Coordination Chemistry of Transition Metals

Transition metals and inner transition metals form bonds by sharing electron pairs with ligands. These occur between a metal and one or more ions or molecules. The number of coordinated ligands in a complex is called its coordination number.

Transition metals are unique among elements because of the manner in which they form bonds. Other elements form bonds to complete their valence shells through shared electron pairs. The p-block elements often complete their valence shells through

n

s²

n

p⁶ electron configurations that follow the octet rule. That is, they share or donate electrons such that the outer valence shell contains eight electrons. Transition metals also form covalent bonds, termed coordinate covalent bonds. In these bonds, pairs of electrons are contributed by an electron donor (a Lewis base) to an electron acceptor (a Lewis acid) in a Lewis acid-base pair configuration. The Lewis acid electron acceptor is known as a central metal ion. The Lewis base electron donor is called a ligand, an ion or molecule that bonds to a central metal atom or ion as part of a coordination complex. A coordination complex is a central metal atom or ion surrounded by ligands, as well as bonded ions if the central part has an overall charge.

Coordination Complex Formation

Transition metals form coordination complexes by acting as a Lewis acid, accepting electron pairs from a Lewis base. The transition metal silver (Ag) forms the central metal ion, and the two ammonia (NH₃) ligands are electron donors.

The part of a coordination complex consisting of the central metal atom or ion and all ligands make up the coordination sphere. When written as part of a chemical formula, the coordination sphere is enclosed in brackets. Any species outside the brackets are not part of the coordination sphere. The coordination number is the number of atoms bound to the single central metal center in a coordination complex. For example, the cationic silver(I) diammine complex, [Ag(NH₃)₂]⁺, has a coordination number of two because two nitrogen atoms (from ammonia, also called an ammine ligand) are bonded to the same silver(I) center. In this case the coordination number and the number of ligands are both two. Each is a monodentate ligand, meaning it is a ligand in a coordination complex that donates a single pair of electrons to the central metal atom or ion. A ligand in a coordination complex that donates two pairs of electrons to the central metal atom or ion is a bidentate ligand. For example, ethylenediamine (H₂NCH₂CH₂NH₂) has two nitrogen atoms that can donate electrons to the central metal ion. Thus when chromium bonds to this ligand, each ethylenediamine can donate two electrons, one from each of the nitrogen atoms. In this coordination complex, the coordination number is six, but only three ligands are present.

Bidentate Ligands

A bidentate ligand forms bonds from two different atoms to the central metal ion. Ethylenediamine (H₂NCH₂CH₂NH₂) has two nitrogen atoms that donate electrons to the central chromium atom.

A ligand in a coordination complex that donates three or more pairs of electrons to the central metal atom or ion is a polydentate ligand. An example is the ligand heme, which makes up a part of hemoglobin. Hemoglobin is a metal-containing protein that supplies oxygen to living tissue through the cardiovascular system. Heme, the iron-binding portion of hemoglobin, contains four nitrogen atoms that each bind to a common central iron ion.

A chelate is a coordination complex formed between a central metal atom or ion and chelating ligands, which are ions or molecules that bond to it with more than one atom. Any bidentate or polydentate ligand is a chelating ligand.

Geometry of Coordination Complexes

Transition metal-ligand bonding leads to the formation of numerous coordination complexes, whose molecular shapes can often be predicted using VSEPR concepts.

Coordination complexes often conform to specific shapes. The two most common shapes are square planar and octahedral. In the square planar complex, four ligands lie within a square plane around the central metal. An octahedral complex has two planes, one containing four ligands in the square planar configuration and another perpendicular plane containing one ligand above and one below the square planar plane. The octahedral configuration in three dimensions looks like an eight-sided die, with two square pyramids sharing a common basal plane.

The two common shapes of coordination complexes are square planar and octahedral. In the diagram, M represents the central metal atom or ion, and each A represents a ligand.

Many other shapes exist, including linear, trigonal planar, square pyramidal, and trigonal bipyramidal. However, these shapes are less common in coordination complexes formed by transition metals. The shape is determined by the coordination number, valence shell electron pair repulsion (VSEPR) considerations, and the number of f electrons of the central metal ion and their interactions with one another.

Geometries of Coordination Complexes

Coordination Number	Geometry
2	Linear: The cation and ligands form a line.
3	Trigonal planar: The ligands are on the same plane around the cation at 120° angles.
4	Tetrahedral: The ligands form a triangular pyramid. The cation is at the center of the pyramid. Square planar: The ligands are on the same plane around the cation at 90° angles.
5	Trigonal bipyramidal: Three ligands arrange themselves as a triangular planar shape. The remaining two ligands arrange themselves perpendicularly to this plane, forming a linear structure with the metal cation. Square pyramidal: Four ligands arrange themselves as a square planar shape. The remaining ligand is perpendicular to this plane.
6	Octahedral: Four ligands arrange themselves as a square planar shape. The remaining two ligands arrange themselves perpendicularly to this plane, forming a linear structure with the metal cation.
7	Pentagonal bipyramidal: Five ligands are on the same plane around the cation at 72° angles. The remaining two ligands arrange themselves perpendicularly to this plane, forming a linear structure with the metal cation.
8	Square antiprism: Four ligands form a square at one side of the cation, and the remaining four ligands form another square at the opposite side of the cation. Dodecahedral: The ligands arrange themselves in 12 triangular planes around the cation.
9 or more	Other configurations

The coordination number of a coordinate complex affects its geometry.

