Several mononuclear Co(II), Ni(II), Cu(II) and Fe(II) complexes of tetradentate salpren-type diimine, obtained from 3,5-di-tert-butyl-2-hydroxybenzaldehyde and 1,3-diaminopropane have been prepared and characterized by analytical, spectroscopic (FTIR, UV-Vis) techniques, magnetic susceptibility measurements and thermogravimetric analyses (TG). The thermodynamic and thermal properties of complexes have been investigated. For further characterization direct insertion probe-mass spectrometry (DIP-MS) was used and the fragmentation pattern and also stability of the ions were evaluated. The characterization of the end products of the decomposition was achieved by X-ray diffraction. The thermal stabilities of metal complexes of N,N′-bis(3,5-di-t-butylsalicylideneimine) propanediamine ligand (L) were found as Ni(II)>Cu(II)>Co(II)>Fe(II).