Organometallic compounds Organometallic chemistry is the study of chemical compounds containing at least one bond between a carbon atom of an organic compound and a metal Organometallic chemistry combines aspects of inorganic chemistry and organic chemistry. Organometallic compounds are widely used in homogeneous catalysis.

Organometallic compounds Organometallic compounds are distinguished by the prefix "organo-" e.g. organopalladium compounds. Examples of such organometallic compounds include all Gilman reagents, which contain lithium and copper. Tetracarbonyl nickel, andferrocene are examples of organometallic compounds containing transition metals. Other examples include organomagnesium compounds like iodo(methyl)magnesium

MeMgI, diethylmagnesium (Et2Mg), and all Grignard reagents; organolithium compounds such as n-butyllithium (n-BuLi), organozinc compounds such as diethylzinc (Et2Zn) and chloro(ethoxycarbonylmethyl)zinc (ClZnCH2C(=O)OEt); and + − organocopper compounds such as lithium dimethylcuprate (Li [CuMe2] ). The term "metalorganics" usually refers to metal-containing compounds lacking direct metal-carbon bonds but which contain organic ligands. Metal beta-diketonates, alkoxides, and dialkylamides are representative members of this class. In addition to the traditional metals, lanthanides, actinides, and semimetals, elements such as boron, silicon, arsenic, andselenium are considered to form organometallic compounds, e.g. organoborane compounds such as triethylborane (Et3B).

Coordination compounds with organic ligands Many complexes feature coordination bonds between a metal and organic ligands. The organic ligands often bind the metal through a heteroatom such as oxygen or nitrogen, in which case such compounds are considered coordination compounds. However, if any of the ligands form a direct M-C bond, then complex is usually considered to be 2+ organometallic, e.g., [(C6H6)Ru(H2O)3] . Furthermore, many lipophilic compounds such as metal acetylacetonates and metal alkoxides are called "metalorganics." Many organic coordination compounds occur naturally. For example, hemoglobin and myoglobin contain an iron center coordinated to the nitrogen atoms of a porphyrin ring; magnesium is the center of a chlorin ring in chlorophyll. The field of such inorganic compounds is known as bioinorganic chemistry. In contrast to these coordination compounds,methylcobalamin (a form of Vitamin B12), with a cobalt- methyl bond, is a true organometallic complex, one of the few known in biology. This subset of complexes are often discussed within the subfield of bioorganometallic chemistry. Illustrative of the many functions of the B12-dependent enzymes, the MTR enzyme catalyzes the transfer of a methyl group from a nitrogen onN5-methyl- tetrahydrofolate to the sulfur of homocysteine to produce methionine. The status of compounds in which the canonical anion has a delocalized structure in which the negative charge is shared with an atom more electronegative than carbon, as in enolates, may vary with the nature of the anionic moiety, the metal ion, and possibly the medium; in the absence of direct structural evidence for a carbon–metal bond, such compounds are not considered to be organometallic.

Structure and properties The metal-carbon bond in organometallic compounds is generally of character intermediate between ionic and covalent. Primarily ionic metal-carbon bonds are encountered either when the metal is very electropositive (as in the case of Group 1 or Group 2 metals) or when the carbon-containing ligand exists as a stable carbanion. Carbanions can be stabilized by resonance (as in the case of the aromatic cyclopentadienyl anion) or by the presence of electron-withdrawing substituents (as in the case of the triphenylmethyl anion). Hence, the bonding in compounds like sodium acetylide and triphenylmethylpotassium is primarily ionic. On the other hand, the ionic character of metal-carbon bonds in the organometallic compounds of transition metals, post-transition metals, and metalloids tends to be intermediate, owing to the middle-of-the-road electronegativity of such metals. Organometallic compounds with bonds that have characters in between ionic and covalent are very important in industry, as they are both relatively stable in solutions and relatively ionic to undergo reactions. Two important classes are organolithiumand Grignard reagents. In certain organometallic compounds such as ferrocene or dibenzenechromium, the pi orbitals of the organic moiety ligate the metal. In metal carbonyl and metal alkenes, back bonding(pi bonding) of electron density from metal to ligand antibonding orbitals makes stronger synergestic bonds.

