- If a compound absorbs light of one color(say orange), then it reflects(transmit) light of blue color. The transmitted or reflected light of blue color attacks on the retina of our eyes and the compound is seen to be blue colored. The color of the transmitted light is called the complementary color of absorbed light.
- At the heart of color theory, complementary color are the opposite hues on the color wheel.
- In their most basic form, they are one primary color and that is created by mixing the other two primary color. For example, the complementary color of yellow is purple which is a mix of blue and red.
- With that knowledge, it is rather easy to remember the first set of complementary color i.e. yellow and purple, blue and orange, red and green as shown in the above diagram.
- If we add the tertiary color, those are made up of one primary and one secondary color, we will find that that color are also complementary i.e. yellow-orange and blue-purple(indigo) orange-red and blue-green(aqua) red-purple(pink) and green-yellow
Limitations of Crystal Field Theory:
- In CFT only d-electrons of the metal ion are considered, the other orbitals such as s, are not taken into consideration.
- This theory has not considered the covalent character in transition metal complexes. It treats the metal-ligand as purely ionic.
- CFT cannot explain the relative strength of ligands as given in spectrochemical series, i.e. it cannot explain why H2O is a stronger ligand as compared to OH– ion.
- CFT has also not considered the Pi bonding in complexes either it is metal to ligand or ligand to metal.
- This theory has no significance to the orbits of the ligands. Therefore it cannot explain any properties related to ligand orbitals and their interaction with metal orbitals.
- It don’t explain the effect of π bond on Δ.
- The compounds like in which metal is in zero oxidation states and the ligand is neutral have no electrostatic attraction between the metal and the ligands.
1.Cr(CO)6 is a volatile compound and Ni(CO)4 is a liquid. This indicates that there is a covalent bonding between metal and ligand instead of the ionic. If there would be an ionic bond, then Cr(CO)6 should be non-volatile and Ni(CO)4 is solid.
2. Electron Spin Resonance(ESR):
- The ESR spectrum of [IrCl6]2- suggests that the single unpaired electron is only 70% localized on the metal atom, and 30% is localized on the chloride ion. This indicates that there is sharing of electron and hence some covalency between metal and ligands.
3. The Nuclear Magnetic Resonance(NMR):
- The fluoride NMR has detected the delocalization of electron in the fluoro complexes, of the paramagnetic metal ion. It is possible only when an unpaired electron spends more than negligible time on 19F nucleus.
4. The Nephelauxetic Effect:
- The electronic repulsion in d-orbitals of transition metal cations gives rise to a number of energy levels depending upon the arrangement of electrons in d-orbitals.
- The energy difference between two energy states can be expressed in terms of interelectronic repulsion parameters, called Racah parameters B and C. The difference in energy between two levels having same spin multiplicity can be expressed in terms of only B, and the difference in energy between two energy levels having different spin multiplicities can be expressed in terms of B and C.
- It is observed experimentally(i.e. from electronic spectra of complexes)that the magnitude of B and C decreases when the complex is formed. The reduced value of B and C indicates that electron density is reduced on metal cation i.e. electron cloud is delocalized over both the metal cation and the ligands. This suggests that there is some covalency between metal cation and ligands.
- The more the value of B and C is reduced, the greater the delocalization of electron cloud and greater the covalency. The delocalization of the electron cloud over the metal cation and the ligand is called the nephelauxetic effect.
5. Nuclear Quadrupole Resonance(NQR);
- The NQR spectrum of some of the complexes containing halide ions as ligands like [PtCl4]2- , [PdCl4]2-suggests that metal-ligand bond is partly ionic and partly covalent.
1.Enthalpy of Hydration:
- When one mole of an ionic crystal is dissolved in water, water molecules gather from the ion and this process is called hydration. In this process, some amount of energy is released which is called Hydration energy.
- Hydration energy of a metal cation increases with the increase in effective nuclear charge and decrease in ionic radii because these two factors bring the water molecules closer to the metal cation resulting in the increased electrostatic attraction between the metal cation and the water molecule.
- For dipositive transition metal cation of 3d-series, the effective nuclear charge increases and ionic radii decrease across a period. So hydration energy should increase regularly from Ca2+ to Zn2+
- So, Hydration energy α charge of the cation ̸ size of the cation
- For example, Hydration energy of Co2+ < Co3+ because here as the size of both the ions are same, the ion having higher charge has greater hydration energy.
- When one mole of an ionic crystal is formed from its constituent gaseous ions, some amount of energy is released which is called lattice energy.
- Or energy required to break one mole of ionic crystal into its surrounding gaseous ion is called lattice energy.
- According to Born Lande’s equation lattice energy of an ionic crystal increases with the increase in the product of Z+and Z–and decrease in the interionic distance(r0).
