# Organomagnesium chemistry

Organomagnesium chemistry, a subfield of organometallic compounds, refers to the study of magnesium compounds that contains Mg-C bonds. Magnesium is the second element in group 2 (alkaline earth metals), and the ionic radius of Mg$^{2+}$ is 86 pm, which is larger than Be$^{2+}$ (59 pm) and smaller than the heavier alkaline earth metal dications (Ca$^{2+}$ 114 pm, Sr$^{2+}$ 132 pm, Ba$^{2+}$ 149 pm), in accordance with periodic trends. Magnesium is less covalent compared to beryllium, and the radius is not large enough for accommodating large number of ligands compared to calcium, strontium and barium. Thus, organomagnesium compounds exhibit unique structure and reactivity in group 2.
The most important type of organomagnesium compound is the Grignard reagents, which are widely used in different fields of synthetic chemistry, especially in organic synthesis, for Grignard reagents serves as a robust source of carbanion. Although most other directions in organomagnesium chemistry are mainly limited to research interest, some areas, such as their application in catalysis and materials, are fast developing. Although most characterized Mg(I) and Mg(0) compounds do not contain Mg-C bonds, which means they cannot be rigorously categorized as organomagnesium compounds, they will be briefly discussed at the end of this page because of their great importance.

## Carbon as anionic σ-ligand
### Grignard reagents (RMgX)

Discovered by Victor Grignard at the university of in 1900, compounds with empirical formula RMgX (R = carbanion, X = Cl, Br, I) are known as Grignard reagents, which are widely used in organic synthesis and ligand preparation. Grignard reagents are a common source of carbanion, which can be used to perform nucleophilic addition, substitution, transmetalation, and metal-halogen exchange reactions. The first crystal structure of Grignard reagents was reported by Guggenberger and Rundle in 1964, from a crystalline EtMgBr(THF)$_2$ (Et = ethyl, THF = tetrahydrofuran). The Mg-C bond length was found to be 2.15(2) Å, which is about the sum of covalent radii of magnesium (141(7) pm) and carbon (76(1) pm at sp$^3$ hybridization).
Although Grignard reagents were discovered and commonly used in 1900s, the corresponding fluoride RMgF was not synthesized until 1970, plausibly because of the difficulty in breaking the strong C-F bond. In 1920 Swarts reported the reduction of amyl fluoride to the corresponding hydrocarbon with activated magnesium, while no intermediates were separated. Alkylmagnesium fluoride was first prepared by Ashly and co-workers in 1970, using metal magnesium and catalytic iodine in refluxing tetrahydrofuran or 1,2-dimethoxyethane from the corresponding alkyl fluoride.
Grignard reagents forms dimers in solutions, and the R and X groups are exchanged between magnesium centers, enabling the Schlenk equilibrium between RMgX and MgR$_2$ and MgX$_2$. Recent ab initio molecular dynamics computations have shown that the formation of such dimers is crucial for explaining the reactivity of Grignard reagents.

### Magnesium dihydrocarbyl and other hydrocarbyl magnesium

Dialkylmagnesium is another convenient precursor of magnesium complexes, which is useful when halides are unwanted. Dialkylmagnesium is usually prepared from Grignard reagents, via precipitation of magnesium halide. Solid state dialkylmagnesium forms one-dimensional chains via Mg-C-Mg 3c-2e bonds, and the Mg-C bond length is 2.24(3) Å in dimethylmagnesium (Me$_2$Mg)$_n$, which is about 10 pm longer than the terminal alkyl-Mg bonds (e.g. 2.15(2) Å in EtMgBr(THF)2). Molecular oligomer of dialkylmagnesium with terminal ligands were also synthesized with similar Mg-C bonding scheme. With large steric hindrance, diaryl magnesium [(2,6-Et$_2$C$_6$H$_3$)$_2$Mg] was found to be a molecular dimer with bridging aryl groups, and the bridging Mg-C distances range between 2.243(7) to 2.296(7) Å. Similar bridging alkynyl groups were found in [(Me$_3$Si)$_2$NMg(C≡CR)(THF)]$_2$ (R = Ph, SiMe$_3$) with the bridging Mg-C distance ranging from 2.189(4) to 2.283(4) Å.
By applying synergistic effect of magnesium and another alkaline metal, deprotonation of hydrocarbon derivatives has become another facile method to achieve the corresponding magnesium complexes. For example, in 2001 Mulvey achieved tetradeprotonation of ferrocene trapped in an amide cationic ring with four magnesium and four sodium, [($^i$Pr$_2$N)$_8$Na$_4$Mg$_4${Fe(C$_5$H$_3$)$_2$}], from free ferrocene and [Na($^i$Pr$_2$N)$_2$Mg($^n$Bu)].

