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

Nitrogen (N) atom in trimethylamine [N(CH3)3] is sp3 hybridised and tetrahedral due to non bonding electron pair of N is localised (more basic). On the other hand N atom of trisilylamine [N(SiH3)3] is sp2 hybridised and trigonal planner.This is due to the participatlon of non-bondlng pair of N into pπ-dπ back banding. Due to this delocalization of non-bonding electron pair of N in trisilylamine it is also less basic.

What is the Si–N–C bond angle in Silyl isothiocyanate and methyl isothiocyanate (H3CNCS)?

The hybridization of Nitrogen in Methyl isothiocyanate (H₃CNCS) is Sp2. Thus bond angle (< C-N-C) is expected to be 120°. But it is slightly greater than 120° due to resonating structure. The resonating structure has N as Sp hybridized. Hence bond angle of the overall structure is found to be about 142°

But in case Silyl isothiocyanate (SiH₃NCO), the structure is found to be (sp hybridized) 

What are the order of extent back bonding, Lewis acid character and nucleophilicity of (BF3, BCl3, BBr3, BI3)boron trihalides?

(1) Back bonding extent in boron tri halides decreases from BF₃ to BI₃ because on increasing of size of p-orbital of halogens atom the strength of back bond decreases thus extent of back bonding:

(2) Lewis acid character of Boron Halides is inversely proportional to extent of back bonding because on decreasing back bonding tendency to accept lone pair from base is increases thus the order of Lewis acid character is:

Hence it is clear that BF3 is weakest Lewis acid due to stronger 2pπ-2pπ back bonding (stronger partial double bond character) in BF₃ (lone pair orbital of fluorine into vacant orbital of boron) and consequently behaves as less electron deficient. The back bonding gradually decreases (From BF₃ to BI₃) and becomes weakest in BI₃. So that BI₃ become strong Lewis acid

(3) The nucleophilicity(affinity towards nucleophile/water) order is inversely proportional to the Lewis acid character thus the nucleophilicity order is:

 Related Questions: 

Why aqueous solution of AlCl3 is acidic in nature ?

What happen when aq AlCl3 react with Acid or Base?

Although anhydrous aluminium chloride is covalent but its aqueous solution is ionic in nature. Why?

Why BF3 do not exist as dimer?. Explain.

Why B-F bond length in BF3 is shorter (130 pm) than B-F bond Iength in BF4- (143 pm)?. Explain.

B-F bond length in BF3 is shorter than B-F bond length in (BF4)- why?

When B2H6 is allowed to react with following Lewis bases, then how may given Lewis base form adduct through symmetrical Cleavage of B2H6.

What is product of reaction between diborane (B2H6) and ammmonia (NH3)?

Why methylation of Diborane (B2H6) replace four hydrogen only ?

What is Use of Boric Acid?

What is use of Orthoboric acids?

What is basicity of "Boric acid" ?

Why Boric acid exist in solid state ?

What is structure of solid Ortho Boric acid ?

What is effect of heat on Borax?

What is the structure of trimetaboric acid and trimetaborate ion?

What is the Sodium per borate ,give the structure and its uses?

Why aqueous solution of borax reacts with two moles of acids ?

What is the molecular formula of Borax ?

Why Boric acid become strong acid in the presence of cis 1,2-diol or 1,3-diol ?

Why Borazine is more reactive than benzene towards Electrophic Aromatic substitution reactions ?

Why Borazine (B3N3H6) is also known as inorganic benzene ?.

Four-center two-electron bond (4C-2e Bond): Structure of AlCl3:

What are the conditions for Back bonding?

 (1) Both of the atoms bonded with back Bonding are must be present in 2nd-2nd or 2nd-3rd period.

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

(3) The donor atom must have localized donatable electron pair. 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 sized orbital’s favour overlap.

EFFECTS OF BACK BONDING:

(1) It always leads to an increase in bond order between the participating atoms.

(2) It always leads to an increase in bond strength between participating atoms.

(3) It always leads to a decrease in bond length between participating atoms.

What is the Back Bonding?

(1) Back bonding is a type of weaker π bond which is formed by sideways overlapping of filled orbital with empty orbital present on adjacent bonded atoms in a molecule.
(2) It is also considered as intermolecular Lewis acid-base interaction as it is a π bond.
(3) Back bonding is found to be effective and considerable in following type of overlapping.