Applications Organometallics find practical uses in stoichiometric and catalytic processes, especially processes involving carbon monoxide and alkene-derived polymers. All the world's polyethylene and polypropylene are produced via organometallic catalysts, usually heterogeneously via Ziegler-Natta catalysis. Acetic acid is produced via metal carbonyl catalysts in theMonsanto process and Cativa process. Most synthetic aldehydes are produced via hydroformylation. The bulk of the synthetic alcohols, at least those larger than ethanol, are produced by hydrogenation of hydroformylation derived aldehydes. Similarly, the Wacker process is used in the oxidation of ethylene to acetaldehyde. Organolithium, organomagnesium, and organoaluminium compounds are highly basic and highly reducing. They catalyze many polymerization reactions, but are also useful stoichiometrically. III-V semiconductors are produced from trimethylgallium, trimethylindium, trimethylaluminium and related nitrogen /phosphorus / arsenic / antimony compounds. These volatile compounds are decomposed along with ammonia, arsine,phosphine and related hydrides on a heated substrate via metalorganic vapor phase epitaxy (MOVPE) process for applications such as light emitting diodes (LEDs) fabrication. Organometallic compounds may be found in the environment and some, such as organolead and organomercury compounds are a toxic hazard.

Concepts and techniques As in other areas of chemistry, electron counting is useful for organizing organometallic chemistry. The 18-electron rule is helpful in predicting the stabilities of metal carbonyls and related compounds.

The 18-electron rule is a rule used primarily for predicting formulae for stable metal complexes.[1] The rule is based on the fact that the valence shells of transition metals consist of nine valence orbitals, which collectively can accommodate 18electrons as either bonding or nonbonding electron pairs. This means that, the combination of these nine atomic orbitalswith ligand orbitals creates nine molecular orbitals that are either metal-ligand bonding or non-bonding. When a metal complex has 18 valence electrons, it is said to have achieved the same electron configuration as the noble gas in the period. The rule and its exceptions are similar to the application of the octet rule to main group elements. The rule is not helpful for complexes of metals that are not transition metals, and interesting or useful transition metal complexes will violate the rule because of the consequences deviating from the rule bears on reactivity. The rule was first proposed by American chemist Irving Langmuir in 1921.

Applicability of the 18-electron rule The rule usefully predicts the formulae for low-spin complexes of the Cr, Mn, Fe, and Co triads. Well-known examples include ferrocene, iron pentacarbonyl, chromium carbonyl, and nickel carbonyl. Ligands in a complex determine the applicability of the 18-electron rule. In general, complexes that obey the rule are composed at least partly of pi-acceptor ligands (also known as π-acids). This kind of ligand exerts a very strong ligand field, which lowers the energies of the resultant molecular orbitals and thus favorably occupied. Typical ligands include olefins,phosphines, and CO. Complexes of π-acids typically feature metal in a low-oxidation state. The relationship between oxidation state and the nature of the ligands is rationalized within the framework of π backbonding.

Consequences for reactivity compounds that obey the 18 VE rule are typically "exchange inert." Examples 2+ 4− include [Co(NH3)5Cl] , Mo(CO)6, and[Fe(CN)6] . In such cases, in general ligand exchange occurs via dissociative substitution mechanisms, wherein the rate of reaction is determined by the rate of dissociation of a ligand. On the other hand, 18-electron compounds can be highly reactive toward electrophiles such as protons, and such reactions are associative in mechanism, being acid-base reactions. Complexes with fewer than 18 valence electrons tend to show enhanced reactivity. Thus, the 18-electron rule is often a recipe for non-reactivity in either a stoichiometric or a catalytic sense. Alternative analysis In the prevalent ligand field analysis, the valence p orbitals on the metal participate in metal-ligand bonding, albeit weakly. Some new theoretical treatments do not count the metal p-orbitals in metal-ligand bonding, although these orbitals are still included as polarization functions. This results in a duodectet (12) rule which accommodates 18- electron rule exceptions such as complexes with pi-donating ligands, as well as low-spin linear 14e complexes and square planar 16e complexes by invoking a hypervalent explanation for >12e species, but has yet to be adopted by the general chemistry community.

