Coordination Compounds - Practice Questions & MCQ

Addition Compounds or Molecular Compounds
These are those compounds which are formed by the combination or simple addition of two or more simple salts. These compounds are of two types, i.e, Double salts and Coordination compounds.

Difference between a double salt and a coordination compound
Both double salts as well as complexes are formed by the combination of two or more stable compounds in stoichiometric ratio. However, they differ in the fact that double salts such as carnallite, KCl.MgCl₂.6H₂O, Mohr’s salt, FeSO₄.(NH₄)₂SO₄.6H₂O, potash alum, KAl(SO₄)₂.12H₂O, etc. dissociate into simple ions completely when dissolved in water. However, complex ions such as [Fe(CN)₆]^4– of K₄[Fe(CN)₆] do not dissociate into Fe²⁺ and CN^– ions.

Coordination entity
A coordination entity constitutes a central metal atom or ion bonded to a fixed number of ions or molecules. For example, [CoCl₃(NH₃)₃] is a coordination entity in which the cobalt ion is surrounded by three ammonia molecules and three chloride ions. Other examples are [Ni(CO)₄], [PtCl₂(NH₃)₂], [Fe(CN)₆]^4–, [Co(NH₃)₆]³⁺

Central atom/ion
In a coordination entity, the atom/ion to which a fixed number of ions/groups are bound in a definite geometrical arrangement around it is called the central atom or ion. For example, the central atom/ion in the coordination entities: [NiCl₂(H₂O)₄], [CoCl(NH₃)₅]²⁺ and [Fe(CN)₆]^3– are Ni²⁺, Co³⁺ and Fe³⁺, respectively. These central atoms/ions are also referred to as Lewis acids.

Coordination sphere
The central atom/ion and the ligands attached to it are enclosed in square bracket and is collectively termed as the coordination sphere. The ionisable groups are written outside the bracket and are called counter ions. For example, in the complex K₄[Fe(CN)₆], the coordination sphere is [Fe(CN)₆]^4– and the counter ion is K⁺

Coordination polyhedron
The spatial arrangement of the ligand atoms which are directly attached to the central atom/ion defines a coordination polyhedron about the central atom. The most common coordination polyhedra are octahedral, square planar and tetrahedral. For example, [Co(NH₃)₆]³⁺ is octahedral, [Ni(CO)₄] is tetrahedral and [PtCl₄]^2– is square planar. 

Homoleptic and heteroleptic complexes
Complexes in which a metal is bound to only one kind of donor groups, e.g., [Co(NH₃)₆]³⁺, are known as homoleptic. Complexes in which a metal is bound to more than one kind of donor groups, e.g., [Co(NH₃)₄Cl₂]⁺, are known as heteroleptic.

Mono or Unidentate ligands
They have one donor atom, i.e, they supply only one electron pair to enctral metal atom or ion. F^-, Cl^-, Br^-, H₂O, NH₃, CN^-, etc. are examples of monodentate ligands.

Bidentate ligands
Ligands which have two donor atoms and have the ability to link with central metal ion at two positions are called bidentate ligands. Some examples include ethylenediamine(en), oxalate(ox), etc.

Tridentate ligands
The ligands having three donor atoms are called tridentate ligands. Some examples include diethylenetriamine(dien), 2.2,2-Terpyridine(terpy).

Tetradentate ligands
These ligands possess four donor atoms. Some examples include nitriloacetate, triethylenetetramine(trien).

Pentadentate ligands
They have five donor atoms. Some examples include ethylenediaminetriacetate ion.

Hexadentate ligands
They have six donor atoms. The most important example is ethylenediaminetetraacetate ion.

Ambidentate ligands
These are those ligands which can bind to the central metal atom through two different sites. For example CN^-, NCS^-, etc. 

Flexidentate ligands
These are the polydentate ligands having many donor sites but according to the availability they change their number of donor sites

Chelating ligands
Some polydentate ligands form coordinate bond with central metal atom through their donor sides forming a closed ring-like structure, these ligands are known as chelating ligands and the complex so formed is known as chelating complex.

• Stability of these ligands can be explained on the basis of entropy change.
• When a chelating ligand bonds to a central metal atom it displaces some number of monodentate ligand equal to its denticity which leads to an increase in the entropy of the system.
• 5 or 6 membered rings are more stable because angle strain is not there.

The oxidation number of the central atom in a complex is defined as the charge it would carry if all the ligands are removed along with the electron pairs that are shared with the central atom. The oxidation number is represented by a Roman numeral in parenthesis following the name of the coordination entity. For example, oxidation number of copper in [Cu(CN)₄]^3– is +1 and it is written as Cu(I).

The coordination number (CN) of a metal ion in a complex can be defined as the number of ligand donor atoms to which the metal is directly bonded. For example, in the complex ions, [PtCl₆]^2– and[Ni(NH₃)₄]²⁺, the coordination number of Pt and Ni are 6 and 4 respectively. Similarly, in the complex ions, [Fe(C₂O₄)₃]^3– and[Co(en)₃]³⁺, the coordination number of both, Fe and Co, is 6 because C₂O₄^2– and en (ethane-1,2-diamine) are didentate ligands.
It is important to note here that coordination number of the central atom/ion is determined only by the number of sigma bonds formed by the ligand with the central atom/ion. Pi bonds, if formed between the ligand and the central atom/ion, are not counted for this purpose.

