Publisher Summary This chapter focuses on the substitution reactions of tetrahedral complexes. The substitution of tetrahedral compounds of carbon can proceed by SNl and SN2 mechanisms, although the latter are not of the limiting type in which five-coordinate intermediates are formed. With tetrahedral complexes of transition-metal ions, an increase in coordination number to five is also possible, substitution reactions being generally very rapid and difficult to study. With the synthesis of optically active organosilicon compounds of the general type R3SiX, where R3Si is, for example, the α-naphthylphenylmethyl group, it has been possible to demonstrate a high degree of stereospecificity in reactions in which X is replaced. With essentially nucleophilic reagents, reactions may be divided into two groups depending on the nature of the leaving group. The chapter discusses the hydrolysis of triarylgermyl halides. The solvolysis reactions of organotin compounds R3SnCl, where R is the ethyl, isopropyl, t-butyl or phenyl group, have been studied in ethanol, propan-2-ol and water dioxan solvents.