Absolute metal-ligand sigma bond enthalpies have been determined for a series of titanocene, zirconocene, and hafnocene halides and dimethyls by iodinolytic titration calorimetry. Absolute metal-iodine bond disruption enthalpies were measured by iodination of the monomeric trivalent group 4 metallocenes (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 enthalpies. 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[(Cp2Hf)-Hf-tt-Me-2] = 47.6(9). That D[(Cp2Zr)-Zr-tt(I)-(I)I approximate to D(I3Zr-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 reliable transferability of D(M-IV-I) in sterically less congested metallocenes. The 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 ligation is pseudotetrahedral, and the ring tert-butyl groups "straddle" the Zr-I bonds to minimize steric interactions. The geometry about Zr in (Cp2ZrI)-Zr-tt is pseudotrigonal, with contracted Zr-ring centroid and Zr-I distances versus (Cp2ZrI2)-Zr-tt, primarily reflecting substantially diminished ligand-ligand repulsive nonbonded interactions in the latter. Cp(2)(tt)Hfl is isomorphous with Cp(2)(tt)Zrl, and the slightly different metrical parameters are in accord with Hf vs Zr ionic radii. The significant differences in interligand repulsive interactions in the trivalent versus tetravalent complexes are confirmed by van der Waals calculations. High-resolution UV PE spectra combined with ab initio relativistic effective core potential calculations provide details of electronic structure. Absolute ionization energy values indicate that iodine behaves as both a strong sigma and pi donor. Trends 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 ligands and, in particular, to competing sigma vs pi M-X bonding.