A new group of room temperature ionic liquids based on triethylalkyl and tri-n-butylalkylphosphonium cations with a bis(trifluoromethylsulfonyl)imide anion as a novel green solvent is presented. The electrochemical behaviors of trivalent, divalent and neutral complexes such as rare earth, noble metal and ferrocene were investigated in phosphonium based ionic liquids in comparison with the corresponding ammonium type of ionic liquids. The diffusion coefficients of these metal complexes were estimated by cyclic voltammetry, chronoamperometry and chronopotentiometry, because it is important for development of the electrochemical devices to understand the available diffusive properties. The redox potentials of these trivalent lanthanide couples suggested that the donor property of phosphonium based ionic liquids was slightly larger than that of nitrogen based ionic liquids. The palladium complexes were more stabilized in phosphonium ionic liquids due to the relatively strong ligand field. The redox reaction of neutral complex, ferrocene, was reversible electrochemically in triethylalkyl and tri-n-butylalkyl phosphonium based ionic liquids. The multivalent and neutral complexes in phosphonium type of ionic liquids were somewhat movable than that in corresponding ammonium ionic liquids because the interaction between the cation and the anion in phosphonium based ionic liquids is weaker.