Triple Bonding to Germanium:  Characterization of the Transition Metal Germylynes (η⁵-C₅H₅)(CO)₂M⋮Ge-C₆H₃-2,6-Mes₂ (M = Mo, W; Mes = −C₆H₂-2,4,6-Me₃) and (η⁵-C₅H₅)(CO)₂M⋮Ge-C₆H₃-2,6-Trip₂ (M = Cr, Mo, W; Trip = −C₆H₂-2,4,6-i-Pr₃) and the Related Single Bonded Metallogermylenes (η⁵-C₅H₅)(CO)₃M-G̈e-C₆H₃-2,6-Trip₂ (M = Cr, W)

The synthesis and characterization of several m-terphenyl stabilized germylyne transition metal complexes and related metallogermylenes are reported. The reaction of Na[M(η⁵-C₅H₅)(CO)₃] (M = Cr, or W) with Ge(Cl)C₆H₃-2,6-Trip₂ (Trip = −C₆H₂-2,4,6-i-Pr₃) below room temperature affords the metallogermylenes (η⁵-C₅H₅)(CO)₃M-G̈e-C₆H₃-2,6-Trip₂ (M = Cr, 1; W, 5). However, for the analogous reaction with Na[Mo(η⁵-C₅H₅)(CO)₃], the metallogermylene was not isolated. Instead, the germylyne complex (η⁵-C₅H₅)(CO)₂Mo⋮Ge−C₆H₃-2,6-Trip₂ (3) was obtained directly with elimination of carbon monoxide. The corresponding chromium and tungsten compounds (η⁵-C₅H₅)(CO)₂M⋮Ge−C₆H₃-2,6-Trip₂ (M = Cr, 2; W, 6) could be obtained by refluxing 1 or 5 in toluene or by photolysis. The related germylyne (η⁵-C₅H₅)(CO)₂W⋮Ge−C₆H₃-2,6-Mes₂ (7), as well as the recently reported molybdenum analogue (η⁵-C₅H₅)(CO)₂Mo⋮Ge−C₆H₃-2,6-Mes₂ (4), which was the first example of a triply bonded germanium species, are also described. All compounds were characterized by ¹H and ¹³C NMR, UV−vis, and IR spectroscopy, and by C, H analysis. The X-ray crystal structures of 1−3, 5, and 7 have also been determined. The M⋮Ge (M = Cr, Mo, W) triple bonds in the germylynes are typically ca. 0.4 Å shorter than the M−Ge single bonds in the metallogermylenes. In addition, angles at germanium in the germylynes are nearly linear (range 170.9(3)−175.99(6)°) whereas in the metallogermylenes the corresponding angles are 114.7(6) and 117.8(2)°, consistent with the presence of a lone pair at germanium. The synthesis of the series of compounds 1−7 has enabled, for the first time, a comparison of single and triple bond lengths between closely related species involving the same coordination number and substituents at the heavier group 14 element.