Dimanganese decacarbonyl

Dimanganese decacarbonyl,^[3] which has the chemical formula Mn₂(CO)₁₀, is a binary bimetallic carbonyl complex centered around the first row transition metal manganese. The first reported synthesis of Mn₂(CO)₁₀ was in 1954 at Linde Air Products Company and was performed by Brimm, Lynch, and Sesny.^[4] Their hypothesis about, and synthesis of, dimanganese decacarbonyl was fundamentally guided by the previously known dirhenium decacarbonyl (Re₂(CO)₁₀), the heavy atom analogue of Mn₂(CO)₁₀. Since its first synthesis, Mn₂(CO)₁₀ has been use sparingly as a reagent in the synthesis of other chemical species, but has found the most use as a simple system on which to study fundamental chemical and physical phenomena, most notably, the metal-metal bond. Dimanganese decacarbonyl is also used as a classic example to reinforce fundamental topics in organometallic chemistry like d-electron count, the 18-electron rule, oxidation state, valency,^[5] and the isolobal analogy.

Dimanganese decacarbonyl

Dimanganese decacarbonyl
Names
IUPAC name bis(pentacarbonylmanganese)(Mn—Mn)
Other names Manganese carbonyl Decacarbonyldimanganese
Identifiers
CAS Number	10170-69-1 Y
3D model (JSmol)	Interactive image
ChemSpider	451751 Y
ECHA InfoCard	100.030.392
EC Number	233-445-6
PubChem CID	6096972
UNII	85SI7K7FWW Y
CompTox Dashboard (EPA)	DTXSID50895049
InChI InChI=1S/10CO.2Mn/c10*1-2;; Y Key: QFEOTYVTTQCYAZ-UHFFFAOYSA-N Y InChI=1/10CO.2Mn/c10*1-2;; Key: QFEOTYVTTQCYAZ-UHFFFAOYAD
SMILES O=C=[Mn](=C=O)(=C=O)(=C=O)(=C=O)[Mn](=C=O)(=C=O)(=C=O)(=C=O)=C=O
Properties
Chemical formula	Mn2(CO)10
Molar mass	389.98 g/mol
Appearance	Yellow crystals
Density	1.750 g/cm3
Melting point	154 °C (309 °F; 427 K)
Boiling point	sublimes 60 °C (140 °F; 333 K) at 0.5 mm Hg
Solubility in water	Insoluble
Structure[1]
Crystal structure	monoclinic
Lattice constant	a = 14.14 Å, b = 7.10 Å, c = 14.63 Å α = 90°, β = 105.2°, γ = 90°
Formula units (Z)	4
Dipole moment	0 D
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	CO source
GHS labelling:[2]
Pictograms
Signal word	Danger
Hazard statements	H301, H311, H331
Precautionary statements	P261, P264, P270, P271, P280, P301+P310, P302+P352, P304+P340, P311, P312, P321, P322, P330, P361, P363, P403+P233, P405, P501
Related compounds
Related compounds	Re2(CO)10 Co2(CO)8 Fe3(CO)12 Fe2(CO)9
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Synthesis

Many procedures have been reported for the synthesis of Mn₂(CO)₁₀ since 1954. Some of these methods serendipitously produce Mn₂(CO)₁₀.

Reduction/carbonylation syntheses

The carbonylation route involves treatment of Mn(II) salt under high pressure of CO and in the presence of a reductant. This is the method reported in 1954 by Brimm, Lynch, and Sesny, albeit in yields of ~1%. They used manganese(II) iodide with magnesium(0) as the reductant under 3000 psi (~200 atm) of carbon monoxide (CO):^[4]

2 MnI₂ + 2 Mg + 10 CO → Mn₂(CO)₁₀ + 2 MgI₂

A more efficient preparation was developed in 1958 and entails reduction of anhydrous manganese(II) chloride with sodium benzophenone ketyl radical under similarly high pressures (200 atm) of CO.^[6] The yield is ~32%.

