CBSE Class 12 Chemistry Coordination Compounds Notes Set 08

Revision Notes

Coordination Compounds : Properties and IUPAC Name

• Coordination Compound: A coordination compound contains a central metal atom or ion surrounded by the number of oppositely charged ions or neutral molecules. There is a coordinate bond between metal atom and these ions or molecules, e.g., \( [Cu(NH_3)_4]^{2+} \).

• Double Salt: When two or more salts are added to form a stable solid and dissociate into constituent ions when dissolved in water or any solvent, e.g., \( FeSO_4(NH_4)_2SO_4 \cdot 6H_2O \) (Mohr’s salt).
• Properties of double salts:
  • (i) They give simple ions in aqueous solution because they are ionic compounds.
  • (ii) They do not contain coordinate bonds.
  • (iii) They exist only in solid state as double salt.
  • (iv) They are soluble in water.
• Coordinate bonds: A type of covalent bond in which one of the atoms supplies both the electrons. Coordinate bonds are also called dipolar bonds or dative bonds.
• Central metal atom or ion: The metal atom or ion surrounded by the fixed number of ions or molecules is called central metal atom or ion, e.g., in \( K_4[Fe(CN)_6] \), \( Fe^{2+} \) is the central metal ion.
• Ligand: The neutral molecules or ions (usually anions) which are attached with the central metal atom or ion in the complex compound. e.g., \( Cl^- \), \( OH^- \), \( CN^- \), \( CO \), \( NH_3 \), \( H_2O \), etc. A ligand may be neutral or charged species. It always act as a Lewis base.

Types of ligands:

(a) On the basis of number of donor sites:

• (i) Unidentate ligands: Contain one donor atom. e.g., \( \ddot{NH}_3 \), \( H_2\ddot{O} \)
• (ii) Bidentate ligands: Contain two donor atoms. e.g., \( (COO^-)_2 \), \( CH_2 - \ddot{NH}_2 \) | \( CH_2 - \ddot{NH}_2 \)
• (iii) Polydentate ligands: Contain more than two donor atoms. e.g., EDTA.

(b) On the basis of charge:

• (i) Cationic ligands: Carry positive charge. e.g., \( NO_2^+ \), \( N_2H_5^+ \).
• (ii) Anionic ligands: Carry negative charge. e.g., \( X^- \) (halo), \( CN^- \) (cyano).
• (iii) Neutral ligands: Do not carry any charge. e.g., \( \ddot{NH}_3 \) (ammine), \( H_2\ddot{O} \) (aqua).

(c) On the basis of nature of ligand:

• (i) Chelate ligands: A bidentate or polydentate ligand uses its two or more donor atoms to bound a single metal ion, then a ring like structure is obtained. It is called chelate and the ligand is known as chelate ligand.
• (ii) Ambidentate ligand: A ligand which contains two donor atoms but only one of them forms a coordinate bond at a time with central metal/ion is called ambidentate ligand. e.g., \( M \leftarrow NO_2 \) (Nitrito—N), \( M \leftarrow SCN \) (Thiocyanato), \( M \leftarrow NCS \) (Isothiocyanate).

