Absolute metal-ligand sigma bond enthalpies in group 4 metallocenes. A thermochemical, structural, photoelectron spectroscopic, and ab initio quantumchemical investigation

Absolute metal-ligand sigma bond enthalpies have been determined for a seri es of titanocene, zirconocene, and hafnocene halides and dimethyls by iodin olytic titration calorimetry. Absolute metal-iodine bond disruption enthalp ies were measured by iodination of the monomeric trivalent group 4 metalloc enes (Cp2TiI)-Ti-tt, (Me5C5)(2)-TiI, (Cp2ZrI)-Zr-tt, and (Cp2HfI)-Hf-tt (Cp -tt = eta(5)-1,3-di-tert-butylcyclopentadienyl). Iodinolysis of (Cp2ZrMe2)- Zr-tt and (Cp2HfMe2)-Hf-tt in turn yields absolute Zr-Me and Hf-Me bond ent halpies. Derived values (kcal/mol) are D[(Cp2Ti)-Ti-tt(I)-I] = 40.6(5); D[( Me5C5)(2)Ti(I)-I] = 52.3(6); D[(Cp2Zr)-Zr-tt(I)-I] = 58.0(5); D[(Cp2Hf)-Hf- tt(I)-I] 61.2(4); (D) over bar[Cp-2(tt)-Me-2] = 43(1); and (D) over bar[(Cp 2Hf)-Hf-tt-Me-2] = 47.6(9). That D[(Cp2Zr)-Zr-tt(I)-(I)I approximate to D(I 3Zr-I) and D[(Me5C5)(2)-Ti(I)-I] approximate to D(I3Ti-I), while D[(Cp2Ti)- Ti-tt(I)-I] approximate to D(I3Ti-I) - 12 kcal/mol, argues for more reliabl e transferability of D(M-IV-I) in sterically less congested metallocenes. T he molecular structures of (Cp2ZrI2)-Zr-tt, (Cp2ZrI)-Zr-tt, and (Cp2HfI)-Hf -tt were determined by X-ray diffraction. In (Cp2ZrI2)-Zr-tt, the Zr ligati on is pseudotetrahedral, and the ring tert-butyl groups "straddle" the Zr-I bonds to minimize steric interactions. The geometry about Zr in (Cp2ZrI)-Z r-tt is pseudotrigonal, with contracted Zr-ring centroid and Zr-I distances versus (Cp2ZrI2)-Zr-tt, primarily reflecting substantially diminished liga nd-ligand repulsive nonbonded interactions in the latter. Cp(2)(tt)Hfl is i somorphous with Cp(2)(tt)Zrl, and the slightly different metrical parameter s are in accord with Hf vs Zr ionic radii. The significant differences in i nterligand repulsive interactions in the trivalent versus tetravalent compl exes are confirmed by van der Waals calculations. High-resolution UV PE spe ctra combined with ab initio relativistic effective core potential calculat ions provide details of electronic structure. Absolute ionization energy va lues indicate that iodine behaves as both a strong sigma and pi donor. Tren ds in the large Cp2MXn,, structural database can be understood in terms of the interplay between electronic and molecular structure factors, which are highly sensitive to the substitution patterns of the cyclopentadienyl liga nds and, in particular, to competing sigma vs pi M-X bonding.