## Carbon as neutral σ-ligand
### Carbonyl complexes

Unlike beryllium, calcium, strontium, and barium, no homoleptic carbonyl complex of magnesium has been found, probably because it lacks available (n-1)d orbitals, and it has low covalency. However, [Mg(O$_3$)$_2$(CO)$_2$] which contains ozonide anion (O$_3$$^−$) was identified when condensing atomic magnesium, oxygen and carbon monoxide in solid argon matrix. The compound shows increased C-O stretching frequency at 2188.9 cm$^{−1}$, compared to free carbon monoxide (2143 cm$^{−1}$), indicating little back-bonding from magnesium to the carbonyl.

### N-heterocyclic carbene complexes

The first characterized N-heterocyclic carbene (NHC) complex of magnesium, [(IMes)MgEt$_2$]$_2$ were synthesized in 1993 by Arduengo and co-workers, by simply mixing the stable carbene with diethylmagnesium. In [(IMes)MgEt$_2$]$_2$ the Mg-C(IMes) bond length was found to be 2.279(3) Å, which is significantly longer than the terminal Mg-C(Et) bond of 2.133(4) Å.
In 1995 Arduengo and co-workers characterized NHC adduct of MgCp*$_2$ (Cp* = pentamethylcyclopentadienyl), which features one η$^5$- and one η$^3$-Cp* ligands. NHCs with side arms were also explored. The amido NHC complex of magnesium was synthesized by Arnold and colleagues in 2004, and the magnesium complex using NHC with phenol arms were synthesized and characterized by Zhang and Kawaguchi in 2006.
Since NHCs are better σ donors than ethers like THF, it provides a scaffold for cationic molecular magnesium complexes, for it is categorized as neutral L-type ligand. In 2019, Dagorne and co-workers reported the first cationic alkyl magnesium supported by NHC ligand, [LMgMe(THF)$_2$]$^+$ BPh$_4$$^−$ (L = IMes, IPr). The synthesis proceeds through an interesting dimeric intermediate with two uncommon μ$^2$-Me bridges. In [(IPr)MgMe(THF)$_2$]$^+$, the Mg-C(IPr) distance was found to be 2.2224(13) Å, which is slightly shorter than the distance in neutral NHC complexes.
Notably, Gilliard and co-workers reported the equilibrium between L$_2$MgMeBr and [L$_3$MgMe]$^+$Br$^−$ (L = 1,3,4,5-tetramethylimidazol-2-ylidene) in d$^5$-bromobenzene, showing the substitution is facile despite its being endothermic.

## Carbon as π-ligand
### Allyl complexes

Allyl Grignard reagents exhibit high reactivity and special selectivity compared to alkyl ones. X-ray determination of single crystal structure and NMR spectroscopy both suggest that the allyl groups favor an η$^1$- instead of η$^3$-coordination pattern. Density functional theory (DFT) computations have shown that the homoleptic complex (C$_3$H$_5$)$_2$Mg adopts a C$_2$-symmetric geometry with two η$^3$-allyl groups, while coordination of THF molecules changed the allyl groups to η$^1$.
Allyl groups can also serve as bridging ligands. In 2001, Balley and co-workers reported a magnesium complex {(Dipp-$^{tBu}$NacNac)Mg(C$_3$H$_5$)}$_6$ (Dipp-$^{tBu}$Nacnac = [HC{C($^t$Bu)NDipp}$_2$]$^−$) featuring six μ-η$^1$:η$^1$ allyl ligands. Bridging μ-η$^1$:η$^2$ allyl ligands were also identified in a dimeric silyl allyl magnesium complex.