High pressure carbonylation synthesis of dianganese decacarbonyl by reduction with sodium ketyl

Low pressure carbonylation

An ambient pressure synthesis of Mn₂(CO)₁₀ was reported from the commercially available and inexpensive methylcyclopentadienyl manganese tricarbonyl (MMT) and sodium(0) as the reductant.^[7] The balanced equation being:

2 Mn(η¹-(₅)−(CH₃C₅H₄)(CO)₃ + 2 Na + 4 CO → Mn₂(CO)₁₀ + 2NaCH₃C₅H₄

The efficiency of the method ranged from 16 to 20% yield, lower than what was previously reported, however, it could be performed more conveniently and on mole scale.

Dimerization syntheses

pentacarbonylhydridomanganese(-I) Mn source, oxidized by Se(PF₂)₂:^[8] Other terminal oxidants achieve the same effect,^[9][10][11] and stable pentacarbonylmanganate (Mn(CO)⁻
₅) salts can substitute for the hydride.^[12][13][14] Thus for example triphenylcyclopropenium tetrafluoroborate reacts with sodium pentacarbonyl manganate to produce the dimer of each:^[15]

Synthesis of dimanganese decacarbonyl by reduction of cyclopropenium ion

Similar methods exist for Mn(CO)₅X compounds where X = Cl, Br, or I; and, more rarely, for Mn(CO)⁺
₆ bound with a weakly coordinating anion.^{[16][17][18][19][20][21]} One additional interesting synthesis of Mn₂(CO)₁₀ occurs by combination of a hexacarbonylmanganese(I) tetrafluoroborate salt with a sodium pentacarbonyl manganate salt. In this instance, manganese is both the oxidant and reductant, producing two formal Mn(0) atoms:^[22] $${\displaystyle {\ce {[Mn(CO)6](BF4) + Na[Mn(CO)5] -> Mn2(CO)10 + Na[BF4] + CO}}}$$

Structure and bonding

This hypothesized structure was confirmed explicitly through x-ray diffraction studies, first in two dimensions in 1957,^[23] followed by its single crystal three-dimensional analysis in 1963.^[24] The crystal structure of Mn₂(CO)₁₀ was redetermined at high precision at room temperature in 1981 and bond lengths mentioned herein refer to results from that study.^[25] Mn₂(CO)₁₀ has no bridging CO ligands: it can be described as containing two axially-linked (CO)₅Mn- subunits. These Mn subunits are spaced at a distance of 290.38(6) pm, a bonding distance that is longer than that predicted.^[26] Two CO ligands are linked to each Mn atom that is coaxial with the Mn-Mn bond and four “equatorial” carbonyls bonded to each Mn atom that are nearly perpendicular to the Mn-Mn bond (Mn’-Mn-CO(equatorial) angles range from 84.61(7) to 89.16(7) degrees). The axial carbonyl distance of (181.1 pm) is 4.5 pm shorter than the average equatorial manganese-carbonyl distance of 185.6 pm. In the stable rotamer, the two Mn(CO)₅ subunits are staggered. Thus, the overall molecule has approximate point group D_4d symmetry, which is an uncommon symmetry shared with S₂F₁₀. The Mn₂(CO)₁₀ molecule is isomorphous with the other group 7 binary metal carbonyls Tc₂(CO)₁₀ and Re₂(CO)₁₀.

Important bond lengths and angles of dimanganese decacarbonyl

Electronic structure

Initial fundamental experimental and theoretical studies on the electronic structure of Mn₂(CO)₁₀ were performed used a mixture of photoelectron spectroscopy, infrared spectroscopy, and an iterative extended-Hückel-type molecular orbital calculation.^[27][28] The electronic structure of Mn₂(CO)₁₀ was most reported in 2017 using the BP86D functional with TZP basis set.^[29] The electronic structure described herein, along with relevant orbital plots, are reproduced from the methods used in that study using Orca (5.0.3)^[30] and visualized using IBOView (v20150427).^[31] The two main interactions of interest in the system are the metal-to-ligand pi-backbonding interactions and the metal-metal sigma bonding orbital. The pi-backbonding interactions illustrated below occur between the t_2g d-orbital set and the CO π* antibonding orbitals. The degenerate d_xz and d_yz backbonding interactions with both axial and equatorial CO ligands is the HOMO-15. More total delocalization occurs onto the axial CO antibonding orbital than does the equatorial, which is thought to rationalize the shorter Mn-C bond length. The primary Mn-Mn σ-bonding orbital is composed of two d_z2 orbitals, represented by the HOMO-9. Other large contributions made in this area were by Ahmed Zewail using ultrafast, femtosecond spectroscopy en route to his 1999 Nobel Prize.^[32] His discoveries elucidated much about the time scales and energies associated with the molecular motions of Mn₂(CO)₁₀, as well as the Mn-Mn and Mn-C bond cleavage events.^[33]

