hydrolyze to the oxide with release of hydrogen sulfide. Nitrides and carbides Titanium nitride (TiN) is a refractory solid exhibiting extreme hardness, thermal/electrical conductivity, and a high melting point. TiN has a hardness equivalent to sapphire and carborundum (9.0 on the Mohs scale), and is often used to coat cutting tools, such as drill bits. It is also used as a gold-colored decorative finish and as a barrier layer in semiconductor fabrication. Titanium carbide (TiC), which is also very hard, is found in cutting tools and coatings. Halides Titanium tetrachloride (titanium(IV) chloride, TiCl4) is a colorless volatile liquid (commercial samples are yellowish) that, in air, hydrolyzes with spectacular emission of white clouds. Via the Kroll process, TiCl4 is used in the conversion of titanium ores to titanium metal. Titanium tetrachloride is also used to make titanium dioxide, e.g., for use in white paint. It is widely used in organic chemistry as a Lewis acid, for example in the Mukaiyama aldol condensation. In the van Arkel–de Boer process, titanium tetraiodide (TiI4) is generated in the production of high purity titanium metal. Titanium(III) and titanium(II) also form stable chlorides. A notable example is titanium(III) chloride (TiCl3), which is used as a catalyst for production of polyolefins (see Ziegler–Natta catalyst) and a reducing agent in organic chemistry. Organometallic complexes Owing to the important role of titanium compounds as polymerization catalyst, compounds with Ti-C bonds have been intensively studied. The most common organotitanium complex is titanocene dichloride ((C5H5)2TiCl2). Related compounds include Tebbe's reagent and Petasis reagent. Titanium forms carbonyl complexes, e.g. (C5H5)2Ti(CO)2. Anticancer therapy studies Following the success of platinum-based chemotherapy, titanium(IV) complexes were among the first non-platinum compounds to be tested for cancer treatment. The advantage of titanium compounds lies in their high efficacy and low toxicity in vivo. In biological environments, hydrolysis leads to the safe and inert titanium dioxide. Despite these advantages the first candidate compounds failed clinical trials due to insufficient efficacy to toxicity ratios and formulation complications. Further development resulted in the creation of potentially effective, selective, and stable titanium-based drugs. History Titanium was discovered in 1791 by the clergyman and geologist William Gregor as an inclusion of a mineral in Cornwall, Great Britain. Gregor recognized the presence of a new element in ilmenite when he found black sand by a stream and noticed the sand was attracted by a magnet. Analyzing the sand, he determined the presence of