Bonding In Coordination Compounds
- 1 Valence Bond Theory For Bonding In Coordination Compounds
- 2 Applications of Valence Bond Theory
- 3 Magnetic Properties of Coordination Compounds and their Geometries
- 4 Limitations of Valence Bond Theory
Valence Bond Theory For Bonding In Coordination Compounds
(1) The central metal ion in the complex makes available a number of empty orbitals for the formation of coordination bonds with suitable ligands. The number of empty orbitals made available for this purpose is equal to coordination number of the central metal ion.
(2) The appropriate atomic orbitals (s, p and d) of the metal hybridise to give a set of equivalent orbitals of definite geometry such as square planar, tetrahedral, octahedral and so on.
The following types of hybridisation are involved for different geometries of the complexes.
(3) The d-orbitals involved in the hybridisation may be either inner d-orbitals i.e. (n – 1) d or outer d-orbitals i.e. nd.
(4) Each ligand has at least one orbital (of donor atom) containing a lone pair of electrons.
(5) The empty hybrid orbitals of metal ion overlap with the filled orbitals of the ligand to form metal-ligand coordinate covalent bonds.
Applications of Valence Bond Theory
Examples of Complexes of C.N. = 6
The chromium (Z = 24) has the electronic configuration 3d5 4s1
The chromium in this complex is in +3 oxidation state and the ion is formed by the loss of one 4s and two of the 3d-electrons.
The inner d-orbitals are already vacant and two vacant 3d, one 4s and three 4p-orbitals are hybridised to form six d2sp3 hybrid orbitals. Six pairs of electrons one from each NH3 molecule (shown by xx) occupy the six vacant hybrid orbitals. The molecule has octahedral geometry.
Since the complex contains three unpaired electrons, it is paramagnetic.
Cobalt atom (Z= 27) has the electronic configuration 3d74s2. In this complex, cobalt is in +3 oxidation state and has the electronic configuration 3d6. This complex has been found to be diamagnetic. The two electrons in 3d-orbitals are paired up leaving two 3d-orbitals empty. These six vacant orbitals (two 3d, one 4s and three 4p) hybridise to form d2sp3 hybrid orbitals. Six pairs of electrons two from each NH3 molecules are donated to these vacant hybrid orbitals. Thus, the complex has octahedral geometry and is diamagnetic.
(3) [CoF6]3– complex ion
Cobalt is in +3 oxidation state and has the electronic configuration 3d6. This complex has been found to be paramagnetic due to the presence of four unpaired electrons.
The electrons in 3d-orbitals are not disturbed and the outer 4d-orbitals are used for hybridisation. The six orbitals (one 4s, three 4p and two 4d) are hybridised forming six sp3d2 hybrid orbitals.
Six pairs of electrons, each one from F ion are donated to the vacant hybrid orbitals forming Co-F bonds.
Thus, the complex has octahedral geometry and is paramagnetic.
Inner and Outer Orbital Entities or Complexes
In octahedral structures, the central metal may use inner (n-1)d orbitals or outer nd-orbitals for hybridisation. Therefore, the complexes may be classified as :
(i) Inner orbital complex or entity
If the complex is formed by the use of inner d-orbitals for hybridisation (written as d2sp3), it is called inner orbital complex. In the formation of inner orbital complex, the electrons of the metal are forced to pair up and hence the complex will be either diamagnetic or will have lesser number of unpaired electrons. Such a complex is also called low spin complex.
For example, [Fe(CN)6]3- and [Co(NH3)6]3+ are inner orbital complexes.(ii) Outer orbital complex or entity
If the complex is formed by the use of outer d-orbitals for hybridisation, it is called an outer orbital complex. The outer orbital complex will have larger number of unpaired electrons since the configuration of the metal ion remains undisturbed. Such a complex is also called high spin complex.For example, [Fe(H2O)6]3+ and [CoF6]3- are outer orbital complexes.
Inner orbital complex (uses inner (n-1) d orbitals) = low spin complex
Iron atom (Z = 26) has the electronic configuration 3d64s2. In this complex, iron is in +3 oxidation state and has the electronic configuration 3d5.
The complex has one unpaired electron. The two electrons in 3d-orbitals are paired up leaving two 3d-orbitals empty. These six vacant orbitals (two 3d, one 4s and three 4p) hybridise to form d²sp³ hybrid orbitals. Six pairs of electrons one from CN¯ ion (shown by xx) occupy the six vacant hybrid orbitals.
The molecule has octahedral geometry and is paramagnetic due to the presence of one unpaired electron.
Since the inner d-orbitals are used in hybridisation, the complex [Fe(CN)6]3- is called an inner orbital or low spin or spin paired complex.
Iron is in +3 oxidation state and has the electronic configuration as 3d5. This complex has been found to be paramagnetic due to the presence of five unpaired electrons.
The electrons in 3d-orbitals are not disturbed and the outer 4d-orbitals are used for hybridisation.