### Cylopentadienyl complexes

Dicyclopentadienyl (Cp) magnesium or magnesocene (Cp$_2$Mg) was first characterized in 1954 by Wilkinson and Cotton, and later crystal structure analysis shows that it features a 5-fold symmetry with two η$^5$-cyclopentadienyl ligands. MgCp$_2$ has an average Mg-C distance of 2.304(8) Å an average C-C distance of 1.39(2) Å, which is in agreement with a later gas-phase diffraction study. For comparison, in ferrocene the Fe-C distance is 2.04(1) Å and the C-C distance is 1.40(2) Å. Magnesocene derivatives generally adopt the ideal structures with staggered parallel Cp rings, though introducing large steric hindrance may distort the geometry, such as [{1,2,4-(Me$_3$Si)$_3$C$_5$H$_2$}$_2$Mg] which has slightly bent sandwich structure.
$^{25}$Mg-NMR spectroscopy suggested the Mg-Cp interaction has significant covalent character. However, because of lacking (n-1)d orbitals and back bonding, the Mg-Cp interaction is weak, enabling cyclopentadienyl magnesium complexes to serve as Cp$^−$ precursor. For example, in the following reaction Cp$_2$Mg transfers two Cp$^−$ ligands to synthesize the [MnCp$_3$]$^−$ anion:
Adding ligands to magnesocene derivatives gives bent Cp$_2$MgL species, and the bonding modes of the cyclopentadiene are sensitive to the changes in the coordination environment. In [(C$_5$Me$_4$H)$_2$MgL] (L = 1,3-di-iso-propyl-4,5-dimethylimidazol-2-ylidene), one of the C$_5$Me$_4$H ligand is slipped by 0.807 Å from the center, which makes difference of 0.69 Å between the shortest and the longest Mg-C distance on the ligand. Thus the complex can be described as [(η$^5$-C$_5$Me$_4$H)(η$^3$-C$_5$Me$_4$H)MgL].

### Magnesium anthracene

Another important complex is the magnesium anthracene, which was first prepared by Ramsden in 1965, using a THF suspension of magnesium and anthracene. From the solution crystalline [(C$_{14}$H$_{10}$)Mg(THF)$_3$] can be obtained, showing two relatively shorter Mg-C distances of 2.225(1) Å, on C9 and C10 of the anthracene. [(C$_{14}$H$_{10}$)Mg(THF)$_3$] reacts like [C$_{14}$H$_{10}$]$^{2-}$ with the two negative charges mainly localized on C9 and C10, and can thus act as nucleophile to give functionalized anthracene or 9,10-dihydroanthracene derivatives. One of its recent application is to synthesize dibenzo-7λ$^3$-phosphanorbornadiene (RPC$_{14}$H$_{10}$), which can be used as phosphinidene transfer reagent.

### Magnesium complexes with neutral π ligands

One of the first magnesium-neutral C=C π interactions was identified in [(DBAP)$_2$Mg]2 (DBAP = dibenzo[b,f]azepinate) by Harder and co-workers in 2017. The structure features one magnesium atom coordinated by four amino groups, and the other magnesium atom is weakly bound to three nitrogen atoms and three C=C π bonds. The Mg-olefin geometry is slightly asymmetric, with the shorter Mg-C distance range from 2.653(3) to 2.832(3) Å with an average of 2.740 Å, and the longer ones vary from 2.792(3) to 2.920(3) Å with an average of 2.848 Å. Previous research has also shown Mg-C distances of 2.559(6) to 2.638(3) Å in a tetrakis(2-aryloxy)ethylene system, but the shorter distance may be attributed to steric restriction.
In 2018, Harder and co-workers identified the first intramolecular Mg-π interaction in a cationic NacNac supported system, i.e., [(Dipp-NacNac)Mg(EtC≡CEt)][B(C$_6$F$_5$)$_4$] and [(Dipp-NacNac)Mg(η$^3$-C$_6$H$_6$)][B(C$_6$F$_5$)$_4$] (Dipp-Nacnac = [HC{C(Me)NDipp}$_2$]$^−$). In the alkyne complex the two Mg-C distances are 2.480(2) and 2.399(2) Å, and in the arene complex the shortest Mg-C bond length is 2.520(2) Å. Later study showed that the binding mode of arenes is sensitive to substitution on the arene, e.g., [(Dipp-NacNac)Mg(η6-mesitylene)][B(C$_6$F$_5$)$_4$] features a η$^6$-like mesitylene with Mg-C distance ranging from 2.5325(17) to 2.6988(16) Å. The first intramolecular Mg-alkene binding was later identified in 2020. In [(Dipp-NacNac)Mg(H$_2$C=CEt$_2$)][B(C$_6$F$_5$)$_4$], Mg is closer to the terminal methylene with Mg-C distance of 2.338(2) Å, and the longer Mg-C distance is 2.944(5) Å. DFT calculations and AIM analysis suggested that the Mg-alkene interaction is less covalent and should be mainly described as ion-induced dipole interactions, and the large asymmetry in the 2-ethylbutene complex should be attributed to charge distribution on the two sp$^2$ carbon atoms.
Although [(C$_6$H$_6$)$_2$Mg]$^{2+}$ remains unknown, the isoelectronic boratabenzene complex has been synthesized by Herberich and co-workers in 2000. In the work [(C$_5$H$_5$BMe)$_2$Mg] and [(3,5-Me$_2$C$_5$H$_3$BNMe$_2$)$_2$Mg] were characterized, with [(C$_5$H$_5$BMe)$_2$Mg] having average Mg-C distance of 2.403 Å (2.359(2) to 2.453(2) Å) and [(3,5-Me$_2$C$_5$H$_3$BNMe$_2$)$_2$Mg] having average Mg-C distance of 2.391 Å (2.350(1) to 2.429(2) Å). The slightly longer Mg-C distance compared to Cp$_2$Mg indicates a weaker donor ability of the boratabenzene, likely due to a more dispersed electron density among six instead of five atoms. Similar to Cp$_2$Mg derivatives, adding ligands like bipyridine (bipy) makes one of the boratabenzene slipped to form [(η$^5$-3,5-Me$_2$C$_5$H$_3$BNMe$_2$) (η$^1$-3,5-Me$_2$C$_5$H$_3$BNMe$_2$)Mg(bipy)], while coordination of THF changes the ligand to N-donor in [(N-3,5-Me$_2$C$_5$H$_3$BNMe$_2$)$_2$Mg(THF)$_2$].

