A Satisfactory theory of bonding in coordination compound must account for properties such as colour and magnetism as well as stereochemistry and bond strength. No single theory as yet does all this for us. Rather several different approaches have been applied to transition metal complexes
A Satisfactory theory of bonding in coordination compound must account for properties such as colour and magnetism as well as stereochemistry and bond strength. No single theory as yet does all this for us. Rather several different approaches have been applied to transition metal complexes. We will consider first of all valance bond theory and after that we explain crystal field theory. CFT is more preferred rather than VBT because it gives colour and magnetism both by single theory.
VALANCE BOND THEORY (VBT)-
The valence bond theory, VBT, was extended to coordination compounds by Linus Pauling in 1931.
The metal ligand bond arises by donation of pair of electrons by ligand to the central metal atom.
To accommodate these electrons the metal ion must possess requisite number of vacant orbitals of comparable energy. These orbitals of the metal atom undergo hybridization to give hybrid orbitals. The basic premise of hybridization is that appropriate linear combinations of non-equivalent orbital’s of an atom give sets of hybrid orbital’s that are equivalent and have specific spatial orientations. For coordination compounds, the hybridizations involving s, p and d orbital’s are important. Different linear combinations of s, p and d orbital’s, like dsp2, dsp3 and d2sp3 yield various types of coordination compounds.
Sometimes the unpaired (n-1) d orbital’s pair up before bond formation making (n-1) d orbital’s vacant. The central metal atom makes available number of d-orbital equal to its co-ordination number.
The metal ligand bonds are thus formed by donation of electron pairs by the ligand to the empty hybridized orbitals. These bonds are equal in strength and directional in nature.
Octahedral, trigonal-bipyramidal or square pyramidal, square planar and tetrahedral complexes are formed as a result of dsp3 (or sp3d2), sp3d, dsp2 and sp3 hybridization respectively.
On the basis of the valence bond theory it is usually possible to predict the geometry of coordination complexes.
In this paramagnetic octahedral compound, Cr ion is in +3 oxidation state and has the electronic configuration of 3d3. The Cr ion undergoes d2sp3 hybridization to give six equivalent hybrid orbitals. Six pairs of electrons, one from each NH3 molecule, occupy the six hybrid orbitals. The complex formed has a octahedral geometry and is paramagnetic because it has 3 unpaired electrons. In the formation of this complex the inner 3d orbital are used in hybridization; that’s why this type of complex, [Cr (NH3)6] +3 is called an inner orbital or low spin or spin paired complex.
In the above diamagnetic octahedral complex, Fe ion is in +2 oxidation state and has the electronic configuration of 3d4. The Fe ion on d2sp3 hybridization gives six equivalent hybrid orbital’s, which are occupied by six pairs of electrons, one from each CN- molecule. Thus the complex consists of an octahedral geometry but is diamagnetic because it has no unpaired electrons. This complex is also called an inner orbital or low spin or spin paired complex.
Drawback of the valance bond theory-
Although valance bond theory successfully explains the geometry (shape) and magnetic behaviour of the coordination compound but it has a number of shortcomings. A few of these are as follow:
It cannot explain why some complexes of a metal ion in a particular oxidation states are low spin, i.e., inner orbital complexes of these some metal ion in the some other complexes of the same metal ion in the same oxidation state are high spin, i.e., outer complexes.
It could not give any satisfactory explanation for the colour in the complexes. It fails to explain the absorption spectra of coordination compounds.
It does not give an exact explanation of thermodynamics or kinetic stabilities of coordination compounds.
It does not distinguish between weak and strong ligands.
In mathematical treatment, it involves a number of assumptions.
In a number of cases ,the experimentally observed values of magnetic moment do not
Exactly coincide with the values calculated from valance bond theory.
CRYSTAL FIELD THEORY-
Crystal field theory (CFT) describes the electronic structure of complex compounds. In order to account for the limitations of VBT theory then a new theory CFT successfully accounts for some magnetic properties, colours, hydration enthalpies, and a spinel structures of transition metal complexes, but it does not attempt to describe bonding. CFT was given by physicists Hans Bethe and John Hasbrouck van Vleck in the 1930s.
In coordination compound transition metal behaves as a central metal ion and sounded by ligands.
Ligands act as a point charge.
Attraction between metal and ligands is 100% ionic. It may be ion-ion and ion -dipole interaction.
Electrons of ligands repels the valance electron of metal atom so these electron preferred to occupy those orbital’s which are not indirection of ligands this result in splitting of these orbital’s.
d orbital’s split into two set one is having lower energy and another set have high energy this is known as crystal field splitting.
Crystal field splitting depends on –
Number of ligands
Arrangement of ligands around central metal ion (geometry)
Crystal field theory explains the magnetic property of complex ion. If in complex compound small splitting occur then high spin is gain and if in complex compound large splitting is occur then low spin is gain.
Colour of the complex ion is explain on the basis of d-d transition at between two set of d orbital’s and result of crystal field theory.