The six orbitals (one 4s, three 4p and two 4d) are hybridised resulting sp3d2 hybridisation. Six pairs of electrons, one from each water molecule occupy the six hybrid orbitals. The molecule is octahedral and is paramagnetic.
Since [Fe(H2O)6]3+ uses outer orbital (4d) in hybridisation, it is therefore, called outer orbital or high spin or spin free complex.
Iron is in +2 oxidation state. The complex is diamagnetic and therefore, it involves d2sp3 hybridisation.It is an inner orbital or low spin complex.
Examples of Complexes of C.N. = 4
The geometry will be tetrahedral or square planar depending upon whether sp3 or dsp2 hybridisation is involved.
The nickel atom has the ground state electronic configuration as 3d84s2.
Nickel is in +2 oxidation state and its electronic configuration is 3d8.
Depending upon the type of hybridisation, there are two possible ways in which the complexes of nickel with coordination number 4 may be formed.
(a) If the complex involves sp3 hybridisation, it would have tetrahedral structure : For the formation of tetrahedral structure the 3d-orbitals remain unaffected and, therefore, the two unpaired d-electrons remain as such. The complex would be paramagnetic.
(b) If the complex involves dsp2 hybridisation, it would have square planar structure The formation of square planar structure through dsp2 hybridisation, one of the 3d-orbitals should be empty and available for hybridisation. This is possible, if the two unpaired d-electrons are paired up thereby making one of the 3d-orbitals empty. There is thus no unpaired electron and the complex would be diamagnetic.
The nickel (II) ion has two unpaired electrons. The magnetic measurements of the complex [NiCl4]2- show that it is paramagnetic and has two unpaired electrons. Therefore, in this case the 3d-orbitals remain undisturbed and sp3 hybridisation occurs resulting in tetrahedral structure of the complex. There are two unpaired electrons in the complex.
The nickel (0) has 3d84s2 as its outer electronic configuration. For complexes with coordination number 4, the central atom may involve sp3 or dsp2 type of hybridisation, for each of which the 4s-orbital must be empty. The electrons of 4s orbitals are forced into 3d-orbitals to pair up with the two unpaired d electrons. Therefore, the complex is diamagnetic. This results in sp3 hybridisation and the complex has tetrahedral structure.
Examples of Complexes of C.N. = 5
The oxidation state of iron in this complex is zero and it has the outer electronic configuration as 3d6 4s2.According to the Hund’s rule, the six electrons shall occupy the five 3d orbitals in such a way that there are four unpaired electrons. For the complexes with coordination number 5, the central atom may involve dsp3 hybridisation, and the 4s orbital must be empty.
The two electrons of 4s orbital and one electron 3d orbital are pushed into 3d orbitals to pair up with the three unpaired 3d electrons. The metal atom involves dsp3 hybridisation (one 3d, one 4s and three 4p) to give vacant dsp3 hybrid orbitals.
Since the complex has no unpaired electron, it will be diamagnetic and it is in agreement with experimental results. Thus, the complex [Fe(CO)5] has trigonal bipyramidal geometry and is diamagnetic.
Magnetic Properties of Coordination Compounds and their Geometries
For metal ions upto three electrons in d-orbitals are available for octahedral hybridisation using 4s and 4p orbitals. The magnetic behaviour of these three free ions and their coordination entities is similar.
When more than three 3d electrons are present, then the two 3d orbitals for octahedral hybridisation are not directly available because of Hund’s rule of maximum multiplicity.
For example: For coordination compound having 3d4 configuration (Cr2+ and Mn2+), to make two 3d orbitals empty, one of the electron will be paired with one of the other orbitals leaving two unpaired electrons. Similarly, for d5 (Mn2+, Fe3+) and d6 (Fe2+, Co2+) configurations, two vacant 3d orbitals result only by pairing of 3d electrons leaving one and zero unpaired electrons respectively.
For example: [Mn(CN)6]3- has magnetic moment corresponding to two unpaired electrons. [MnCl6]3- has magnetic moment corresponding to four unpaired electrons.
Similarly, [Fe(CN)6]3- has magnetic moment of a single unpaired electron while [FeF6]3- has magnetic moment corresponding to 5 unpaired electrons.
[CoF6]3- is paramagnetic with four unpaired electrons whereas [Co(C2O4)3]3- is diamagnetic.
For example: the complexes [Mn(CN6]3- , [Fe(CN)6]3- and [Co(C2O4)3] are inner orbital complexes involving d2sp3 hybridisation and are low spin complexes. The first two complexes are paramagnetic while the latter is diamagnetic.
[MnCl6]3- , [FeF6]3- and [CoF6]3- are outer orbital complexes involving sp³d² hybridisation
Limitations of Valence Bond Theory
(i) It involves a number of assumptions.
(iii) It does not explain the detailed magnetic properties of the complexes.
(iv) This theory does not explain the spectral properties of the coordination compounds.
(v) It does not explain the thermodynamic and kinetic stabilities of different coordination compounds.
(vi) It does not make exact predictions regarding the tetrahedral or square planar structures of 4-coordinate complexes.
(vii) It does not distinguish between weak and strong ligands.