Mg  Mg Bonds DOI: 10.1002/anie.200802960 MgCl and Mg 2 Cl 2 : From Theoretical and Thermodynamic Considerations to Spectroscopy and Chemistry of Species with Mg  Mg Bonds** Ralf Köppe,* Patrick Henke, and Hansgeorg Schnöckel* Recently, compounds containing metal–metal bonds between main-group elements have attracted interest to a surprising extent. [1–5] In spite of the latest structural results for two crystalline Mg 2 R 2 compounds (R1 = [ArNC(NiPr 2 )NAr]  ; R2 = [{ArNCMe} 2 CH]  ; Ar = 2,6-diisopropylphenyl, iPr = isopropyl) [2] and in spite of a number of theoretical contribu- tions on molecules with Mg Mg bonds, [10–12] several important questions still remain to be answered, for example, concern- ing the unexpected disproportionation stability of the above- mentioned Mg I compounds. Herein, we present the following results: 1) On the basis of quantum-chemical-assisted ther- modynamic calculations, a feasible synthesis of radical monomeric Mg I halides at temperatures near 900 8C is presented. 2) The nature of the Mg Mg bond is investigated by spectroscopic examination of MgCl and its linear dimer Mg 2 Cl 2 in solid inert-gas matrixes; the Mg Mg dissociation energy is obtained. 3) By comparing the thermodynamics of Mg 2 Cl 2 (1), Mg 2 Cp 2 (2, Cp = cyclopentadienyl), and a model compound Mg 2 R* 2 (3, R* = C(NH 2 )(NCH 3 ) 2 ) analogous to crystalline Mg 2 (R1) 2 , the enormous disproportionation sta- bility of the latter compound is explained. For further investigations, for example in the field of metal-rich Mg n R m clusters (n > m), the synthesis of reactive starting materials like MgCl is absolutely necessary. Initial results on the synthesis of MgCl are reported. After the first evidence for stable alkaline earth(I) halides were shown to be erroneous about 50 years ago, [13] about 40 years ago the first ESR spectroscopic evidence was given for MgF, among other examples, as a gaseous molecule at a temperature of 2300 8C trapped in an inert-gas matrix. [18] For this purpose, MgF 2 was vaporized at about 1250 8C; subse- quently, the gaseous MgF 2 molecules were dissociated to MgF and F atoms at about 2350 8C. [19] To find a feasible synthesis technique in analogy to the synthesis of AlCl at approx- imately 900 8C, [6] there should be no further component (e.g. Mg(g)) in the equilibrium composition besides gaseous MgCl and MgCl 2 , otherwise the entropically favored formation of monohalides would be put at risk. In contrast to the entropi- cally favored endothermic formation of AlCl (Al(l) + AlCl 3 (g)!3AlCl(g)), the Mg vapor pressure is already 1.5 mbar at 800 8C; thus, we looked for a solid magnesium- containing compound with a significantly reduced Mg activ- ity; for example, with a low decomposition pressure. We chose MgB 2 , a well-known compound with unexpected supercon- ducting capabilities, [21–23] as its Mg partial pressure had been examined in detail and amounts to not more than 10 3 mbar at 800 8C. [24–27] In principle, the following reactions to produce MgCl 2 and MgCl could take place when HCl is passed over heated MgB 2 [Eq. (1), (2)]. [28] 2 MgB 2 ðsÞþ 2 HClðgÞ¼ H 2 ðgÞþ 2 MgClðgÞþ 4BðsÞ ð1Þ MgB 2 ðsÞþ 2 HClðgÞ¼ H 2 þ MgCl 2 ðgÞþ 2BðsÞ ð2Þ The subtraction of Equation (2) from Equation (1) yields the relevant components for the equilibrium presented in Equation (3). [29] MgB 2 ðsÞþ MgCl 2 ðgÞ¼ 2 MgClðgÞþ 2BðsÞ ð3Þ Finally, supported by several experiments on the thermal stability of MgB 2 [24–26, 30] as well as by quantum-chemical calculations on MgCl and Mg 2 Cl 2 , the temperature-dependent partial pressure curves of MgCl 2 , MgCl, and Mg 2 Cl 2 are obtained (Figure 1). Figure 1 also contains the decomposition pressure curve for MgB 2 ; that is, for all hypothetical model reactions discussed herein, the applied HCl pressure must be higher than the partial pressure of magnesium. When HCl is passed over MgB 2 at approximately 700 8C and when this gas phase, together with a large surplus of inert gas (N 2 , Ar), is trapped on a copper surface cooled to 10 K, the Raman spectrum shown in Figure 2 results. An IR spectrum is obtained analogously (see the Supporting Information). Besides the well-known bands in MgCl 2 and its oligomers, [31] this spectrum displays only two further absorptions, which are assigned to monomeric and dimeric MgCl (Table 1). In the case of linear (D 1h ) Mg 2 Cl 2 , the IR spectrum shows the antisymmetric MgCl vibration (n 3 ) at 485 cm 1 ; its 35 Cl/ 37 Cl shift (although partially hidden) is in accordance with the calculated values (see Table 1 and the Supporting Informa- tion). The vibration of monomeric MgCl is, as expected, red- [*] Dr. R. Köppe, P. Henke, Prof. Dr. H. Schnöckel Institut für Anorganische Chemie, Universität Karlsruhe (TH) Engesserstrasse 15, Geb. 30.45, 76128 Karlsruhe (Germany) Fax: (+ 49)721-608-4854 E-mail: Hansgeorg.Schnoeckel@chemie.uni-karlsruhe.de Homepage: http://www.aoc.uni-karlsruhe.de/17.php [**] We thank the Deutsche Forschungsgemeinschaft (DFG), the DFG Research Center of Functional Nanostructures (CFN), the Karlsruhe Institute of Technology (KIT) and the Fonds der Chemischen Industrie for financial support. We thank as well Prof. Bärnighausen for helpful suggestions regarding solid alkaline earth subhalides and Bruker Bio Spin for the measurement of the ESR spectra. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.200802960. Communications 8740 # 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2008, 47, 8740 –8744