Synthesis and characterization of some diorganotin(IV) complexes of Schiff bases derived from a non-protein amino acid. Crystal structures of {HO2CC6H4[N=C(H)}{C(CH3)CH(CH3)-3-OHI-p} and its di-n-butyltin(IV) complex ((Bu2Sn)-Bu-n{O2CC6H4[N=C(H)}{C(CH3)CH(CH3)-3OHI-p}2)