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Friday, August 6, 2021

Why Back bonding does not take place in P(SiH3)3 instead of phosphorus has lone pair and Silicon has vacant d-orbital but in N(SiH3)3 molecule back bonding does take place?

Conditions for back bonding:

(1) Both of the atoms bonded with back Bonding are must be present in 2nd-2nd or 2nd-3rd period. 4rth period onward back bonding does not take place.

(2) One of the atoms has lone pair (donor atom) and another (acceptor atom) have vacant Orbital and direction of back Bonding depends upon vacant Orbital.

(3) The donor atom must have localized donatable electron pair and there should be inter electronic repulsion (smaller size). In general these are later half second period P - block elements (F, O, N and C).

(4) The acceptor atom must have low energy empty orbital which generally are np or nd orbitals. Small and similar size orbital’s favour overlap.

(5) Back bonding is a weak pi bond thus only effective overlapping will be form back bonding.

(6) Back bonding is found to be effective and considerable in following type of overlapping.

 (i) 2p-2p

(ii) 2p-3p

(iii) 2p-3d 

(7) The extent of overlapping order is [2p-2p> 2p-3d >2p-3p]

(8) dx2-y2 and dx2 Orbital’s does not participate in back Bonding. 

Solution:

In case of  trisilyl phosphine [P(SiH3)3]  Phosphorous (P) being larger in atomic size so it does not face much interelectronic repulsion and so, not much eager to donate it’s loan pair electron and comfortable with it. So, it will not donate to Si atom in this case. Thus, back bonding will not take place in trisilyl phosphine [P(SiCH3)3].

But in the case of trisilyl amine [N(SiH3)3], the nitrogen (N) atom is smaller in atomic size leading to high interelectronic repulsion, so it wants ease by donating it’s loan pair electron to other atoms near to it. And Si has vacant d-orbital, so needy for it. Since both conditions are follow here,  hence back bonding will take place.

 Related Questions:

(1) Why trimethylamine amine ( N(CH3)3) is tetrahedral while trisilyl amine (N(SiH3)3) planner.?









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