Laurel E. Schock

A new approach to measuring absolute metal-ligand bond disruption enthalpies in organometallic compounds. The [(CH3)3SiC5H4]3 U-system

Absolute uranium—ligand bond disruption enthalpies in the series Cp3UR(Cp = η5-Me3SiC5H4) have been measured by halogenolytic isoperibol titration calorimetry of Cp3U/Cp3UI/Cp3UR ensembles. Derived D(Cp3UR) values in toluene solution are (kcal mol−1, R): 62.4(0.4), I; 28.9(1.7), n-Bu; 25.6(3.1), Bz; 39.3(2.3), CH2SiMe3; 44.8(1.1), Me; 48.5(2.2), vinyl; 86.7, CCPh. D(Cp3U ← L) values (kcal mol−1, 95% confidence limits) in toluene solution were also determined for L = CO [10.3(0.2)] and THF [9.8(0.2)]. The magnitudes of the D(UR) values appear to reflect a combination of steric and electronic factors, and suggest that D(UI) is less sensitive to ancillary ligation (more transferable) then D(U-alkoxide). The complex Cp3U(vinyl) crystallizes in the triclinic space group P with two molecules in a unit cell of dimensions a = 11.298(1), b = 13.997(2), c = 9.460(2) A, and α = 97.00(1), β = 105.98(1) and γ = 96.94(1)°. Least-squares refinement led to a value for the conventional R index (on F) of 0.0196 for 4373 reflections having 2θMo-Kα 3σ(I). The molecule possesses a conventional pseudotetrahedral Cp3MX geometry with UC(ring) = 2.759(4) A (average), UC(α, vinyl) = 2.436(4) A, < UC(α, vinyl)C(β, vinyl) = 137.7(3)°, < CgUCg = 116.4, 117.2, 120.0 and < CgUC(α, vinyl) = 95.1, 100.0, 100.1°. All hydrogen atoms were located and UH(Cα, vinyl) = 2.93(4) A. The metrical parameters evidence severe non-bonded repulsions between the vinyl ligand and the Cp ligands, as well as between different Cp ligands. The quantity D(MI)—D(MCH3) is proposed as a gauge of metal—ligand bonding.