# Intro to Coordination Chemistry

Discussion Questions

• How did the study of coordination compounds started?
• How can the number of isomers be used to determine the structure of a coordination compound?

Coordination chemistry is the study of the compounds that form between metals and ligands, where a ligand is any molecule or ion that binds to the metal. A metal complex is the unit containing the metal bound to its ligands. For example, [PtCl2(NH3)2] is the neutral metal complex where the Pt+2 metal is bound to two Cl- ligands and two NH3 ligands. If a complex is charged, it is called a complex ion (ex. [Pt(NH3)4]+2 is a complex cation). A complex ion is stabilized by formation of a coordination compound with ions of opposite charge (ex. [Pt(NH3)4]Cl2). It is convention to write the formula of a complex or complex ion inside of square brackets, while counterions are written outside of the brackets. In this convention, it is understood that ligands inside the brackets are bound directly to the metal ion, in the metal's first coordination sphere (a.k.a inner coordination sphere). Ions written outside of the brackets are assumed to be in the second coordination sphere, and they are not directly bound to the metal.

• Neutral Complex: $$[CoCl_3(NH_3)_3]$$
• Complex Cation: $$[CO(NH_3)_6]^{3+}$$
• Complex Anion: $$[CoCl_4(NH_3)_2]^-$$
• Coordination Compound: $$K_4[Fe(CN)_6]$$

A common metal complex is Ag(NH3)2+, formed when Ag+ ions are mixed with neutral ammonia molecules.

$Ag^+ + 2 NH_3 \rightarrow Ag(NH_3)_2^+$

A complex Ag(S2O3)23- is formed between silver ions and negative thiosulfate ions:

$Ag^+ + 2 S_2O_3^{2-} \rightarrow Ag(S_2O_3)_2^{3-}$

The geometry and arrangement of ligands around the metal center affect the properties of a coordination compounds. Compounds with the same molecular formula can appear as isomers with very different properties. Isomers are molecules that have identical chemical formulas, but have different arrangements of atoms in space. Isomers with different geometric arrangements of ligands are called geometric isomers. Isomers whose structures are mirror images of each other are called optical isomers.

## How did the study of coordination compounds started?

The coordination chemistry was pioneered by Nobel Prize winner Alfred Werner (1866-1919). He received the Nobel Prize in 1913 for his coordination theory of transition metal-amine complexes. At the start of the 20th century, inorganic chemistry was not a prominant field until Werner studied the metal-amine complexes such as $$[Co(NH_3)_6Cl_3]$$. Werner recognized the existence of several forms of cobalt-ammonia chloride. These compounds have different color and other characteristics. The chemical formula has three chloride ions per mole, but the number of chloride ions that precipitate with Ag+ ions per formula is not always three. He thought only ionized chloride ions will form precipitate with silver ion. In the following table, the number below the Ionized Cl- is the number of ionized chloride ions per formula. To distinguish ionized chloride from the coordinated chloride, Werner formulated the Complex formula and explained structure of the cobalt complexes.

Figure 1: Proposed Structure of Cobalt Ammonia Complexes from Number of Ionized Chloride
Solid Color Ionized Cl- Complex formula
CoCl36NH3 Yellow 3 [Co(NH3)6]Cl3
CoCl35NH3 Purple 2 [Co(NH3)5Cl]Cl2
CoCl34NH3 Green 1 trans-[Co(NH3)4Cl2]Cl
CoCl34NH3 Violet 1 cis-[Co(NH3)4Cl2]Cl

The structures of the complexes were proposed based on a coordination sphere of 6. The 6 ligands can be ammonia molecules or chloride ions. Two different structures were proposed for the last two compounds, the trans compound has two chloride ions on opposite vertices of an octahedral, whereas the the two chloride ions are adjacent to each other in the cis compound. The cis and trans compounds are known as geometric isomers.

Other cobalt complexes studied by Werner are also interesting. It has been predicted that the complex Co(NH2CH2CH2NH2)2ClNH3]2+ should exist in two forms, which are mirror images of each other. Werner isolated solids of the two forms, and structural studies confirmed his interpretations. The ligand NH2CH2CH2NH2 is ethylenediamine (en) often represented by en.

Example 1

Sketch the structures of isomers Co(en)33+ complex ion to show that they are mirror images of each other.

Solution
The images are shown on page 242 Inorganic Chemistry by Swaddle. If the triangular face of the end-amino group lie on the paper, you can draw lines to represent the en bidentate ligand. These lines will show that the two images are similar to the left-hand and right-hand screws.

