Bonding Of Co-ordination Compounds

1. Valency Bond Theory

(i) Uses hybrid orbitals to hold the donated electrons pairs for formation of the coordinate covalent bonds.

(ii) Can explain the structure and magnetic properties.

(iii) Select low energy empty orbitals to hybridize for the appropriate geometry.

(iii) If there are not enough orbitals, pair up any unpaired metal electrons to free up orbitals.

(iv) Hybrid orbitals sets:

d²sp³ octahedral

sp³ tetrahedral

dsp² square planar

Hybrid Orbitals

1. Octa hedral complexes

(a) Inner orbital complexes: When the complex formed involves (n – 1)d – orbitals for hybridization (d²sp³), the complexes is called inner orbital complex or low spin complex.

Example: [Co(NH₃)₆]³⁺, [Co(CN)₆]^4-, [Fe(CN)₆]^3-

(b) Outer orbital complexes: When the complex formed involves outer nd – orbitals for hybridization (sp³d²), the complexes is called outer orbital complex or high spin complex.

Example: [Ni(NH₃)₆]³⁺, [Fe(H₂O)₆]³⁺, [FeF₆]^3-

(i) Octahedral hybridization uses $d_{xy},d_{yz},d_{zx},d_{x^2-y^2},d_{z^2}$atomic orbitals to form six d²sp³ hybrid orbitals.

(ii) Consider the formation of [Fe(CN)₆]^3-

Fe = [Ar]3d⁶4s²

(iii) To form${\left[Fe{\left(CN\right)}_6\right]}^{3-}$, Fe need 6 empty orbitals to hold the electron pairs donated by the CN atoms, so must pair up 2 electrons:

(iv) Now we can form bonds with two d orbitals, one orbitals, and three p orbitals, giving d²sp³ hybridization.

(v) ${\left[Fe{F}_6\right]}^{3-}$has 5 unpaired electrons, not 1, so we have to modify this approach, using 4d orbitals instead of 3d orbitals, with no electron pairing.

2. Tetrahedral complexes

(i) Formation of [Ni(CO)₄]⁰. Oxidation state of nickel in this complex is '0'. Its electronic configuration is [Ar]3d⁸4s². Hence, we have

sp³ hybrid orbitals accommodate four pairs of electrons from four CO molecules and the resulting tetrahedral complex is diamagnetic due to absence of unpaired electrons.

Square Planar Complexes:

(ii) Formation of [Cu(NH₃)₄]²⁺. Oxidation state of Cu in[Cu(NH₃)₄]²⁺ = +2. Hence, we have

Limitations of Valence Bond Theory

(a) Explains, but does not predict the shape.

(b) Qualitative explanations; does not explain relative stability.

(c) Can not explain colour and spectra.

(d) Can not explain relative stability of structural isomers.

2. Crystal Field Theory

(i) Assumes electrostatic interactions between the metal ion and the ligands, rather than covalent bond formation.

(ii) Bonding attractions between positively charged ion and negatively charged electron pairs on the ligands.

(iii) Electrons on the metal (d electrons) are repelled by the ligands electrons.

(iv) Some d orbitals are repelled to a greater extent than others, which can explain magnetic properties and colour.

(a) Between ligands repulsion

$\bullet$d_xy

$\bullet$d_xz

$\bullet$d_yz

(b) Toward ligands repulsion

$\bullet$$d_{z^2}$$\bullet$$d_{x^2-y^2}$

Splitting in Crystal Field Theory

(A) Octahedral crystal field

(i) Ligands in an octahedral structure are located on the axes, so they repel the$d_{x^2-y^2}$and $d_{z^2}$ orbitals more that the others.

(ii) Octahedral field thus splits the orbitals into two energy levels.

(iii) The splitting of the energy levels is called D, the crystal field splitting energy.

(iv) It is possible to promote electron form a lower energy orbitals to a higher energy orbital, with absorption of light of a wavelength corresponding to an energy of

(B) Tetrahedral crystal field

(i) In a tetrahedral structure, the ligands are located at opposite corners of a cube enclosing the axes.

(ii) The orbitals are split into two energy levels, but in the reverse order of an octahedral field.

Tetrahedral Complexes

4 of the eight corners of the cube are occupied by ligands.

Orbitals occupancy in a crystal field

(i) Depends on the magnitude of $\Delta$.

(ii) Varies with the structure.

${\Delta }_{oct}>{\Delta }_{tetr}\left(tetra\sim 45\%ofoct\right)$

(iii) Varies with principal energy level.

$4d>3d\left(4d\sim 1.45\times 3d\right)$

$5d>4d\left(5d\sim 1.74\times 3d\right)$

Illustration : What is crystal field splitting?

Solution: In a free transition metal ion, all the five d-orbitals are degenerate but when it is involved in a complex formation, the degeneracy is split. This is called crystal field splitting.