Question

Find the total number of chemical specie(s) in which effective $p\pi - d\pi$ type of back bonding is observed.

N(SiH$_3$)$_3$, O(SiH$_3$)$_2$, :CCl$_2$, P(CH$_3$)$_3$, N(GeH$_3$)$_3$, B(OMe)$_3$, B(OEt)$_3$, SiF$_4$, P(SiH$_3$)$_3$

• A. 2
• B. 3
• C. 4
• D. 5

Answer 1

Answer: (A)

2 species — namely SiF₄ (8th species) and P(SiH₃)₃ (9th species).

Answer 2

We must choose only those species where the central atom is from period 3 (or beyond) so that low‐lying vacant d‐orbitals are available for donation of electron density from a pπ orbital on the substituent. Examining the list:

1. In N(SiH₃)₃, O(SiH₃)₂, :CCl₂, N(GeH₃)₃, B(OMe)₃, and B(OEt)₃ the central atoms (N, O, C, B) belong to the second period (or do not have suitable orbitals).
2. For P(CH₃)₃, the central atom P (period 3) has vacant 3d‐orbitals but the CH₃ groups do not possess lone pairs to donate.
3. In SiF₄ the central Si (period 3) has vacant 3d‐orbitals and the F atoms (with lone pairs) can donate electron density via pπ orbitals.
4. In P(SiH₃)₃ the phosphorus center (period 3) has available 3d‐orbitals and the SiH₃ groups (though sigma donating) are known to allow back‐bonding; indeed, trisilylphosphine is nearly planar—attributed to effective pπ–dπ donation from Si.

Thus effective pπ–dπ back bonding is observed in SiF₄ and P(SiH₃)₃.