CRYSTAL FIELD SPLITTING IN TETRAHEDRAL COMPLEXES:

Tetahedral complex (sp³):

In a tetrahedral field : Consider a cube such that a metal atom or ion is situated at its centre of symmetry through which the axis of geometry are passing and joining the face centres of this cube. Therefore, lobes of eg orbitals will be directed towards the face centres but those of t₂g orbitals will be pointing towards edge centres. Now consider 4 monodentate ligands approaching the metal, the 4 alternate corners of this cube so as to make a tetrahedron.

Thus it is clear that t₂g orbitals are nearer to the ligands than the eg orbitals. Hence t2g orbitals will experience more repulsion than eg orbitals. Therefore, crystal field splitting will be reversed of octahedral field which can be shown as below.

In tetrahedral complexes none of the ligand is directly facing any orbital so the splitting is found to be small in comparison to octahedral complexes. For the same metal, the same ligands and metal-ligand distances, it can be shown that del.tetra = (4/9) del.oct. This may attributes to the following two reasons.

(1) There are only four ligands instead of six, so the ligand field is only two thirds the size; as the ligand field spliting is also the two thirds the size and

 (2) The direction of the orbitals does not concide with the direction of the ligands. This reduces the crystal field spliting by roughly further two third.

Consequently, the orbital splitting energies are not sufficiently large for forcing pairing and, therefore, low spin configurations are rarely observed.

FACTORS FAVOURING TETRAHEDRAL COMPLEXES:

Tetrahedral complexes are favoured by steric requirements, either simple electrostatic repulsion of charge ligands or vander wall's repulsions of large one. A valence bond (VB) point of view ascribed tetrahedral structure to sp3 hybridisation.

Tetrahedral complexes are thus generally favoured by large ligands like Cl^-, B^-, I^- and PPh₃ and metal ions of six types;

(1) Those with a noble gas configuration such as Be²⁺ (ns⁰);

(2) Those with pseudo noble gas configuration (n-1) d¹⁰ns⁰np⁰, such as Zn^2+, Cu⁺ and Ga^3+, and

(3) Those transition metal ions which do not strongly favour other structure by virtue of the CFSE, such as Co^2+, d⁷.

(4) Those transition metal which have lower oxidation state.

(5) Those metals generally with electronic configuration d⁰, d⁵ and d¹⁰ prefer to form such complexes.

(6) It is observed that

OTHER EXAMPLES :

SN	Complex	Nature
1	[Ni(CO)4]	Diamagnetic
2	[Ni(Cl)4]2-	Paramagnetic with two unpaired electron
3	[NiCl2(pph3)2]	Paramagnetic with two unpaired electron
4	[MnCl4]2-	Paramagnetic with five unpaired electron
5	[FeCl4]2-	Paramagnetic with four unpaired electron
6	[Cu(py)4]+	Diamagnetic
7	Cs2[CuCl4]	Paramagnetic with two unpaired electron (Orange tetrahedral) Sp3
8	NH3[CuCl4]	Paramagnetic with two unpaired electron (Yellow Square Planer) dsp2
9	[Zn(NH3)4]2+	(d10) CFSE=0 , Diamagnetic
10	[Zn(CN)4]2-	(d10) CFSE=0 , Diamagnetic

CRYSTAL FIELD THEORY (CFT):

The Crystal Field Theory (CFT) was originally proposed for explaining the optical properties of crystalline solids. It was applied to the study of coordination compounds in the 1950s. CFT assumes the ligands to be point charges and the interaction between them and the electrons of the central metal to be electrostatic in nature. The five d-orbitals in an isolated gaseous metal atom/ion have same energy, i.e., they are degenerate. This degeneracy is maintained if a spherically symmetrical field of negative charges surrounds the metal atom/ion. However, when this negative field is due to ligands (either anions or the negative ends of dipolar molecules like NH₃ and H₂O) in a complex, it becomes asymmetrical and the degeneracy of the d-orbitals is lifted. It results in splitting of the d-orbital energies. The pattern of splitting depends upon the nature of the crystal field. We will first consider:

(1) CRYSTAL FIELD SPLITTING IN OCTAHEDRALCOMPLEXES:

For convenience, 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 to that of the free ion just as would be the case in a spherical filed. Next, the orbitals lying along the axes (dz²and dx²-y² d) get repelled more strongly than d_xy, d_yz and d_xz orbitals, which have lobes directed between the axes. The d_xy , d_yz , d_xz orbitals are lowered in energy relative to the average energy in the spherical crystal filed. Thus, the degenerate set of d orbitals get split into two sets: the lower energy orbitals set, t₂g and the higher energy, eg set. The energy separation is denoted by del.oct (the subscript o is for octahedral.

CRYSATAL FIELD STABLISATION ENERGY:

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(2) CRYSTAL FIELD SPLITTING IN TETRAHEDRAL COMPLEXES:
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(3) CRYSTAL FIELD SPLITTING IN SQUARE PLANER COMPLEXES:

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