Lanthanide oxidation..

various lanthanide oxidation states are described here

1. OXIDATION STATES OF LANTHANIDES

2. OXIDATION STATE The total number of electrons that an atom either gains or loses in order to form a chemical bond with another atom. The underlying principle for oxidation state is conservation of mass . Since electrons cannot be created or destroyed a proper accounting of where electrons go is central to understanding chemical reactions. It also determines the ability of an atom to oxidise (to lose electrons) or to reduce (to gain electrons) other atoms or species . If we assign oxidation states before and after a reaction, the we can understand where the electrons flowed during the reaction. Electrons are where all the action is in chemical reactions. Oxidation results in an increase in the oxidation state and reduction result in a decrease in the oxidation state.

3. LANTHANIDES Rare Earth Element Occurrence: 3× 10 -4 . % of earth crust Available in monzite sand as lanthanide orthophophates Norwegain mineralogist victor Gold schmidt in 1925 Z= 58 to 71 Fifteen metallic elements ( La to Lu ) Highly dense elements (6.1 to 9.8g per cc) Mp: 800 – 1600 Bp: 1200 - 3500 Valance electrons lies in 4f orbitals Electronic configuration [ Xe ]4f 1-14 5d 0-1 6s 2

4. All of the elements in the series closely resemble lanthanum and each another in their chemical and physical properties . They have a lustre and are silvery in appearance . They are soft metals and can even be cut with a knife The elements have different reaction tendencies depending on basicity. Some are very reactive while some take time to react . Lanthanides can corrode or become brittle if they are contaminated with other metals or non-metals . They all mostly form a trivalent compound . Sometimes they can also form divalent or tetravalent compounds . They are magnetic .

5. OXIDATION STATES OF LANTHANIDES

6. All the elements in the lanthanide series show an oxidation state of + 3 Lanthanides show variable oxidation states. They also show  +2, +3, and +4 oxidation states But the most stable oxidation state of Lanthanides is +3. Elements in other states hence try to lose or gain electrons to get +3 state By that those ions become strong reducing or oxidizing agents respectively Uneven distribution of oxidation state among the metals is attributed to the high stability of empty, half-filled or fully filled f-sub shells Ln , Pm, Ho, Eb , Lu +3 Ce , Pr, Tb, Dy +3,+4 Sm , Eu , Tm, Yb +2,+3 Nd +2,+3,+4

7. WHY +3 OXIDATION STATE ???

8. Greater stabilization of the 4f orbitals compared to 5d and 6s The order of penetration of the orbitals into the inner electron core is 4f>5d>6S The 4f electrons are the closest to nucleus and attracted by it the most. Now as successive ionisation increases the net charge on the lanthanide cation being closest to the nucleus ,4f electrons are pulled even more closer than 5d and 6s electrons. Thus is Ln3+,4f electrons are just too strongly pulled by the nucleus to be ionised further as it requires huge energy for it So states higher than +3 is generally not seen.

9. ANY OTHER OXIDATION STATE??

10. OCCURRENCE OF +4 OXIDATION STATE Ce 4+ noble gas configuration but it reverts to a +3 oxidation state and thus acts as a strong oxidant and can even oxidize water, although the reaction will be slow . Pr 4 + , Nd 4 + Earlier in the lanthanide series so the effective nuclear charge is not so high to be able to attract the 4f electrons much more than 5d and 6s Tb 4 + , Dy 4 + (4f 7 valence shell) Half shell effect more stable than other filled orbitals. Ce 4+ , Pr 4+ , Nd 4+ , Tb 4+ , Dy 4+

11. OCCURRENCE OF +2 OXIDATION STATE Eu2+ [4f7 ], Sm2+ [4f6 ], Yb2+ [4f14] clear influences of electronic shell structure Europium   (Z=63 ) [ Xe ] 4f 7   6s 2 half-filled 4f 7  configuration and hence it readily forms Eu 2+ ion . Eu 2+  then changes to the common oxidation states of lanthanides (+3) and forms Eu 3+ , acting as a strong reducing agent . Ytterbium   (Z=70) fully filled f-orbital It has similar reasons for being a strong reducing agent , in the Yb 2+   state.

12. Oxidation state in Aqueous Solution In aqueous solution, Sm2+, Eu2+ and Yb2+ loose electron, i.e get oxidized and are good  reducing agents. On the other hand Ce4+, Pr4+, Tb4+ gain electron – gets reduced and are good oxidizing agents . Higher oxidation states (+4) of elements are possible only with oxides.  Example :  Pr, Nd , Tb and Dy . CONSEQUENSES OF OXIDATION STATES Colour of the ions Ionization energy Lanthanide contraction Separation of lanthanides Basic strength of hydroxides complex formation

13. COLOUR OF IONS Lanthanides ions can have electrons in f-orbital and also empty orbitals like the d-block elements. When a frequency of light is absorbed, the light transmitted exhibit a colour complementary to the frequency absorbed . Inner transition element ions can absorb the frequency in the visible region to use it for f-f electron transition and produce visible colour. Many of the lanthanide metals are silver-white . The lanthanide ions with +3 oxidation state are coloured both in solid-state and in aqueous solution. The colour of a cation depends on the number of unpaired f electrons Lanthanides, with xf electrons, have the same colour as of (14-x) electron elements.

14. IONISATION ENERGY Ionization energy E nergy needed to remove the valence electron from the atom/ion and is directly related to the force of attraction on the electron . Across the periodic table, Also, the ionization energy will be more for half-filled and fully filled orbitals . Atomic no: Nuclear charge: Size: Ionization Energy :

15. LANTHANIDE CONTRACTION : The atomic size or the ionic radii of tri positive lanthanide ions decrease steadily from La to Lu due to increasing nuclear charge and electrons entering inner (n-2) f orbital. This gradual decrease in the size with an increasing atomic number is called  lanthanide contraction The lanthanide contraction is the result of a poor shielding effect of 4f electrons Shielding effect the inner-shell electrons shield the outer-shell electrons so they are not effected by nuclear charge. POOR SHIELDING Positively charge nucleus decreasing the has greater attraction to electrons atomic radius as the Z Shielding effect: s > p > d > f

16. SEPERATION OF LANTHANIDES Since all the elements exhibit the +3 oxidation state as common, they have similar properties Thus, the separation of elements in its pure state is difficult. BASIC STRENGTH OF HYDROXIDES As the size decreases, charge to size ratio increase , the ionic character decreases or covalent character increases making hydroxides les and less basic More the charge to size ratio, the electron cloud of anion is more polarized, more covalent character .  As the size of lanthanides decreases from La to Lu, the covalent character of the hydroxides increases and hence their basic strength decreases. Thus, La (OH)3 is more basic and Lu(OH)3 is the least basic .

17. COMPLEX FORMATION Lanthanides exhibiting 3+ oxidation state is the larger and hence low charge to radius ratio . This reduces the complex-forming ability of lanthanides compared to d-block elements. Still they, form complexes with strong chelating agents like EDTA, β- diketones , oxime etc. They do not form Pπ -complexes. ELECTRODE POTENTIAL Formation Ce 4+ of is favoured by the noble gas configuration. E value for Ce 4+ / Ce 3+ is +1.74 V which is enough to oxidize the water. Thus the reduction potential ( tendency to accept electrons) is more They act as a good oxidant and the reaction rate is slow Thus act as a Good analytical reagent