In this blog post, you will learn about why are most of the transition elements paramagnetic and why do they show variable oxidation state.
(1)Why do transition elements show paramagnetic behavior?
·The paramagnetic behavior is due to the presence of unpaired electron in (n-1)d orbital.
·The paramagnetic behavior increases as the number of unpaired electron increases.
·In 3d series, the paramagnetic behavior increases from beginning to the middle (Sc to Mn) and then decrease to zinc.
·The paramagnetism is expressed in magnetic moment µ.
·The value of µ increases as the number of unpaired electrons increases and consequently, paramagnetism also increases.
·√n(n+2) BM; where n=no. of unpaired electrons.
·For compound with 2 unpaired electrons-: mu=2√2=paramagnetic
(2)Why do transition elements shows variable oxidation state?
·Variable oxidation state is due to the participation of outer ns and penultimate (n-1)d electrons in bonding since these orbitals have almost equal energies.
·The most common oxidation state shown by 3d series is +2.
·After removing the ns-electron, the remainder is called core. In case of d-block elements, the core is usually unstable and hence loses 1 more electron.
·Thus the losing of 1 or more electron gives rise to the variable oxidation states to transition metals.
·For eg-: Sc has +2 and +3 oxidation state +2 because it has both s-electron used for bonding. And 3 because 2s & 1d orbitals are involved in bonding. Its stable Oxidation state is +3.
(3)Why transition metals do forms complex compounds?
- The color of the transition elements is due to the presence of unpaired electron in (n-1)d orbitals
- The colour of the compound is explained by the crystal field theory.
- In an isolated transition metal, there are five d orbitals in the inert shell which have equal energies and in lower energy state and are said to be in ground state degenerate orbitals.
- When the combining ions or ligands come little closer to the metal ion, the ground state degenerate d-orbitals become excited state which also have the equal energy and is said to be in the excited state degenerate d-orbitals.
- On approaching the combining ion or ligands very close to the metal ion, the excited state degenerate d-orbitals don’t have equal energies.
- The d-orbital then splits into 2 sets of orbitals, one set consisting of 3d-orbitals( dxy,dyz,dzx) which forms the t2g set and another set consists of 2-d orbitals d(x²-y²) and dz² which forms eg set.
- The 1-set of d-orbital has higher energy compared to another. In case of tetrahedral complexes,t2g set has higher energy than the eg set while in case of octahedral complexes, eg set has higher energy than t2g set.
- The splitting of d-orbitals in the influence of combining ions or ligands is called as crystal field splitting. This phenomena is called crystal field splitting theory.
- The difference in energies between the 2 cells is called as crystal field energy symbolized by Δ=(crystal).
Ti(H2O)6]3+ has purple color, why?
Oxidation state of Ti atom is +3
The electronic configuration of Ti atom is 3d1. That single electron will occupy the lowest energy orbital. Then t2g and eg remains vacant. Then energy difference of t2g and eg will lie on visible range and absorption of visible radiation lead to electronic transition from t2g to eg.
In this case, the absorption of green color takes place so that its ion has complementary color purple. This type of transition is called d-to-d transition.
|ions||complexes||outer electronic configuration||no.of unpaired electrons||color of ions|
Variable Oxidation State Table-:
|Elements||Outer electronic configuration||Oxidation state|