Isomerism

Isomers are molecules that have the same chemical formula but different arrangement of atoms.

The geometry of coordination complexes can be symmetrical if all the ligands are identical. However, this is not always the case. When different ligands attach to the central metal ion or different atoms of the same ligand attach to the central metal ion, the geometries vary according to the positions of atoms relative to the central metal ion.

An isomer is one of two or more molecules that share the same chemical formula but have different arrangements of atoms. Isomers can have very different properties from each other despite having the same chemical formula.

In the square planar complexes, four ligands are bound to a metal (M). M may be an ion or neutral. If three of the ligands are the same (A) and one is different (B), no isomers exist because they are related by rotation about the same axis. Rotation of the model gives the same molecule.

Square Planar Rotation

The diagram shows two types of ligands, A and B, around a central metal atom or ion. When only one ligand in a square planar complex is unique, isomers cannot form. Each arrangement is produced by simply rotating the complex.

However, if two B ligands exist, two different geometric isomers are possible. The two B ligands may be across from each other (at a 180° angle), or they may be adjacent to each other (at a 90° angle). This arrangement forms a geometric isomer, which is one of two or more molecules that have the same molecular formula and bond structure but differ in the positions of substituents around a rigid part of the molecule. Another name for a geometric isomer is a cis-trans isomer. A cis configuration is a geometric arrangement in which two identical substituents are on the same side of the central part of a molecule or an ion, at a 90° angle. A trans configuration is a geometric arrangement in which two identical substituents are on opposite sides of the central part of a molecule or an ion, at a 180° angle. In coordination complexes, a complex that has two similar ligands adjacent to one another is a cis isomer, and a complex with similar ligands opposite from each other is a trans isomer.

Square Planar Cis and Trans Isomers

When two similar ligands are on the same side of the central metal ion, the isomer has cis configuration. When two similar ligands are on opposite sides of the central metal ion, the isomer has trans configuration.

Two geometric isomers are found in diamminedichloridoplatinum(II) (Pt(NH₃)₂Cl₂). The cis isomer is known as cisplatin, a chemotherapy drug used to treat many forms of cancer. The trans isomer, however, has no therapeutic properties and is toxic to humans. The cis and trans ligand configurations can also be found in octahedral complexes.

The diagram shows two types of ligands, A and B, around a central metal atom or ion, M. Similar ligands on the same side of the central metal ion form the cis configuration in octahedral complexes. Similar ligands on opposite sides of the central metal ion form the trans configuration. Note that rotation of the complex gives either orientation of the isomer.

If the ligands are all different species, another type of isomerism, called optical isomerism, comes into play. An optical isomer, also called an enantiomer, is one of a pair of mirror-image molecules that are not identical and cannot be superimposed.

The diagram shows ligands of different species, represented by A, B, C, and D, around a central metal atom or ion, M. When the mirror image of a coordination complex cannot be superimposed on the complex, the isomers are optical isomers, also called enantiomers.

A linkage isomer is one of two or more coordination complexes that differ in how a ligand is bonded to the central metal atom or ion. The bonding atom in a ligand is different in each isomer.

A linkage isomer is formed when the atom within a ligand that bonds to the central metal ion differs.

An ionization isomer is one of two forms of a coordination complex that differ only in which of two species is a ligand and which is outside the coordination sphere. One of the ligands switches positions with another species that is part of the coordination complex but is outside the coordination sphere. This other species can be either an ion or a neutral molecule. The coordination complex changes so that it has a different ligand in the new coordination sphere.

An ionization isomer is formed when the ligand is swapped with a species outside the coordination complex. This species may be an ion or may be neutral.

Nomenclature

The naming of coordination complexes follows five rules: name the cation, name the ligands, use prefixes for multiple ligands, name the central metal, give the central metal's oxidation state.