From the description above, sketch the structures.

Exercise 1

• How many isomers does the complex Co(NH2CH2CH2NH2)2ClNH3]2+ have? Draw the structures of the isomers.
• How many isomers does Co(en)2Cl2+ have? Sketch the structures of the isomers.
• How many isomers does Co(NH3)4Cl2+ have? Sketch the structures of the isomers.

## How are coordination compounds named?

Structures of coordination compounds can be very complicated, and their names long because the ligands may already have long names. Knowing the rules of nomenclature not only enable you to understand what the complex is, but also let you give appropriate names to them.

Often, several groups of the ligand are involved in a complex. The number of ligand molecules per complex is indicated by a Greek prefix: mono-, di- (or bis), tri-, tetra-, penta-, hexa, hepta-, octa-, nona-, (ennea-), deca- etc for 1, 2, 3, ... 10 etc. If the names of ligands already have one of these prefixes, the names are placed in parentheses. The prefices for the number of ligands become bis-, tris-, tetrakis, pentakis- etc.

For neutral ligands, their names are not changed, except the following few:

• $$H_2O$$: aqua
• $$NH_3$$: ammine (not two m's, amine is for organic compounds)
• $$CO$$: carbonyl
• $$NO$$: nitrosyl

Normal names that will not change

• NH2CH2CH2NH2, ethylenediamine
• C5H4N-C5H4N, dipyridyl
• P(C6H5)3, triphenylphosphine
• NH2CH2CH2NHCH2CH2NH2, diethylenetriamine

The last "e" in names of negative ions are changed to "o" in names of complexes. Sometimes "ide" is changed to "o". Note the following:

• Cl-, chloride -> chloro
• OH-, hydroxide -> hydroxo
• O2-, oxide -> oxo
• O2- peroxide, -> peroxo
• CN-, cyanide -> cyano
• N3-, azide -> axido
• N3-, nitride -> nitrido
• NH2-, amide -> amido
• CO32-, carbonate -> carbonato
• -ONO2-, nitrate -> nitrato (when bonded through O)
• -NO3-, nitrate -> nitro (when bonded through N)
• S2-, sulfide -> sulfido
• SCN-, thiocyanate -> thiocyanato-S
• NCS-, thiocyanate -> thiocyanato-N
• -(CH2-N(CH2COO-)2)2, ethylenediaminetetraacetato (EDTA)

The names of complexes start with the ligands, the anionic ones first, followed with neutral ligands and the metal. If the complex is negative, the name ends with "ate". At the very end are some Roman numerals representing the oxidation state of the metal.

To give and remember all rules of nomenclature are hard to do. Pay attention to the names whenever you encounter any complexes is the way to learn.

• [Co(NH3)5Cl]Cl2, Chloropentaamminecobalt(III) chloride
• [Cr(H2O)4Cl2]Cl, Dichlorotetraaquochromium(III) chloride
• K[PtCl3NH3], Potassiumtrichloroammineplatinate(II)
• PtCl2(NH3)2, Dichlorodiammineplatinum
• Co(en)3Cl3, tris(ethylenediamine)cobalt(III)chloride
• Ni(PF3)4, tetrakis(phosphorus(III)fluoride)nickel(0)

A bridging ligand is indicated by placing a m- before its name. The m- should be repeated for every bridging ligand. For example,

(H3N)3Co(OH)3Co(NH3)3, Triamminecobalt(III)-m-trihydroxotriamminecobalt(III)

Example 2

Solution

The structure is

(C6H5)3P    Cl    Cl
\  /  \  /
Pd    Pd
/  \  /  \
Cl    Cl    P(C6H5)3


Exercise 2

• How many ionized chloride ions are there per formula?
• How many chloride ions act as bridges per formula in this complex?
• How many types of chloride ligands are there in this complex?

Example 3

Name the complex:

        NH
(en)2Co<  >Co(en)2 Cl3
OH


Solution

The name is Bis(ethylenediamine)cobalt(III)-m- imido-m-hydroxobis(ethylenediamine)cobalt(III) ion.

DISCUSSION

When this compound dissolves in water, is the solution a conductor? What are the ions present in the solution of this compound? How many moles of chloride ions are present per mole of the compound?

When potassiumtrichloroammineplatinate(II) dissolves in water, what ions are produced? What about chloropentaamminecobalt(III) chloride?

## How the number of isomers be used to determine the structure of a coordination compound?

Before modern structure determination methods were developed, the study of complexes were mostly done by chemical methods and deduction. The fact that CoCl2(NH3)4Cl has only two isomer suggests that