Ligands are attached with the central metal ion through donor atoms. Each donor atom donates one electron pair to the central metal ion, i.e, the central metal atom or ion gains electrons from the donor atoms. In order to explain the stability of the complex, Sidgwick proposed effective atomic number denoted as EAN, which is defines as the resultant number of electrons with the metal atom or ion after gaining electrons from the donor atoms of the ligands. The effective atomic number (EAN) generally coincides with the atomic number of next noble gas in some cases. EAN is calculated by the given relation:

EAN = Atomic number of the metal - number of electrons lost in ion formation + number of electrons gained from the donor atoms of the ligands.

The EAN values of various metals in their respective complexes are tabulated below:

Just as the octet is useful in formulating the bonding in compounds of the light elements, the notion of an EAN provides a rough guide for bonding in coordination compounds. Almost all the metals achieve EAN of a noble gas through coordination. The EAN concept has been particularly successful for complexes of low valent metals.

The formula of a compound is a shorthand tool used to provide basic information about the constitution of the compound in a concise and convenient manner. Mononuclear coordination entities contain a single central metal atom. The following rules are applied while writing the formulas:
(i) The central atom is listed first.
(ii) The ligands are then listed in alphabetical order. The placement of a ligand in the list does not depend on its charge.
(iii) Polydentate ligands are also listed alphabetically. In case of abbreviated ligand, the first letter of the abbreviation is used to determine the position of the ligand in the alphabetical order.
(iv) The formula for the entire coordination entity, whether charged or not, is enclosed in square brackets. When ligands are polyatomic, their formulas are enclosed in parentheses. Ligand abbreviations are also enclosed in parentheses.
(v) There should be no space between the ligands and the metal within a coordination sphere.
(vi) When the formula of a charged coordination entity is to be written without that of the counter ion, the charge is indicated outside the square brackets as a right superscript with the number before the sign. For example, [Co(CN)₆]^3–, [Cr(H₂O)₆]³⁺, etc.
(vii) The charge of the cation(s) is balanced by the charge of the anion(s).

The names of coordination compounds are derived by following the principles of additive nomenclature. Thus, the groups that surround the central atom must be identified in the name. They are listed as prefixes to the name of the central atom along with any appropriate multipliers. The following rules are used when naming coordination compounds:
(i) The cation is named first in both positively and negatively charged coordination entities.
(ii) The ligands are named in alphabetical order before the name of the central atom/ion. (This procedure is reversed from writing formula).
(iii) Names of the anionic ligands end in –o, those of neutral and cationic ligands are the same except aqua for H₂O, ammine for NH₃, carbonyl for CO and nitrosyl for NO. While writing the formula of coordination entity, these are enclosed in brackets ( ).
(iv) Prefixes mono, di, tri, etc., are used to indicate the number of individual ligands in the coordination entity. When the names of the ligands include a numerical prefix, then the terms, bis, tris, tetrakis are used, the ligand to which they refer being placed in parentheses. For example, [NiCl₂(PPh₃)₂] is named as dichloridobis(triphenylphosphine)nickel(II).
(v) Oxidation state of the metal in cation, anion or neutral coordination entity is indicated by Roman numeral in parenthesis.
(vi) If the complex ion is a cation, the metal is named same as the element. For example, Co in a complex cation is called cobalt and Pt is called platinum. If the complex ion is an anion, the name of the metal ends with the suffix – ate. For example, Co in a complex anion, [Co(SCN)₄]²⁻ is called cobaltate. For some metals, the Latin names are used in the complex anions, e.g., ferrate for Fe.
(vii) The neutral complex molecule is named similar to that of the complex cation.

In the naming of complex ion the name of ligands are written in alphabetical order followed by the name of central metal atom with its oxidation number in roman numerals. If the complex part contains two or more same type of ligands then di, tri, tetra,etc. are used.

For example, [Co(NH₃)₃Cl₃] is written as Triamminetrichloridocobalt(III).

• The naming of the metal is replaced by placing the suffix  - "ate".
• Some special names are given to some metals.
   

  Metals	Cationic	Anionic
  Ag	Silver	Argentate
  Fe	Iron	Ferrate
  Cu	Copper	Cuprate
  Au	Gold	Aurate
  Pb	Lead	Plumbate
  Sn	Tin	Stannate

• For example, [Ag(NH₃)₂]Cl is named as diamminesilver(I)chloride.

The IUPAC of [PtCl₂(NH₃)₄]⁺²[PtCl₄]^-2 is written as Tetraamminedichloridoplatinum(IV)tetrachloroplatinate(II).

1. Use prefix -μ for ligands present in bridging.
2. If the charge on complex is odd, then distribute this charge to the metal atoms and it the charge is even, then divide this charge equally to the central metal atoms.
3. Now, the naming can be written according to the rules discussed earlier.