Dimanganese decacarbonyl
Dimanganese decacarbonyl, which has the chemical formula Mn210, is a binary bimetallic carbonyl complex centered around the first row transition metal manganese. The first reported synthesis of Mn210 was in 1954 at Linde Air Products Company and was performed by Brimm, Lynch, and Sesny. Their hypothesis about, and synthesis of, dimanganese decacarbonyl was fundamentally guided by the previously known dirhenium decacarbonyl, the heavy atom analogue of Mn210. Since its first synthesis, Mn210 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, and the isolobal analogy.
Synthesis
Many procedures have been reported for the synthesis of Mn210 since 1954. Some of these methods serendipitously produce Mn210.Reduction/carbonylation syntheses
The carbonylation route involves treatment of Mn 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 of carbon monoxide :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 of CO. The yield is ~32%.
Low pressure carbonylation
An ambient pressure synthesis of Mn210 was reported from the commercially available and inexpensive methylcyclopentadienyl manganese tricarbonyl and sodium(0) as the reductant. The balanced equation being: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 Se2:Similar methods exist for Mn5X compounds where X = Cl, Br, or I; and, more rarely, for Mn bound with a weakly coordinating anion.
One additional interesting synthesis of Mn210 occurs by combination of a hexacarbonylmanganese tetrafluoroborate salt with a sodium pentacarbonyl manganate salt. In this instance, manganese is both the oxidant and reductant, producing two formal Mn atoms:
Structure and bonding
This hypothesized structure was confirmed explicitly through x-ray diffraction studies, first in two dimensions in 1957, followed by its single crystal three-dimensional analysis in 1963. The crystal structure of Mn210 was redetermined at high precision at room temperature in 1981 and bond lengths mentioned herein refer to results from that study. Mn210 has no bridging CO ligands: it can be described as containing two axially-linked 5Mn- subunits. These Mn subunits are spaced at a distance of 290.38 pm, a bonding distance that is longer than that predicted. 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 angles range from 84.61 to 89.16. The axial carbonyl distance of is 4.5 pm shorter than the average equatorial manganese-carbonyl distance of 185.6 pm. In the stable rotamer, the two Mn5 subunits are staggered. Thus, the overall molecule has approximate point group D4d symmetry, which is an uncommon symmetry shared with S2F10. The Mn210 molecule is isomorphous with the other group 7 binary metal carbonyls Tc210 and Re2(CO)10.Electronic structure
Initial fundamental experimental and theoretical studies on the electronic structure of Mn210 were performed used a mixture of photoelectron spectroscopy, infrared spectroscopy, and an iterative extended-Hückel-type molecular orbital calculation. The electronic structure of Mn210 was most reported in 2017 using the BP86D functional with TZP basis set. The electronic structure described herein, along with relevant orbital plots, are reproduced from the methods used in that study using Orca and visualized using IBOView. 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 t2g d-orbital set and the CO π* antibonding orbitals. The degenerate dxz and dyz 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 dz2 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. His discoveries elucidated much about the time scales and energies associated with the molecular motions of Mn210, as well as the Mn-Mn and Mn-C bond cleavage events.
Reactivity
Mn210 is air stable as a crystalline solid, but solutions require Schlenk techniques. Mn210 is chemically active at both the Mn-Mn and Mn-CO bonds due to low, and similar, bond dissociation energies of ~36 kcal/mol and ~38 kcal/mol, 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 and Mn 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 radical.Oxidative cleavage
Selective mono-oxidation of the Mn-Mn bond is most often done via addition of classical metal oxidants or weak homonuclear single covalent bonds of the form X-X. These reactions yield the + cation with a bound weakly coordinating anion, or the Mn5X complex. The general reaction schemes for each are seen as balanced equations below:Reductive cleavage
Reductive cleavage is almost always done with sodium metal, yielding the − anion with the sodium counterion. The balanced general reactions are given below:Further reduction gives Na3Mn4.