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In 1890, L. Mond discovered that when carbon monoxide burned after passing through active nickel powder, it produced a green-colored flame; cooling the resulting gas yielded a colorless liquid (with a melting point of 298 K and a boiling point of 316 K). If this gas was passed through a heated glass tube, metal nickel would deposit on the walls of the tube. This gas is nickel tetracarbonyl, Ni(CO)4. Since the 1960s, more than a hundred such carbonyl compounds and their derivatives have been synthesized, and almost all transition metals can form such compounds. This special class of complexes formed by transition metals and carbon monoxide ligands is called metal carbonyl compounds, or carbonyl complexes. Metal carbonyl compounds hold an important position in modern inorganic chemistry, both in theoretical research and practical applications. The preparation of metal carbonyl compounds generally involves the following methods: (1) direct synthesis method. Most carbonyl complexes are formed by the direct combination of a metal with carbon monoxide, but the metal must be a newly reduced product and in a highly activated state. (2) Reduction carbonylation under high pressure. Under high pressure, a reducing agent is used to induce a carbonylation reaction between the metal and carbonyl groups. The reducing agents used are mainly hydrogen, active metals, phenylmagnesium bromide (C6H5MgBr), etc. (3) Certain polynuclear carbonyl compounds can be obtained through thermal decomposition or decomposition under ultraviolet light. (4) The interaction of carbonyl compounds of two different metals can yield heteronuclear carbonyl compounds. Metal carbonyl compounds come in different types and can generally be divided into (1) simple carbonyl complexes, namely mononuclear carbonyl complexes, with the general formula M(CO)x. Most of these complexes consist of transition elements from the later groups in the extended periodic table, and the metal atoms in them have a charge of zero. (2) Multinuclear carbonyl complexes. A carbonyl complex that contains two or more metal atoms is called a polynuclear carbonyl complex; it is a type of metal cluster compound. (3) Carbonyl hydrides, such as HCo(CO)4, H2Fe(CO)4, HMn(CO)5, etc. (4) Heteronuclear carbonyl complexes, such as [((CO)4MnCo(CO)4)] and the like. (5) Mixed carbonyl complexes, such as [Fe(CO)4Cl], [Cr(CO)5(NH3)], etc. According to the coordination field theory, the experimental data from photoelectron spectroscopy, together with the molecular orbital energy levels and the outer molecular orbitals, confirm that metal carbonyl compounds formed between metals and carbon monoxide can exist stably, mainly due to the formation of covalent bonding between them. Carbon monoxide molecules can not only provide lone pairs of electrons to the σ orbitals of metal atoms to form σ-covalent bonds, but also possess empty antibonding π* orbitals that can accept electrons from the filled d orbitals of metal atoms to form back-donation π bonds (feedback π bonding). These two types of bonding are collectively referred to as σ-π bonding. Due to the synergistic effect of σ-π bonding, the strength of the M—C bond in metal carbonyl compounds is enhanced, while the C—O bond is weakened. In mononuclear carbonyl complexes, the carbonyl groups are all terminally coordinated as M←CO; whereas in polymeric carbonyl complexes, in addition to terminal coordination, there may also be bridging carbonyl groups that bind the metals together through covalent bonds. In summary, metal carbonyl compounds have three characteristics: (1) although carbon monoxide is not a very strong Lewis base, it can form strong chemical bonds with transition metals. (2) In such complexes, the central atom exhibits a lower oxidation state, usually zero. It can sometimes exhibit a lower positive oxidation state, and even occasionally a negative oxidation state. (3) The vast majority (99%) of such complexes obey the effective atomic number rule. Except for the mononuclear carbonyl complexes of iron-group elements and Ni(CO)4, which are liquids at room temperature, all other known metal carbonyl complexes are solids. All mononuclear carbonyl compounds are colorless, whereas polymeric complexes are colored, and their color deepens as the number of metal atoms in the molecule increases. Solid carbonyl complexes have low melting points and are prone to sublimation; they are all typical covalent compounds. They are all insoluble in water and soluble in non-polar solvents [with the exception of Fe2(CO)9]. They are generally flammable and sensitive to air; some, such as Fe(CO)5 and Ni(CO)4, are highly toxic, so great care must be taken when handling them. Carbonyl complexes undergo a variety of reactions, among which the important types include: (1) thermal decomposition reactions; (2) redox reactions; (3) substitution reactions; (4) reactions to form carbonylate anions, etc. The main applications of metal carbonyl compounds include: the production of pure metals; use as anti-knock agents in gasoline; use as catalysts in carbonylation reactions; and serving as important raw materials for organometallic compounds in the chemical industry.