Exceptions to the 18-electron rule π-donor or σ-donor ligands with small interactions with the metal orbitals lead to a weak ligand field which increases the energies of t2g orbitals. These molecular orbitals become non-bonding or weakly anti-bonding orbitals (small Δoct). Therefore, addition or removal of electron has little effect on complex stability. In this case, there is no restriction on the number of d-electrons and complexes with 12 -22 electrons are possible. Small Δoct makes filling eg* possible ( > 18e-) and π-donor ligands can make t2g antibonding ( < 18 e-). These types of ligand are located in low to medium of 2− 4+ 0 3+ 3+ 6 thespectrochemical series. For example: [TiF6] (Ti , d , 12 e), [Co(NH3)6] (Co , d , 2+ 2+ 9 18 e), [Cu(OH2)6] (Cu , d , 21 e) In tems of metal ions, Δoct increases down a group as well as increasing oxidation number. Strong ligand fields lead to low-spincomplexes which cause some exceptions to 18-electron rule. 16e complexes A popular class of complexes that violate the 18e rule are the 16e complexes with d8 configurations. All high-spin d8metal ions are octahedral (or tetrahedral), but the low- spin d8 metal ions are all square planar. Important examples of square-planar low-spin d8 metal Ions are Ni(II), Pd(II), and Pt(II). At picture below is shown the splitting of the d sub-shell in low-spin square-planar complexes. Examples are especially prevalent for derivatives of the cobalt and nickel triads. Such compounds are typically square-planar. 2− The most famous example is Vaska's complex (IrCl(CO)(PPh3)2), [PtCl4] , and Zeise's 2 − salt[PtCl3(η -C2H4)] . In such complexes, the dz2 orbital is doubly occupied and nonbonding. Many catalytic cycles operate via complexes that alternate between 18e and square-planar 16 configurations. Examples include Monsanto acetic acid synthesis, hydrogenations, hydroformylations, olefin isomerizations, and some alkene polymerizations. Other violations can be classified according to the kinds of ligands on the metal center. Bulky ligands Bulky ligands can preclude the approach of the full complement of ligands that would allow the metal to achieve the 18 electron configuration. Examples:

 Ti(neopentyl)4 (8 VE)

 Cp*2Ti(C2H4) (16 VE)

 V(CO)6 (17 VE)

 Cp*Cr(CO)3 (17 VE) t  Pt(P Bu3)2 (14 VE)

 Co(norbornyl)4 (13 VE) +  [FeCp2] (17 VE) Sometimes such complexes engage in agostic interactions with the hydrocarbon framework of the bulky ligand. For example:  W(CO)3[P(C6H11)3]2 has 16 VE but has a short bonding contact between one C-H bond and the W center.

 Cp(PMe3)V(CHCMe3) (14 VE, diamagnetic) has a short V-H bond with the 'alkylidene-H', so the description of the compound is somewhere between

Cp(PMe3)V(CHCMe3) and Cp(PMe3)V(H)(CCMe3). High-spin complexes High-spin metal complexes have singly occupied orbitals and may not have any empty orbitals into which ligands could donate electron density. In general, there are few or no π-acidic ligands in the complex. These singly occupied orbitals can combine with the singly occupied orbitals of radical ligands (e.g., oxygen), or addition of a strong field ligand can cause electron-pairing, thus creating a vacant orbital that it can donate into. Examples:

 CrCl3(THF)3 (15 VE) 2+  [Mn(H2O)6] (17 VE) 2+  [Cu(H2O)6] (21 VE,) Complexes containing strongly pi-donating ligands often violate the 18-electron rule. These ligands include fluoride (F−), oxide (O2−), nitride (N3−), alkoxide (RO−), and imide (oxide (RN2−). Examples:

2−  [CrO4] (16 VE)

 Mo(=NR)2Cl2 (12 VE) In the latter case, there is substantial donation of the nitrogen lone pairs to the Mo (so the compound could also be described as a 16 VE compound). This can be seen from the short Mo-N bond length, and from the angle Mo - N - C(R), which is nearly 180°. Counter-examples:

 trans-WO2(Me2PCH2CH2PMe2)2 (18 VE)

 Cp*ReO3 (18 VE) In these cases, the M=O bonds are "pure" double bonds (i.e., no donation of the lone pairs of the oxygen to the metal), as reflected in the relatively long bond distances. Pi-donating ligands Ligands where the coordinating atom bear nonbonding lone pairs often stabilize unsaturated complexes. Metal amides and alkoxides often violate the 18e rule.

Combinations of effects The above factors can sometimes combine. Examples include

 Cp*VOCl2 (14 VE)

 TiCl4 (8 VE) Higher electron counts Some complexes have more than 18 electrons. Examples:

 Cobaltocene (19 VE)  Nickelocene (20 VE) 2+  The hexaaqua copper(II) ion [Cu(H2O)6] (21 VE) Often, cases where complexes have more than 18 valence electrons are attributed to electrostatic forces - the metal attracts ligands to itself to try to counterbalance its positive charge, and the number of electrons it ends up with is unimportant. In the case of the metallocenes, the chelating nature of the cyclopentadienyl ligand stabilizes its bonding to the metal. Somewhat satisfying are the two following observations: (i) cobaltocene is a strong electron donor, readily forming the 18- electron cobaltocenium cation and (ii) nickelocene tends to react with substrates to give 18-electron complexes, e.g. CpNiCl(PR3) and free CpH. In the case of nickelocene, the extra two electrons are in orbitals which are weakly metal-carbon antibonding, this is why it often participates in reactions where the M-C bonds are broken and the electron count of the metal changes to 18.