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i methyl amine has 3 methyl groups attached to the nitrogen. these - CH3 groups are aliphatic and hence electron donating in nature. They tend to donate electrons to nitrogen which increases the electron density at Nitrogen. Trimethylamine can hence easily donate electrons and hence is a stronger base
6 4 6
N(SiH3)3 compouding is planar with an sp2 nitrogen, while the geometry of N(CH3)3 is pyramidal with an sp3 nitrogen. The planar geometry in N(SiH 3)3 is due to pp- dp bonding between a nitrogen 2p orbital and empty 3d orbitals on Si. This p-bonding decreases the Lewis basicity of N(SiH3)3 by delocalizing the nitrogen lone pair.