An organometallic compound is generally defined as one that possesses a metal – carbon bond. Organometallic chemistry can be viewed as bridge between organic and inorganic chemistry. For example, all chemists would undoubtedly characterize nickel Tetracarbonyl, Ni(CO)4, as an Organometallic compound even though carbon monoxide is hardly a typical organic compound. Likewise organoboron, organosilicon, organoarsenic, and organotellurium compounds are included in organometallic chemistry even though boron, silicon, arsenic, and tellurium are borderline metals. Traditional inorganic chemicals such as sodium cyanide, although possessing a metal-carbon bond, are not normally categorized as organometallic compounds.
Hapticity A single organic ligand may interact with a central metal atom using one or more of its atoms simultaneously. The number of atoms in a ligand attached to the metal atom is denoted by the prefix η followed by a superscript indicating the number of ligand atoms attached to the metal atom. This called hapticity. Most ligands attach through one atom only, therefore, they are called as monohapto (η1). Cyclopentadienyl ligand, C5H5- or Cp, for example, can attach to metal atom through one, three or five carbon atoms. Therefore, it may act as mono (η1), tri(η3)- or pentahapto (η5)-ligand.
Eighteen Electron Rule The EAN and 18-electron rules are similar. The 18-electron rule is a rule of thumb and is more advantageous than EAN because there is no need to remember the atomic number of the noble gases. However, there are some exceptions which obey neither EAN nor 18-electron rule. The mononuclear compounds in which the number of total valence electrons is odd never obey either EAN or 18-electron rule. Stable complexes with both more than and fever than 18 outershell electrons are fairly common. Insight into the connection between stability of organometallic compounds and the 18-electron rule – and a basis for rationalizing the exceptions – can be gained by reviewing the molecular orbital description of bonding in complexes.
Counting Electrons in Complexes :
By counting number of valence electrons surrounding each metal atom or metal, ion, in a complex it is possible only to predict the complex would be stable but also in some cases, whether there will be M—M bonds, or not or the ligands will be terminal or bridging etc.
There are two methods for counting electrons:
The neutral atom method will be more useful because it does not require the correct assignment of oxidation states, which may be sometimes difficult for organometallic compounds. The oxidation state method is widely used for the reactions in which oxidation state of the central metal atom is changed. For both the electron counting methods, it is necessary to know how many electrons each ligands in a complex donates to the metal. The contribution of electrons for a variet y of ligands for both the neutral atom and the oxidation state methods is shown in Table. In the neutral atom method, each metal atom and ligand is treated as neutral.
Illustration: Cr(CO)6, Ni(PF3)4 and Fe(CO)4PPh3
Illustration: Mn2(CO)10, Co2(CO)8