• Coordination number: Number of monodentate ligands attached to central metal ion in a complex is called coordination number. It may also be defined as total number of chemical bonds formed between central metal ion and donor atom of ligand e.g., in \( [Ni(NH_3)_6]^{2+} \), the coordination number of \( Ni \) is 6.
• Coordination polyhedron: The spatial arrangement of the ligand atoms which are directly attached to the central atoms or ions define a coordination polyhedron about the central atom e.g., \( [PtCl_4]^{2-} \) has square planar geometry.
• Charge on the complex ion: The charge on the complex ion is equal to the algebraic sum of the charges on all the ligands coordinated to the central metal ion.
• Donor atom: An atom in the lewis base that forms the bond with the central atom/ion is called donor atom because it donates the pair of electrons.
• Denticity: The number of ligating groups or coordinating atoms in a ligand is called denticity e.g., unidentate, bidentate, etc.
• Applications of chelates:
  • (i) In the softening of hard water.
  • (ii) In the separation of lanthanoids and actinoids.
  • (iii) In the detection as well as estimation of some metal ions such as nickel (II) ion.
• Coordination sphere: The central atom/ion and the ligands attached to it are enclosed in square bracket and is collectively termed as coordination sphere e.g., in the complex \( K_4[Fe(CN)_6] \), the coordination sphere is \( [Fe(CN)_6]^{4-} \).
• Flexidentate character of ligands: Certain polydentate ligands have flexible character and are called flexidentates. e.g., EDTA is hexadentate in nature but in some cases, it may act as pentadentate or tetradentate ligand.
• Oxidation number of central atom: It is defined as the charge that central metal atom carry if all the ligands are removed along with the electron pairs that are shared with the central atom. It is represented by Roman numeral.
• Homoleptic and Heteroleptic complexes: Complexes in which the metal atom or ion is linked to only one type of ligands are called homoleptic complexes, e.g., \( [Co(NH_3)_6]^{3+} \) and \( [Fe(CN)_6]^{4-} \). The complexes in which the metal atom or ion is linked to more than one kind of ligands are called heteroleptic complexes. e.g., \( [Co(NH_3)_4Cl_2]^+ \) and \( [Cr(en)_2Cl_2]^+ \).
• Homonuclear and Polynuclear complexes: Complexes in which only one metal atom is present are known as homonuclear complexes. e.g., \( [Co(NH_3)_6]Cl_3 \) and \( [Cu(NH_3)_4]SO_4 \). Complexes in which more than one metal atom is present are known as polynuclear complexes.
• Counter ions: The ions which are not included in the primary coordination sphere are known as counter ions. e.g., in \( K_4[Fe(CN)_6] \), \( K^+ \) ions are counter ions.
• Coordination ions: The coordination entity with charge is called as coordination ion.

IUPAC Nomenclature of Coordination compounds:

• (i) The cation whether simple or complex is named first followed by anion.

• (ii) Ligands are named in alphabetical order.
• (iii) For indicating the number of each kind of ligand within the coordination entity, two kinds of numerical prefixes are used (di, tri, tetra, etc.). For ligands containing any of these prefixes in their names, their numbers are indicated by prefixes bis, tris, tetrakis, etc. Anionic ligands end with–o. Neutral ligands retain their names while cationic end with ium.
• (iv) The coordination sphere is written in square bracket.
• (v) In IUPAC naming, ligands are named first in the alphabetical order followed by the metal atom and then the oxidation state of metal by a Roman numeral in parenthesis.
• (vi) Name of coordination compounds starts with a small letter and the complex part is written as one word.
• (vii) Oxidation number of central atom is indicated in Roman numerals. No space is left between the number and the rest of the name.

Werner’s Theory, Bonding, VBT, CFT

Werner’s Theory of Coordination compounds:

Different postulates of Werner’s coordination theory are given below:

• (i) Metal ions possess two types of valency (a) Primary or ionisable valency and (b) Secondary or non ionisable valency.
• (ii) Every metal ion has a fixed number of secondary valency and this is known as coordination number.
• (iii) Primary valencies are satisfied by the anions while secondary valancies are satisfied by the negative group or neutral molecules with lone pair of electrons.
• (iv) Secondary valencies are directed in space towards the internal positions.

Limitations of Werner’s theory: This theory does not explain the following queries—

• (i) Why is the complex forming tendency limited to a few elements only?
• (ii) Why bonds in the coordination complexes are of directional nature?
• (iii) Why are certain complexes of magnetic nature and show geometrical and optical isomerism?

Valence Bond theory:

It was developed by Pauling. The brief points are:

• (i) A suitable number of vacant orbitals must be present in the central metal atom or ion for the formation of coordinate bond with the ligands.
• (ii) Central metal ion can use appropriate number of \( s, p \) or \( d \)-orbitals for hybridisation depending upon total number of ligands.
• (iii) The hybridised orbitals are allowed to overlap with those ligand orbitals that can donate an electron pair for bonding.
• (iv) The outer orbitals (high spin) or inner orbitals (low spin) complexes are formed depending upon whether outer \( d \)-orbitals or inner \( d \)-orbitals are used.

Limitations of Valence bond theory:

• (i) It cannot explain the detailed magnetic properties of complex compounds.
• (ii) It cannot explain the optical absorption spectra of coordination compounds.
• (iii) It cannot predict properly whether a particular coordinate complex is square planar or tetrahedral in geometry.
• (iv) It fails to make distinction between strong and weak ligands.
• (v) It does not explain thermodynamic or kinetic stabilities of coordination compounds.

Crystal field theory (CFT):

• (i) The ligand is considered as point charge or point dipole.

