Molybdenum and Tungsten Germylyne Complexes of the General Formula trans-[X(dppe)₂M⋮Ge-(η¹-Cp*)] (X = Cl, Br, I; dppe = Ph₂PCH₂CH₂PPh₂; Cp* = C₅Me₅):  Syntheses, Molecular Structures, and Bonding Features of the Germylyne Ligand

The halogermylenes (Cp*GeX)_n (1a, X = Cl, n = 1; 1b, X = Br, n = 2; Cp* = pentamethylcyclopentadienyl) were prepared in high yield by a redistribution reaction of GeCp*₂ with GeX₂(1,4-dioxane). The ionic compounds [Cp*Ge][GeX₃] (2a, X = Cl; 2b, X = Br) were also formed to a small extent in this reaction. Treatment of trans-[Mo(dppe)₂(N₂)₂] (3) (dppe = Ph₂PCH₂CH₂PPh₂) with 1 equiv of 1a,b affords in refluxing toluene the germylyne complexes trans-[X(dppe)₂Mo⋮Ge−(η¹-Cp*)] (4a, X = Cl; 4b, X = Br). Concomitant formation of GeCp*₂ and the dihalo complexes trans-[Mo(dppe)₂(X)₂] (5a, X = Cl; 5b, X = Br) is observed in this reaction. In comparison, the reactions of trans-[W(dppe)₂(N₂)₂] (6) with 1a,b and (Cp*GeI)_∞ (1c) afford selectively the germylyne complexes trans-[X(dppe)₂W⋮Ge−(η¹-Cp*)] (7a−c). The molecular structures of 4a·0.5pentane and 4b·0.5pentane reveal very short Mo−Ge bonds, almost linear Mo−Ge−C(Cp*) sequences, and an η¹-bonded Cp* substituent indicating the presence of a molybdenum−germanium triple bond. The structural features of the germylyne ligand in 4a and 4b are compared with those in the corresponding tungsten complexes trans-[X(dppe)₂W⋮Ge−(η¹-Cp*)] (7a−c), and the bonding parameters of the Cp* substituent are shown to be similar to those in the germanes (η¹-Cp*)GeI₃ (8) and (η¹-Cp*)₂GeCl₂ (9).