A new approach to measuring absolute metal—ligand bond disruption enthalpies in organometallic compounds. The [(CH 3) 3SiC 5H 4] 3 U-system

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Absolute uranium—ligand bond disruption enthalpies in the series Cp 3UR(Cp = η 5-Me 3SiC 5H 4) have been measured by halogenolytic isoperibol titration calorimetry of Cp 3U/Cp 3UI/Cp 3UR ensembles. Derived D(Cp 3U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), CH 2SiMe 3; 44.8(1.1), Me; 48.5(2.2), vinyl; 86.7, CCPh. D(Cp 3U ← 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 Cp 3U(vinyl) crystallizes in the triclinic space group P 1 with two molecules in a unit cell of dimensions a = 11.298(1), b = 13.997(2), c = 9.460(2) Å, 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 2θ Mo-K α < 50° and I > 3σ( I). The molecule possesses a conventional pseudotetrahedral Cp 3MX geometry with UC(ring) = 2.759(4) Å (average), UC(α, vinyl) = 2.436(4) Å, < 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) Å. The metrical parameters evidence severe non-bonded repulsions between the vinyl ligand and the Cp̋ ligands, as well as between different Cp̋ ligands. The quantity D(MI)—D(MCH 3) is proposed as a gauge of metal—ligand bonding.