Ligand Sphere Conversions in Terminal Carbide Complexes

Carbide complexes remain a rare class of molecules. Their paucity does not reflect exceptional instability but is rather due to the generally narrow scope of synthetic procedures for constructing carbide complexes. The preparation of carbide complexes typically revolves around generating LnM-CEx fragments, followed by cleavage of the C-E bonds of the coordinated carbon-based ligands (the alternative being direct C atom transfer). Prime examples involve deoxygenation of carbonyl ligands and deprotonation of methyl ligands, but several other p-block fragments can be cleaved off to afford carbide ligands. This Review outlines synthetic strategies toward terminal carbide complexes, bridging carbide complexes, as well as carbide-carbonyl cluster complexes. It then surveys the reactivity of carbide complexes, covering stoichiometric reactions where the carbide ligands act as C1 reagents, engage in cross-coupling reactions, and enact Fischer-Tropsch-like chemistry; in addition, we discuss carbide complexes in the context of catalysis. Finally, we examine spectroscopic features of carbide complexes, which helps to establish the presence of the carbide functionality and address its electronic structure.

Transition Metal Carbide Complexes.

Raman spectra of the mercurimethanes C (HgX)4 (X = Cl, Br, and I), CH (HgCl)3, and CH2 (HgX)2 (X = Cl and Br) at high pressures up to about 6 GPa were studied. v (CH) and v (CHg) valence vibrations show strong and medium pressure dependences in the ranges (13.8 ± 2.7) and (7.6 ± 1.7) cm−1 · GPa−1, respectively. It is concluded that obviously anharmonic vibrations of light atoms with large vibrational amplitudes should exhibit high pressure sensitivities.

Raman Spectra of Mercurimethanes CH4-n (HgX)n at High Pressures†

The complete series of tetrakis(halomercuri)methanes C(HgX)₄ (X = F, Cl, Br and I) was prepared by metathesis of tetrakis(acetoxymercuri)methane and aqueous hydrofluoric acid and solutions of halides (Cl-, Br-, and I-), respectively. All compounds are insoluble in most common solvents, but partly soluble in dimethylsulfoxide. Complete vibrational spectra indicate more or less ideally tetrahedral molecules C(HgX)₄ in all cases. The originally triply degenerate valence vibrations of the CHg₄ entities and the force constants for the C-Hg bonds show distinct correlations with the electronegativities of the halides X. On the other hand the totally symmetrical valence vibrations of CHg₄ were found to be highly sensitive to variations in X, mainly due to coupling with the valence vibrations of the Hg-X-groups of the same symmetry species. Some analogies in the vibrational behaviour of the CHg₄ fragments and of the isosteric NHg₄ core in tetramercurioammonium complexes were observed and discussed.

Tetraquecksilbermethane, II [1]Tetrakis(halogenoquecksilber)-methane C(HgX)4 (X = F, Cl, Br und I): Synthesen und Schwingungsspektren / Tetramercurimethanes, II [1]Tetrakis(halomercuri)methanes C(HgX)4 (X = F, Cl, Br und I): Syntheses and Vibrational Spectra

Knut Grabetz

Papers

Tetraquecksilbermethane, II [1]Tetrakis(halogenoquecksilber)-methane C(HgX)4 (X = F, Cl, Br und I): Synthesen und Schwingungsspektren / Tetramercurimethanes, II [1]Tetrakis(halomercuri)methanes C(HgX)4 (X = F, Cl, Br und I): Syntheses and Vibrational Spectra