• (ii) Interaction between metal ion and ligand is considered as electrostatic in nature.
• (iii) Metal ion is supposed to be present at the origin of the axis. Ligands approach to the metal ion along the axis of octahedral complex between the axis of tetrahedral complex and in the case of square planar complex four ligand approach to metal ion along x, y plane.
• (iv) Due to the electrostatic interaction between ligands electrons and metal \( d \)-orbital electron, degeneracy of \( d \)-orbital is lost and splitting of \( d \)-orbitals occurs.
• (v) Some ligands are able to produce strong fields in which the splitting will be large whereas others produce weak fields and consequently result in small splitting of \( d \)-orbitals. In general, ligands can be arranged in a series in the order of increasing field strength as given below and called as spectrochemical series.

Properties of Metal carbonyls:

• (i) Metal carbonyls are mostly solids at room temperature and pressure. Exceptionally, iron and nickel carbonyls are liquids.
• (ii) The mononuclear carbonyls are volatile and toxic.
• (iii) Most of metal carbonyls are soluble in hydrocarbon solvents except \( [Fe_2(CO)_9] \).
• (iv) Mononuclear carbonyls are either colourless or light coloured.
• (v) They are highly reactive due to central metal and the CO ligands.
• (vi) Metal carbonyls are used as industrial catalyst and as precursor in organic synthesis.

Bonding in metal carbonyls: It also involves both \( \sigma \)- and \( \pi \)-bond. \( \sigma \)-bond is formed by overlapping of lone pair on Co to the vacant \( d \)-orbitals of metal whereas \( \pi \)-bond is formed by the back donation of pair of \( d \)-electrons to vacant antibonding orbital of carbonyl.

Factors affecting the stability of coordination complexes:

• (i) Nature of the central ion: Greater the charge density on the central metal ion, greater is the stability of the complex.

• (ii) Nature of the ligand: More basic ligands have a tendency to donate the electron pairs to the central metal ion more easily resulting in a stable complex.
• (iii) Chelate effect: Entropy increases when chelation occurs and so the formation of the complex becomes more favourable.

Mnemonics

• Concept: Ligands causing Crystal Field Splitting are arranged in order of increasing field strength-Spectrochemical Series

• Mnemonic: I Brought SCaNned Classnotes to Study Fundamentals of Chemistry.
• He Nurtured Excellence in NHew (New) CoordinatioN COmpounds chapter.
• Interpretation: \( I^- < Br^- < SCN^- < Cl^- < S^{2-} < F^- < OH^- < C_2O_4^{2-} < H_2O < NCS^- < EDTA^{4-} < NH_3 < CN^- < CO \)
• \( I^- = I \)
• \( Br^- = Brought \)
• \( SCN^- = SCaNned \)
• \( Cl^- = Classnotes \)
• \( S^{2-} = Study \)
• \( F^- = Fundamentals \)
• \( OH^- = Of \)
• \( C_2O_4^{2-} = Chemistry (Oxalate ion) \)
• \( H_2O = He \)
• \( NCS^- = Nurtured \)
• \( EDTA^{4-} = Excellence \)
• \( NH_3 = NHew \)
• \( CN^- = CoordinatioN \)
• \( CO = COmpounds (Carbonyl group) \)

Know the Terms

• Coordination chemistry: The study of the coordination compounds is known as coordination chemistry.

• Labile complex: A complex in which ligand substitution is fast.
• Inert complex: A complex in which ligand substitution is slow.
• Synergic bonding: A ligand donates a pair of electrons to the metal atom or ion and then accepts a pair of electrons back in its vacant orbital also from \( d \)-orbitals of the metal or ion. This is called synergic bonding and the ligands involved are known as \( \pi \)-donor ligands.
• Effective Atomic Number (EAN): It can be calculated for the metal atom or ion in the coordination complex by using following relation: EAN = Atomic no. (Z) of metal atom – Oxidation number + 2 C.N. where, C.N. is coordination number.
• Perfect or penetrating complexes: These are the complexes in which complex ion is fairly stable and either completely or feebly dissociates in solution.
• Imperfect or abnormal complexes: These are the complexes in which the complex ion is less stable and is dissociated reversibly to give enough simple ions.
• Homogeneous catalysis: Organometallic compounds or intermediates derived from soluble transition metal complexes catalyse a variety of reaction in solutions. This is known as homogeneous catalysis.
• Macrocyclic effect: Multidentate ligands happen to be cyclic in nature without causing any steric hindrance, the stability of the complexes is further increased. This is known as macrocyclic effect.
• Metal carbonyl: Organometallic compounds in which carbon monoxide acts as the ligand.