For inorganic chemists, homogeneous catalysis is often synonymous with organometallic catalysts. Inorganic chemistry deals with synthesis and behavior of inorganic and organometallic compounds. Dienes are widely used chelating ligands in organometallic chemistry. Diarylmercurials are often used to synthesize lanthanide organometallic complexes. Various organometallic and inorganic reagents are effective. Tetracarbonyl nickel, and ferrocene are examples of organometallic compounds containing transition metals. Allenes serve as ligands in organometallic chemistry. Coordination complexes and organometallic compounds are also photoreactive. There are several classes of cationic initiators including&colon; Onium salts, organometallic compounds and pyridinium salts. The metal-carbon bond in organometallic compounds is generally highly covalent. HMPA is a specialty solvent for polymers, gases, and organometallic compounds. They are used widely as ligands in organometallic complexes. NHCs are widely used as ancillary ligand in organometallic chemistry. The term is used almost exclusively to describe solutions and implies catalysis by organometallic compounds. Benzene is an excellent ligand in the organometallic chemistry of low-valent metals.