Coordination complexes are named according to five rules outlined by Swiss chemist and Nobel Laureate Alfred Werner:

1. Name the cation first and the anion second if the coordination complex is ionic.
2. Name the ligands in alphabetical order, followed by the central metal ion. For anionic ligands, add the suffix -ido. For neutral ligands, use the name of the molecule, except for water (H₂O, use aqua), ammonia, (NH₃, use ammine), carbon monoxide (CO, use carbonyl), and nitrogen monoxide or nitric oxide (NO, use nitrosyl).
3. If more than one ligand of a single type is present, use Greek prefixes (di-, tri-, tetra-, and so on) to name them, unless the ligand already contains this sort of prefix (e.g., ethylenediamine). In this case or if the ligands are polydentate, use the prefixes bis-, tris-, tetrakis-, pentakis-, and so on.
4. Name the central metal. If the central metal is a cation, use the name of the element. If the central metal is part of an anion, use the suffix -ate (e.g., platinate, cuprate, ferrate).
5. After the central metal name, give its oxidation state in Roman numerals in parentheses.

For example, to name the complex [Co(NH₃)₆]Cl₃, first name the complex cation: [Co(NH₃)₆]³⁺. The ligands are all the same, ammonia. Ammonia is one of the exceptions. When ammonia coordinates to a metal center, it is called an ammine ligand. In this case, six ammines are coordinated to the same Co(III) cation, so the Greek prefix hexa- is used to describe that six ${\rm{Co}}^{\rm{III}}\!-\!\!{\rm{N}}$ bonds are present; this is called a hexammine complex. Its oxidation is determined by examining the charge of the complex cation and the anion within it. The complex cation must be 3+, because three Cl^– anions are bonded to it. The ammine ligand is neutral; therefore, the charge for the cation cobalt must be 3+. The cation of the coordination complex is therefore hexamminecobalt(III). Finally, name the anion, chloride. The coordination complex is therefore hexamminecobalt(III) chloride.

Names of Sample Coordination Complexes

Coordination Complex Type	Chemical Formula	Name
Cation complexes	[Pt(NH₃)₅Cl]Br₃	pentaamminechloridoplatinum(IV) bromide
Cation complexes	[Co(H₂NCH₂CH₂NH₂)₃]₂(SO₄)₃	tris(ethylenediamine)cobalt(III) sulfate
Anion complexes	K₄[Fe(CN)₆]	potassium hexacyanidoferrate(II)
Anion complexes	Na₂[NiCl₄]	sodium tetrachloridonickelate(II)
Neutral complexes	[Ni(H₂NCH₂CH₂NH₂)₂Cl₂]	dichloridobis(ethylenediamine)nickel(II)
Neutral complexes	Fe(CO)₅	pentacarbonyliron(0)

Consider K₄[Fe(CN)₆]. This complex is ionic, with K⁺ as the cation and the complex Fe(CN)₆^4- as the anion. The name starts with the name of the cation: potassium. To name the complex anion, start with naming the ligands. There are six CN ligands in the complex. CN is cyanide. This is an anionic ligand, so it gets the suffix -ido and becomes cyanido: potassium cyanido. There are six ligands, so the name of the ligand becomes hexacyanido: potassium hexacyanido. Next the metal should be named. The metal is part of an anion, so ferrate should be used instead of iron: potassium hexacyanoferrate. To determine the oxidation state of the iron ion, consider the charge of the cyanido groups and charge of the complex anion. Each cyanido ligand has a charge of 1–. The anion has an overall charge of 4–. This puts the charge of the iron ion to 2+. The name then becomes potassium hexacyanidoferrate(II).

Uses of Coordination Compounds

Coordination compounds are used in a variety of ways, including as biological agents, as pigments and dyes, as industrial catalysts, and for medicinal purposes.

Many coordination compounds occur naturally and play important biological roles. For example, hemoglobin, which carries oxygen in the blood, and chlorophyll, which is the pigment responsible for photosynthesis, both rely on coordination compounds. Vitamin B₁₂ is also a coordination compound. Because coordination compounds tend to be brightly colored, they are also frequently used as dyes and pigments, such as indigo, cochineal, and safflower, all of which are naturally occurring.

Industrially, coordination complexes with polydentate ligands are often used as chelating agents. That is, they remove heavy metals from solutions in which metals are undesirable, such as drinking water.

Further, coordination complexes can function as catalysts in both natural and industrial processes. An important pair of catalysts, titanium trichloride and triethylaluminum, catalyze the formation of polymers from organic compounds. This process is used to form many synthetic fibers, films, and plastics.

Coordination complexes also have medicinal uses. Cisplatin, cis-Pt^IICl₂(NH₃)₂, is a neutral coordination complex that is used to treat many types of cancers. The nitroprusside ion, [Fe^II(CN)₅NO]^2-, is a vasodilator used to treat patients suffering from hypertension, along with nitric oxide (NO). Lanthanum carbonate (La₂(CO₃)₃) is used to treat patients with renal failure, as a means of filtering excess phosphate. Nonsteroidal anti-inflammatory compounds, commonly referred to as NSAIDs, are ligands that can be combined with various metals for a variety of therapeutic uses. For example, when combined with copper, NSAIDs such as ibuprofen have enhanced anti-inflammatory effects and reduced side effects.

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Transition Metals and Coordination Chemistry

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