Reactivity

Mn₂(CO)₁₀ is air stable as a crystalline solid, but solutions require Schlenk techniques. Mn₂(CO)₁₀ is chemically active at both the Mn-Mn and Mn-CO bonds due to low, and similar, bond dissociation energies of ~36 kcal/mol (151 kJ/mol)^[34] and ~38 kcal/mol (160 kJ/mol),^[35] respectively. For this reason, reactivity can happen at either site of the molecule, sometimes selectively.

Mn-Mn bond cleavage reactions

The Mn-Mn bond is sensitive to both oxidation and reduction, producing two equivalents of the corresponding Mn(I) and Mn(-I) species, respectively. Both of the potential resultant species can be derived further. Redox neutral cleavage is possible both thermally and photochemically, producing two equivalents of the Mn(0) radical.

Oxidative cleavage

Selective mono-oxidation of the Mn-Mn bond is most often done via addition of classical metal oxidants (e.g. Ce^IV, Pb^IV, etc) or weak homonuclear single covalent bonds of the form X-X (X is group 16 or 17 element).^{[36][37][38][39][40]} These reactions yield the [Mn(CO)₅]⁺ cation with a bound weakly coordinating anion, or the Mn(CO)₅X complex. The general reaction schemes for each are seen as balanced equations below:$${\displaystyle {\ce {Mn2(CO)10 + 2 M^{n}X_{n}-> 2Mn(CO)5X + 2M^{(n-1)}X_{(n-1)}}}}$$or for two-electron oxidants$${\displaystyle {\ce {Mn2(CO)10 + M^{n}X_{n}-> 2Mn(CO)5X + M^{(n-2)}X_{(n-2)}}}}$$and$${\displaystyle {\ce {Mn2(CO)10 + RE-ER -> 2Mn(CO)5(ER)}}}$$for E = O, S, Se, Te$${\displaystyle {\ce {Mn2(CO)10 + X-X -> 2Mn(CO)5X}}}$$for X = F, Cl, Br, I

Reductive cleavage

Reductive cleavage is almost always done with sodium metal,^[41][42] yielding the [Mn(CO)₅]⁻ anion with the sodium counterion. The balanced general reactions are given below:$${\displaystyle {\ce {Mn2(CO)10 + 2 Na^{0}-> 2Na[Mn(CO)5]}}}$$The resultant manganate anion is a potent nucleophile, which can be protonated to give the manganese hydride,^[43][44] or alkylated with organic halides^[45][46][43] to give a large swath of organomanganese(I) complexes.

Redox-neutral cleavage

Homolytic cleavage, usually via light,^[47] but sometimes heat,^[48] gives the Mn(0) metalloradical, which can react with itself to reform Mn₂(CO)₁₀, or combine with other radical species that usually result in formal oxidation to Mn(I). This reactivity is comparable to that of organic, carbon-based radicals via the isolobal analogy. The homolytic cleavage is given by:$${\displaystyle {\ce {Mn2(CO)10 + h\nu -> 2[Mn(CO)5]^{.}}}}$$The use of the produced radical species, [Mn(CO)₅]*, has found several applications as a radical initiator for various organic methodologies^[49][50][51] and polymerization reactions.^[52][53][54]

Ligand substitution reactions

Ligand substitution reactions that do not disrupt the Mn-Mn bonding is done by using strongly sigma donating L-type ligands that can outcompete CO without participating in redox reactivity.^[55] This requirement usually necessitates phosphines^[56][57] or N-heterocyclic carbenes (NHCs),^[58] with substitution occurring at the axial position according to the reactions below:

Representative axial ligand substitution reactions of dimanganese decacarbonyl

Safety

Mn₂(CO)₁₀ is a volatile source of a metal and a source of CO.