This book consists of four chapters. The first chapter deals with the general introduction along with literature survey up to 2006 of Mn (III) complexes with oxygen and nitrogen donor atom ligands. 317 references have been cited in this chapter. The second chapter consists of the experimental details for the synthesis of the complexes containing β-diketone (acetylacetone) and ethylenediamine, propylenediamine, hexamethylenediamine. The separation and purification procedures are also given in this chapter. In the third chapter the main results obtained during the present work are given and they are discussed in detail. The characterization on the basis of elemental analysis, conductivity measurements, infrared spectral data, electronic spectral data, magnetic momentum measurements and thermogavimetric analysis have been presented in this chapter. Antifungal and antibacterial activities along with the probable structures for all the complexes have also been presented in the third chapter.Delignification properties are also discusse in this chapter. Fourth chapter is the summary of all the work done.