₂⁻) is the basis for its use in the extraction and purification of these metals. As a monodentate ligand, CN⁻ coordinates (bonds) through carbon as the donor atom, but, as a didentate ligand, it usually coordinates at both ends (C and N) and acts as a bridging ligand (―CN―) to form infinite linear (chain) polymers as in Prussian blue, AgCN, AuCN, Zn(CN)₂, and Cd(CN)₂.

The cyanide ion forms complexes with transition metals and with zinc, cadmium, and mercury, usually by substitution in aqueous solution with no change in oxidation state. The most important complexes are anionic with the formula [Mⁿ⁺(CN)_x]^{(x− n)−}, where Mⁿ⁺represents a transition metal ion. Examples are [Ni(CN)₄]²⁻, [Pt(CN)₄]²⁻, [Fe(CN)₆]^{4− or 3−}, [Co(CN)₆]³⁻, [Pt(CN)₆]²⁻, and [Mo(CN)₈]^{5−, 4−, or 3−}. The free anhydrous parent acids of many of these anions—for example, H₄[Fe(CN)₆] and H₃[Rh(CN)₆]—have been isolated.

Cyanide complexes exhibit a variety of coordination numbers and configurations. Metal ions with a d¹⁰structure form linear complexes of coordination number 2—as, for example, [M(CN)₂]⁻ (where M = Hg, Ag, or Au)—while the isoelectronic complexes [Cu(CN)₄]³⁻, [Ag(CN)₄]³⁻, [Zn(CN)₄]²⁻, [Cd(CN)₄]²⁻, and [Hg(CN)₄]²⁻ are tetrahedral. All the hexacoordinate complexes are octahedral, while octacoordinate complexes are cubic, dodecahedral, or square antiprismatic. (The dodecahedron and square antiprism are two structures that can be obtained by distorting the simple cube.) For d², d⁴, d⁶, d⁸, and d¹⁰ transition metal ions, the octa-, hepta-, hexa-, penta-, and tetracoordinate complexes, respectively, are species with maximum coordination number.

Mixed complexes of type [M(CN)₅X]ⁿ⁻ (where X = H₂O, NH₃, CO, NO, H, or a halogen) also exist. The cyanide ion has the ability to stabilize metal ions in low oxidation states (probably by accepting electron density into its π^*orbitals)—e.g., [Ni(CN)₄]⁴⁻, which contains nickel in the formal oxidation state of zero. (See the article chemical bonding for a discussion of π^*orbitals.) Cyanide complexes have figured prominently in numerous kinetic studies. For example, fast electron-transfer reactions between [Fe(CN)₆]³⁻ and [Fe(CN)₆]⁴⁻ and between [Mo(CN)₈]³⁻ and [Mo(CN)₈]⁴⁻ established the outer-sphere mechanism for redox reactions (see oxidation-reduction reaction); replacement of water in [Co(CN)₅H₂O]²⁻ established the dissociative mechanism for substitution at a Co(3+) ion.

Transition metals also form complexes with organic cyanides (RCN or ArCN, called nitriles) and organic isocyanides (RNC or ArNC, called isonitriles)—where R and Ar are alkyl and aryl groups, respectively—by reaction of a metal halide, carbonyl, or other complex with the nitrile or isonitrile, respectively. Nitriles and isonitriles appear to be stronger donors of σ electrons than carbon monoxide, but they are capable of extensive back acceptance of π electrons from metals in lower oxidation states—as in Cr(CNR)₆ or Cr(CNAr)₆ and Ni(CNR)₄ or Ni(CNAr)₄, which are analogous to the corresponding carbonyls Cr(CO)₆ and Ni(CO)₄, respectively. Although a bridging isonitrile group has been reported in (π−C₅H₅)₂Fe₂(CO)₃(CNC₆H₅), this type of bonding is unusual.

Organometallic complexes

Organometallic complexes are complexes formed between organic groups and metal atoms. They can be divided into two general classes: (1) complexes containing metal-carbon σ bonds and (2) π-bonded metal complexes of unsaturated hydrocarbons—that is, compounds with multiple bonds between carbon atoms (see organometallic compound).