## Applications

For the wide applications of Grignard reagents, please refer to the corresponding page.
The luminescence properties of magnesium compounds have been studied since the pioneering work by Chandrasekhar and co-workers in 2005, where a phosphorus-based tris-hydrazone complex of Mg(II) was synthesized and determined to have an intense fluorescence emission peak at 442 nm in dichloromethane solution.
In 2018, Roesky and co-workers developed a diamidophosphine ligand, with the ligated dimagnesium(II) compound having a fluorescence quantum yield of 34% in the solution. Munz and co-workers developed a pincer-like carbazole-mesoionic carbene ligand in 2021, delivering its magnesium complex a quantum yield of 14%. In 2022, Sen and colleagues reported two 2,2′-pyridylpyrrolide supported magnesium complexes, one mononuclear and one dinuclear, with quantum yield of 14% and 22%, respectively.
Catalytic systems for hydroboration reactions based on magnesium complexes have been investigated, with the pioneering work by Hill in 2012, where a Nacnac supported magnesium(II) hydride dimer was used to catalyze the hydroboration of ketones. Magnesium complexes have also been found to catalyze nucleophilic cyclization reactions.

## Low-oxidation state magnesium complexes

The first molecular Mg(I) compound, which contains a Mg-Mg bond, was synthesized by Jones, Stasch, and co-workers in 2007, from potassium metal reduction of Nacnac or priso ligand supported magnesium halide. In the [(Dipp-Nacnac)Mg]$_2$ the Mg-Mg distance is 2.8457(8) Å, and in the [(Dipp-$^{NMe2}$Priso)Mg]$_2$ (Dipp-$^{NMe2}$Priso = [Me$_2$NC(NDipp)$_2$]$^−$) the Mg-Mg distance is 2.8508(12) Å, both approximately equal to twice of the covalent radius of magnesium. In 2016, Jones and co-workers reported Mg(I) compounds with super bulky amido ligands, delivering a two-coordinate Mg(I). In [{2,6-(Ph$_2$CH)$_2$-4-$^i$Pr-C$_6$H$_2$}Mg]$_2$, the Mg-Mg distance of 2.8223(11) Å is significantly shorter than the N,N’-chelated Mg(I) dimers.
The first Mg(0) compound was reported in 2021 by Harder and co-workers, by reducing a similar Nacnac chelated in a harsher condition using newly prepared Na/NaCl.
In 2010, Platts and co-workers characterized a non-nuclear attractor in the Mg-Mg bond of [(Dipp-Nacnac)Mg]$_2$ from experimental electron density, which suggest the specialty of the Mg-Mg bond.
Mg(I) compounds have been proven to be useful reductants in synthetic chemistry. They have been found to be doing reversible addition to C=C double bonds, C-F bond activation, CO reduction, defluorination of PTFE, and reduction of OCP$^−$.