- The lattice energy for the halides of dipositive metal ions of the 3d-series transition element should increase from Ca2+ to Zn2+ion and a straight line should be observed.
3.Ionic radii of Divalent Metal ions of 3d-series transition element:
- The ionic radii of dipositive and tripositive metal cations of 3d-series transition metals in the low spin or high-spin octahedral field might be expected to decrease regularly from Ca2+to Zn2+ .
- The reason is that there is an increase of force of attraction between metal cations and ligands due to the increase in effective nuclear charge and the poor shielding effect of d-electrons due to which ligands and metal cation approach each other more closely.
- In weak field octahedral complex of 3d-series transition metal with oxidation number less than equal to +3, the value of is ∆0 small and there will be no pairing of electrons. Therefore in weak field complexes of d4, d5, d6, and d7 configuration, there is no pairing of electrons. These complexes have the maximum number of unpaired electrons are called high spin or spin free complexes. The term high spin or spin free is used because these complexes have the same number of spin as in d-orbital of free metal cations.
- In strong field octahedral complex of 3d-series transition metal with oxidation number, in general, greater than equal to +2, the value of ∆0 is large. In strong field complexes of d4, d5, d6, and d7 configurations, the pairing of d-electrons will take place in according to Hund’s rule. These complexes have the maximum number of paired electrons are called low spin or spin paired complexes. The term low spin or spin paired is used because these complexes have more number of paired electrons(or spin)than that of the free metal cation.
- It is to be notated that week field octahedral complexes are always not the high spin complexes. The metal cation of 3d-transition series with the oxidation number of greater than equal to +4 and 4d and 5d series transition metal cations always form low spin complexes with weak ligands.
- For example, [NiF6]2- ion (oxidation state of Ni is +4) is low spin and diamagnetic, though F–is a weak ligand. [Rh(H2O)6]3+is low spin and diamagnetic, though is a weak ligand.
- An exception is observed for 3d-series transition metals in which Co3+ form low spin complexes with H2O and O2- though H2O and O2-are weak ligands.
1. Oxidation State of the Metal Cation:
For example ∆0 for [Co(H2O)6]2+ = 9200 cm-1
2. Same Oxidation State of Metal Cation but the number of d-electrons is Different:
For example ∆0 for [Co(H2O)6]2+ = 9200 cm-1(3d7)
3. Principal Quantum Number(n) of the d-orbital of the Metal Cation:
∆0 for [Ir( NH3)6]2+= 41200 cm-1
4. Nature of Ligands:
(weak end)O22−< I− < Br− < S2− < SCN− (S–bonded) < Cl− < N3− < F−< NCO− < OH− < C2O42− < H2O < NCS− (N–bonded) < CH3CN < gly (glycine) < py (pyridine) < NH3 < en (ethylenediamine) < bipy (2,2′-bipyridine) < phen (1,10-phenanthroline) < NO2− < PPh3 < CN− < CO < CH2(strong end)
The order of the field strength of common ligands is independent of the nature of the metal cation and the geometry of the complex.
5. Number of Ligands:
Crystal field splitting in Octahedral complex:
- In a free metal cation all the five d-orbitals are degenerate(i.e.these have the same energy.In octahedral complex say [ML6]n+the metal cation is placed at the center of the octahedron and the six ligands are at the six corners. These six corners are directed along the cartesian coordinates i.e. along the x, y, and z-axis. When all the ligands are at an infinite distance from the metal cation, the five d-orbital of the metal cation will not be affected by the ligand electrostatic field and will remain degenerate. When the ligands move towards the metal cation, there are two electrostatic forces i.e.
(1)The attraction between metal cation and ligand
(2)Electrostatic repulsion between d-electrons of the metal cation and lone pairs of ligands
- Greater the repulsion between metal cation and ligands, ligands will be more closer to the metal cation and hence more will be the repulsion between the metal d-electrons and the lone pair of electrons on the ligand. When the ligands are closer to the metal cation an electrostatic force of repulsion also exists among the ligands.These two repulsion cause to adopt the octahedral geometry that locates the ligand at the internuclear distance from the metal cation and as far apart from one another as possible.
- The force of repulsion between metal d-electron and the ligand electrons cause to increase in potential energy of metal d-electrons. Remember that greater the force of repulsion higher will be the potential energy. If all the six ligands approaching the metal cation surrounds it spherically symmetric i.e. all the six ligands are at equal distance from each of the d-orbitals.
- The energy of each d-orbital will raised by the same amount and all the five d-orbital will remain degenerate. This is a hypothetical situation and has the average energy of a set of d-orbitals.In an actual octahedral complex, a spherically symmetric field is never obtained. Therefore all the five d-orbitals are not affected by the same extent.
