Trihalides of Nitrogen are lewis bases

Trihalides of P , Sb , As , Bi are lewis acids?

Explain the trend :

i)NF3 < NCl3 < NBr3 < NI3 ..in accordance with Lewis Base Strength

ii) PF3 > PCl3 > PBr3 > PI3 ...in accordance with the Lewis Acid Strength

1. The trihalides of nitrogen are lewis bases because nitrogen atom has a lone pair of electrons/ unshared pair of electrons.

The electronic configuration of nitrogen is 1s² 2s² 2p³ . So, when it forms trihalides, It still has lone pair electrons of 2s² to donate. Hence, they act as lewis base.

2. P, Sb, As, and Bi have d orbitals in their valence shell. So, in these cases, the central atom, i.e. group 15 elements have tendency to accept electrons rather than donate and hence, they are lewis acids.

3.  Lewis bases need to be able to donate electrons. Fluorine is the most electronegative element in the halogens followed by chlorine, bromine and iodine. Due to fluorine being strongly electronegative, it draws the electron density towards itself which makes it difficult for nitrogen atom to donate its lone pair of electrons. So, NF is least basic. This trend follows the strength of electronegativity of the halides. Since, iodone is least electronegative, it is the most basic trihalide of nitrogen.

SO, we have the trend, in decreasing order of basic strength:

NF₃< NCl₃ < NBr₃ < NI₃

4. The sequence for the Lewis acid strength for:

 PF₃ > PCl₃ > PBr₃ > PI₃

because: Lewis acids have a tendency to accept electrons. As mentioned above, phosphorous has empty d orbital in its valence shell to accept electrons. Also, the electronegativity of the halides decreases as F< Cl<Br<I. So, fluorine has stronger affinity towards electrons making PF₃ stronger lewis acid while making PI₃ weakest lewis acid.