Splitting in octahedral field-
The most common type of complex is octahedral in each complex which has six ligands form an octahedron around the metal ion. In the complexes formed in a structure of octahedral the compounds formed have splitting energy named as crystal fielding splitting energy and is denoted by âˆ†oct. In these compounds the dxy, dyz and the dxz orbitals have relatively less energy than the dz2 and dx2-y2 This is due to the fact that the ligands come along the axis which causes a electrostatic repulsion in between the electrons of d orbitals and the ligand lone pairs which causes the energy gap, therefore dz2 and dx2-y2 experience more repulsion but the dxy, dxz and dyz orbitals will experience relatively less repulsion. The higher two energy orbitals are denoted by Eg and the lower three orbitals are denoted as t2g orbitals. The general energy diagrams are as represented below:-
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The size of the energy gap Δ between the two sets of orbitals depends on many factors, including the ligands and geometry of the complex. Weaker field ligands always produce a small value of Δ means small splitting in d orbitals, while strong field ligand always gives a large splitting. The reasons behind this can be explained by ligand field theory. The spectrochemical series is a list of ligands ordered by the size of the splitting Δ that they produce.
What is meaning of high spin and low spin-
Strong field ligands are the ligands which cause a great or large difference on the formed d orbitals, for example CN− and F- are strong field ligand. The electrons in a complex which is formed by a strong field ligand are placed in orbitals in a way so as minimum spins are occurring. Due to this all the orbitals which are having relatively lower energy are filled earlier then as to fill the orbitals having higher energy, even if the pairing starts. This hence follows the Afbau principle. The complexes hence formed are called Low spin complexes since some of their spins are generally cancelled. Like for example CN- is a strong field ligand and it is a cause of the higher splitting energy observed.
On the other hand the ligands like -I and -Br are weak field ligands since they cause a very less splitting of the d orbitals. As a result it is relatively very easy to put the electrons into different orbitals as putting in the same orbital needs some amount of energy. Hence each of the d orbital acquires an electron which leads to the formation of a high spin complex in accord to the rule of maximum multiplicity given by Hund. As for example Br is a weak field ligand and it will be a cause of formation of high spin complex.
To have a low spin complex formed the energy required for placement of an electron in the already occupied orbital should be less than the energy required to have the electron in a new orbital. As discussed earlier eg are the higher energy orbitals in case of the octahedral orbits. If the requirement of energy for pairing of the electrons is relatively less than the energy required for a new orbital electron then the low spin complexes are formed.
Splitting in tetrahedral field-
In tetrahedral coordination entity formation, the d orbital splitting is inverted and is smaller as compared to the octahedral field splitting. For the same metal, the same ligands and metal-ligand distances, it can be shown that Δt = (4/9) Δ0. Consequently, the orbital splitting energies are not sufficiently large for forcing pairing and, therefore, low spin configurations are rarely observed.
After the octahedral compounds the tetrahedral compounds are most common, in these compounds the metal ion is surrounded by ligands which help in making the coordination number to four. The case of a tetrahedral compounds is just the opposite to the case of octahedral, as here dxy, dyz and dxz are having higher energies than the dx2-y2 and dz2 orbitals and again the splitting is in two parts. As the ligands come along the axis of the tetrahedral the energy of the t2g orbitals is higher due to the repulsion of the electrons. In case of tetrahedral the ligands are not oriented along the axis which results in less splitting of the tetrahedral orbitals then in case of octahedral ones. Square geometries are also be described by CFT. Hence the energy required to place the electron in higher energy orbital is lower than the energy required to pair the electrons as a result the tetrahedral compounds are generally high spin.
These energy diagrams help to understand the formation of high spin and low spin complexes. This accounts for the information of diamagnetic and paramagnetic materials. A compound that is having unpaired electrons in its d orbitals has paramagnetic nature and will cause attraction to the magnetic fields present. On the other hand the compounds which do not have an unpaired electron or are low spin, are diamagnetic in nature causing less attraction by the magnetic fields present out there.
Splitting in squre planner geometray –
The splitting pattren for squre-planer complexes is the most complicated of three cases. The squre planar geometry may be considered to be derived from octahedral by removing negative charges from z-axies. As these negative charges are removed , dz2 dxz and dyz orbitals, all of which have a z- component become more stable as given fig.Clearly , the dx2-y2 orbitals possesses the higest energy (as in octa hedral case) , and the dxy orbitls the next highest. However , the relative placement of the dz2 and the d and orbitals cannot be dettermined simply by inspection and must be calculated.
This type of spiltings may also be explained as follow :
As the lobes of dx2-y2 point towards the ligands, this orbitals has highest energy. The lobes of dxy orbitals lie between the ligand but are coplaner with them , hence this orbitals is next highest in energy. The lobes of dz2 orbitals point out of the plane of the complex but the belt around the centre of the orbital lies in the plan. Therefore , dz2 orbitls is next higest energy. The lobes of dxy and dyz orbitals point out of the plane of the complex , hence they are least affected by the electrostatic field of the ligands, they are degenerate and lowest in energy.
Limitations of Crystal Field Theory-
The crystal field model is successful in explaining the formation, structures, colour and magnetic properties of coordination compounds to a large extent. However, from the assumptions that the ligands are point charges, it follows that anionic ligands should exert the greatest splitting effect. The anionic ligands actually are found at the low end of the spectrochemical series. Further, it does not take into account the covalent character of bonding between the ligand and the central atom. These are some of the weaknesses of CFT, which are explained by ligand field theory (LFT) and molecular orbital theory which are beyond the scope of the present study.
(a). R.chang chemistray
(b). Pardeep 12 th books
(c). Bse chemistray books
(d). Class notes
5. http://www.tutorvista.com/content/chemistry/chemistry-iv/coordination- compounds/coordination-compoundsindex.php
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