The chemistry of organometallic compounds is considered to be both an old and a new part of science. Compounds that possess at least one covalent bond between a carbon atom and a metal atom are called organometallic compounds. Metal in metal carbonyls of the same coordination number as the oxidation state is subject to the 18 - electronic rule. The Mingos rule finds application in counting the total number of valence electrons of d-block organometallic compounds. By these rules, the number of electrons determines the structure of a particular compound. Organometallic compounds of transition elements are unstable due to unfilled d-orbitals and the tendency of organic groups to be eliminated as olefins with the formation of metal hydrides (elimination of hydrogen in the β-position). The problem of instability is solved by selecting adequate ligands such as stabilizing (CO) or those that do not have β-hydrogen (W(CH 2 SiMe 3 ) 6 ) or those whose β-hydrogen is difficult to access (Cr(CMe 3 ) 4 ). Ligands that form complexes with transition metals are alkene, allyl, diene, polyene and cyclic polyenes. The most important organometallic reactions are CO ligand substitution, β-hydrogen elimination, α-hydrogen elimination, oxidative addition, and hydrogen and alkyl migration.