A series of novel phosphinate-based ionic liquids have been synthesised using various organic phosphorus acids, such as mono-n-octylphosphinic acid (octylPO₂H₂), mono-n-hexylphosphinic acid (hexylPO₂H₂), di-n-octylphosphinic acid ((octyl)₂PO₂H), di-n-hexylphosphinic acid ((hexyl)₂PO₂H) and bis(2,4,4-trimethylpentyl)phosphinic acid ((TMP)₂PO₂H), by reaction with tetrabutylphosphonium hydroxide (Bu₄POH), tetrabutylammonium hydroxide (Bu₄NOH), pyridine (py) and triethylamine (Et₃N). Reactions simply involve neutralizing the organophosphorus acids with bases. The only byproduct, water, and excess amine, py/Et₃N can be easily removed under vacuum (Chapter 2). Some of the physical and chemical properties of these ionic liquids have been characterized by NMR, ES-MS, TGA, UV-Visible spectroscopy etc. The formed tetraalkylphosphonium/tetraalkylammonium phosphinate ionic liquids are viscous, hygroscopic liquids, but are more thermally stable than trialkylammonium and pyridinium analogues. For example, for the most stable ionic liquid, [Bu₄P][(octyl)₂PO₂], its onset temperature of decomposition in thermal gravimetric analysis (TGA) is 358 °C (Chapter 2). One of the applications of these novel phosphinate-based ionic liquids has been investigated. Au(0), Rh(0) and Ir(0) nanoparticles have been prepared in some of these phosphinate-based ionic liquids as a media and (or) reducing agents and have been characterized by SEM, TEM, Zetasizer, UV-Visible spectroscopy and IR spectroscopy. In general, the average particles size of Au(0), and Ir(0) were ca. 25 nm, but some of the particles average size formed was less than 2 nm, such as Rh(0) and Ir(0) nanoparticles formed in [Bu₄P][(octyl)₂PO₂] or [Bu₄P][(TMP)₂PO₂]. Generally, these phosphinate-based ionic liquids are good media to prevent nanoparticle aggregation (Chapter 3).