$${\rm I}$$ and $${\rm I}$$$${\rm I}$$$${\rm I}$$ follow $${S_N}1$$ mechanism

$${\rm I}$$ and $${\rm II}$$ follow $${S_N}2$$ mechanism

Compound $${\rm IV}$$ undergoes inversion of configuration

The order of reactivity for $${\rm I},$$ $${\rm I}{\rm I}{\rm I}$$ and $${\rm IV}$$ is : $${\rm I}V > {\rm I} > {\rm I}{\rm I}{\rm I}$$

$$C{r_2}{O_3},BeO,SnO,Sn{O_2}$$

$$C{r_2}{O_3},CrO,SnO,PbO$$

$$NO,{B_2}{O_3},PbO,Sn{O_2}$$

$$ZnO,A{l_2}{O_3},PbO,Pb{O_2}$$

$$Al{\left( {C{H_3}} \right)_3}$$ has the three-centre two-electron bonds in its dimeric structure

$$B{H_3}$$ has the three-center two-electron bonds in its dimeric structure

$$AlC{l_3}$$ has the three-center two-electron bonds in its dimeric structure

The Lewis acidity of $$BC{l_3}$$ is greater than that of $$AlC{l_3}$$