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Nitrogen Group (Group 5) Trends

Summary of Nitrogen Group (Group VA) Trends:

1. The hydrides become less thermally stable and more reactive down the column. The M-H and M-CH3 mean bond enthalpies decrease in strength down the group and consequently the hydrides and alkyls become less stable.

2. The majority of nitrogen compounds are covalent. The main exceptions being those based on the nitride ion.

3. Multiple bond formation:

  • Nitrogen forms very strong multiple pπ-pπ bonds to itself and neighboring elements belonging to the same row, e.g.CN-, N2, NO+. Compounds of P, As, and Bi with multiple bonds may be obtained if large groups are introduced into the molecules, e.g. P2R2 and As2R2. Similar multiply bonded compounds of Sb are not known.

4. Coordination numbers:

  • The coordination numbers increase down the group. For nitrogen3 and 4 coordination predominate. Phosphorous and arsenic in addition form octahedral complexes and higher coordination numbers are observed for Sb and Bi.

5. Increased metallic character

  • The elements become increasingly metallic down the column in their chemical and physical properties. Down the group the oxo cations, e.g. SbO+ and BiO+ become more prevalent.

6. The oxides become more basic down the column.

  • Phosphorous and arsenic oxides are acidic, antimony oxide is amphoteric, and that of bismuth is basic.

7. The halides become more ionic and increasingly adopt infinite structures in preference to molecular ones.

8. Catenation occurs in the order: N < P > As > Sb > Bi. Phosphorous forms a wide range of ring and cage compounds because of the favorable P-P bond enthalpy.

9. Negative oxidation states:

  • For nitrogen the N3- ion is well established in the ionic nitrides of the electropositive elements. The anionic derivatives of the heavier elements frequently retain element-element bonds, e.g Sb42-, Bi42-, Sb73-, and As113-

10. The donor/acceptor behavior of R3M:

  • Donor/Lewis base ability: N < P > As > Sb > Bi
  • Steric effects: N > P ~ As > Sb > Bi and increase with bulk of substituents: PR3 > PR2H > PRH2 > PH3
  • π-acidity (see lecture): R3P > R3As > R3Sb > R3Bi
  • Lewis acidity of the +5 fluorides: PF5 > AsF5 > SbF5

11. Hydrolysis of halides:

  • In the +5 oxidation state PF5 is not readily hydrolyzed, AsF5 hydrolyses, SbF5 vigorously reacts with water. BiF5 reacts explosively with water. In the +3 oxidation state NF3 is unreactive, PF3 reacts only with OH- not OH2, AsF3, SbF3 are soluble in water, BiF3 is insoluble in water but soluble in inorganic acids.

12. Stabilization of the +3 oxidation state relative to +5 oxidation state. The trend is less well defined than that of group IVa and in fact an alternation in stabilities is observed. This may be illustrated by the following halide stabilities:

  N P As Sb Bi
EF5 unknown stable stable stable stable
ECl5 unknown stable known but unstable stable unknown
EBr5 unknown stable unknown stable unknown
EI5 unknown stable unknown stable unknown
EF3 stable stable stable stable stable
ECl3 known but unstable stable stable stable stable
EBr3 known but unstable stable stable stable stable
EI3 known but unstable stable stable stable stable
  • The + 5 oxidation state halides are unknown for N, well defined for P, only stable for As as fluoride, well defined for the fluoride and chloride of antimony, and only known for the fluoride for Bi.
  • For both oxidation states the stability order is F > Cl . Br >I as anticipated by the ordering of the mean enthalpies, i.e.fluoride forms the strongest bonds and iodine the weakest.
  • The oxides show a similar trend. BiV and NV oxides and oxoacids are strongly oxidizing whereas PV oxides and oxoacids are very stable and AsV and SbV oxides and oxoacids are mildly oxidizing.