- Since the two d-orbitals( dx2-y2 and dz2 ) points directly towards the ligands and the three d- orbitals( dxy ,dyz and dzx ) point in between the path of the approaching ligand. Therefore the dx2-y2 and dz2 orbitals will be more strongly repelled than the dxy ,dyz and dzx orbitals. Therefore the energy of the dx2-y2 and dz2 orbitals will be raised and that of the other three orbitals which lie far away from the ligand will be decreased relative to the hypothetical energy state.
- The five d-orbital which were degenerate in a free metal cation is now split into two sets of d-orbitals of different energies, a higher energy level with two orbitals(dx2-y2 and dz2)having the same energy and a lower level with three equal energy orbitals(dxy,dyz, and dzx). The set of dx2-y2 and dz2orbitals are referred to as eg set which is doubly degenerate and the set of dxy,dyz, and dzx is referred to as t2g set which is triply degenerate.
- Since the distance between metal cation and the ligands has remained the same, the net potential energy(or average energy) of the system must remain the same as that of the spherical field before splitting. This state of average energy is called the barycentre.
- The separation of five d-orbitals of metal cation into two sets of different energies is called crystal field splitting. The energy difference between two sets of orbitals which arise from an octahedral field is measured in terms of the parameter ∆0 or 10Dq where o in ∆0 stands for octahedral.
- Since the energy of barycentre remains constant, the total energy decrease of the t2g set must be equal to the total energy increase of the eg set. Therefore since there are two eg orbitals, they must increase by 0.6∆0 or 6Dq and the three t2g orbitals must decrease by 0.4∆0 or 4Dq per electron. The decreased energy t2g of orbitals stabilizes the complex by 0.4∆0 and the increase in energy of eg orbitals destabilizes the complex by 0.6∆0.
Crystal Field Theory:
- Ionic ligands such as Cl–,OH–,CN– are regarded as negative point charges(or simply point charges) and the neutral ligands such as H20,NH3,Py are regarded as dipole(or simply dipoles) because these ligands are dipolar.If the ligand is neutral molecule like the negative end of the dipole is directed towards the metal ion.
- Metal-ligand bond is not covalent i.e. there is no overlapping of orbitals.Instead of bonding in complexes is purely electrostatic in nature.In complexes two types of electrostatic forces come into account,
(1)One is the attraction between metal cation and the negatively charged ligand or the negative end of the polar ligand(i.e. dipole)
- Another repulsion also come into account that occurs among the ligands.
- the five d-orbital in a free metal ion are degenerate(i.e.have same energy).When a complex is formed, the electrostatic field of ligands destroy the degeneracy of these d-orbitals i.e. these orbitals now have same energies.
- The orbital lying in the direction of the lidands are raised in energy more than those lying away from the ligands because of the repulsion between the d-electrons and the ligands.
- In order to understand CFT, it is necessary to know the geometry and orientation of the five d-orbitals.
Outer orbital octahedral complex:
- In this complex ion oxidation state of Fe is +1, because NO exists in +1 oxidation state in a complex of Fe and it is valence shell electronic configuration is 3d6, 4s1
- Magnetic moment measurement indicates that its experimental magnetic moment is 3.89 B.M. which corresponds to three unpaired electrons in the d-orbital of complexion.
- The single NO+ strong ligand has little tendency to pair up only two unpaired electrons. Since H2O is a weak ligand, therefore, it has no tendency to pair up electrons and none of the five 3d orbitals is vacant.
- Therefore the 4s, 4p and two of the five 4d orbitals (i.e.4dx2-y2 and 4dz2 )combine to give six sp3d2 hybrid orbitals.
- These hybrid orbital form bonds with six ligands by accepting six pairs of electrons, one pair from each of the six ligands.
- Here as outer d-orbital is involved in hybridization it gives outer orbital octahedral geometry.
- The octahedral complex of d1, d2, and d3 metal cation are always inner orbital octahedral complexes whether the ligands are strong or weak.
- The octahedral complex of d8, d9, and d10 metal cation are always outer orbital complexes either the ligands are strong or weak.
- The complexes of d4, d5,d6, and d7 metal cation are outer orbital complex if the ligands are weak.
- In this complex oxidation state of cobalt is +3
Valence shell electronic configuration is
- Magnetic moment measurement indicates that [Co(CN)6]3-C ion is diamagnetic(given). So that all the d-electrons arranged in such a way, that no unpaired electron is left in the d-orbital.
- In other words as CN– ion is a strong ligand, it causes the pairing of 3d-electrons.
- Here two vacant 3d-orbital combine with the vacant 4s and 4p orbital to form six d2sp3
- Then six hybrid orbital overlap with six filled orbital of CN– ligand, one on each of the ligand and thus six coordinate covalent bonds are formed which gives d2sp3 hybridization.
- As inner d-orbitals are involved in hybridization, it gives inner orbital octahedral geometry.