Chemistry BONDING-VALANCE BOND APPROACH AND BASIC IDEAS OF CRYSTAL FIELD THEORY
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BONDING-VALANCE BOND APPROACH AND BASIC IDEAS OF CRYSTAL FIELD THEORY

Valence Bond theory of Co-ordination Compounds :

(i) The suitable number of atomic orbitals of central metal ion (`s`, `p`, `d`) hybridise to provide empty hybrid orbitals.

(ii) These hybrid orbitals accept lone pair of electrons from the ligands and are directed towards the ligand positions according to the geometry of the complex.

(iii) When inner `d`-orbitals i.e. (`n-1`) `d` orbitals are used in hybridization, the complex is called-inner orbital or spin or hyperligated complex.

(iv) A substance which do not contain any unpaired electron is not attracted by `2` magnet. It is said to be diamagnetic. On the other hand, a substance which contains one or more unpaired electrons in the `d`-orbitals, is attracted by a magnetic field. [exception `O_2` and `NO`). It is said to be paramagnetic.

Paramagnetism can be calculated by the expression,

`mu_s` = `sqrt[n(n + 2)]`, where `mu` = magnetic moment.
`s` = spin only value and `n`= number of unpaired electrons.

On the basis of value of magnetic moment, we can predict the number of unpaired electrons present in the complex. If we know the number of unpaired electrons in the metal complex, then it is possible to predict the geometry of the complex species.

(v) There are two types of ligands namely strong field and weak field ligands. A strong field ligand is capable of forcing the electrons of the metal atom/ion to pair up (if required). Pairing is done only to the extent which is required to cause the hybridization possible for that Coordination number. A weak field ligand is incapable of making the electrons of the metal atom/ion to pair up.

Strong field ligands : `CN^-`, `CO`, `en`, `NH_3`, `H_2O`, `NO^-`, `Py`.

Weak field ligands : `I^-`, `Br^-`, `Cl^-`, `F^-`, `NO_3^-`, `OH^-`, `C_2O_4^(2-)`, `NH_3`, `H_2O`

Draw back of valence bond Theory :

(i) It describes bonding in coordination compounds only qualitatively.

(ii) It does not offer any explanation for the optical absorption spectra of complex.

(iii) It does not describe the detailed magnetic properties of coordination compounds.

Geometry(shape) & magnetic nature of some the complexes (Application of valence bond theory) is given in fig.

Ligand Field Theory :

According to this theory when the ligands come closer to metal atom or ion, a field is created. This field tends to split the degenerate `d`-orbitals of the metal atom into different energy levels. The nature and number and of lignads determine the extent
of splitting on the basis of which the magnetic and spectroscopic properties of the complex can be explained.

The shape of complexes depends upon hybridization state of central atom, it is described as follows :

(a) `text(Octahedral complexes)` : On the basis of hybridized orbitals it can be of two type as `d^2sp^3` (inner orbital) or `sp^3d` (outer orbital) hybridized. Let us assume that the six ligands are positioned symmetrically along the cartesian axes, with the metal atom at the origin. As the ligands approach, first there is an increase in the energy of `d` orbitals relative to that of the free ion just as would be the case in a spherical field. Next, the orbitals lying along the axes ( `d_(z^2)` and `d_(x^2 - y^2)`) get repelled more strongly than `d_(xy)`, `d_(yz)` and `d_(zx)` orbitals, which have lobes directed between the axes. The `d_(z^2)` and `d_(x^2 -y^2)` orbitals get raised in energy and `d_(xy)`, `d_(yz)`, `d_(zx)` orbitals are lowered in energy relative to the average energy in the spherical crystal field. Thus, the degenerate set of `d` orbitals get split into two sets: the lower energy orbital set, `t_(2g)` and the higher energy, `e_g` set. The energy separation is denoted by `Delta_o` (the subscript `o` is for octahedral): See fig.1

`text(Inner orbital complexes)` : We have already discussed that in these type of complexes the `d`-orbitals used are of lower quantum
number i.e. (`n- 1`) various examples are as follow
(i) Complexes fonned by the use of inner orbitals are diamagnetic or have reduced paramagnetism.
(ii) These are called as low spin or spin paired complexes

Examples : See fig.2

`text(Outer orbital complexes)` :
(i) In these complexes `s`, `p` as well as `d` orbitals involved in hybridization, belong to the highest quantum number(`n`).
(ii) Complexes formed by the use of outer `n``d` orbitals will be paramagnetic.
(iii) These complexes are called high-spin or spin free complexes.
(iv) The outer orbital complexes have greater number of unpaired electrons.

Examples : See fig.3

(b) `text(Tetrahedral Complexes)` : In tetrahedral coordination entity formation, the `d` orbital splitting is inverted and smaller as compared to the octahedral field splitting. For the same metal, the same ligands and metal-ligand distances, it can be shown that `Delta_t = -(4//9)Delta_o`. Consequently, the orbital splitting energies are not sufficiently large for forcing pairing and, therefore, low spin configurations are rarely observed. See fig.4

Examples : See fig.5

(c) `text(Square planar complex)` : These are formed due to `dsp^2` hybridisation. These complexes tend to be formed when the central ion has only one `d` orbital available in the inner shell.

Example : See fig.6

Stability of Complex :

(a) A complex is formed in solution by the stepwise addition of ligands to a metal ion.

(b) This can be expressed as follows `M + L ⇋ ML`, where `M` = metal and `L` is ligand.

(c) The stability constant `K` for this reaction is as shown

`K = [ML]/([M][L])`

(d) This metal can again get a ligand

`ML + L ⇋ ML_2`

(e) The forthcoming stabthty constant `K_1` then `K_1 = [ML_2]/([ML][L])` its value is less than `K`

(f) The higher the value of stability constant stabler is the complex.

(g) The value of stabilityconstants for some of the complexes are given in the table

Factors influencing the stability of complex :

(a) `text(Nature of central ion)` :

(i) The complex will be more stable for higher values of charge density(`text[Charge]/text[radius]`).
(ii) The higher the electronegativity of the central ion, the greater is the stability of its complexes.
(iii) The higher the oxidation state of the metal the more stable is the compound.

(b) `text(Nature of ligand)` :

(i) A basic ligand is likely to easi ly donate its e lectrons. Thus a more basic ligand will form more stable complex.

(ii) Chelating ligands form more stable complexes as compared to monodentate ligands.

Spectrochemical series :

Ligands can be arranged in increasing order of their strength (ability to cause crystal field splitting) and the series so obtained is called as spectrochemical series.

`I^(-) < Br^(-) < Cl^(-) < F^(-) < OH^(-) < OX^(-2) < H_2O < Py < NH_3 < en < NO_2^(-) < CN^(-) < CO`

Ligands arranged left to `NH_3` are generally regarded as weaker ligands which can not cause forcible pairing of electrons within `3d` level and thus form outer orbital octahedral complexes.

On the other hand `NH_3` and all ligands lying right to it are stronger ligands which form inner orbital octahedral complexes after forcible pairing of electrons within `3d` level.
 
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