In this paper we report a comparative experimental thermochemical and ab initio quantum chemical study of metal−ligand bonding and bonding energetics in the group 3, lanthanide, group 4, and group 5 zerovalent bis(arene) sandwich complexes Sc(TTB)2 (1) Y(TTB)2 (2), Gd(TTB)2 (3), Dy(TTB)2 (4), Ho(TTB)2 (5), Er(TTB)2 (6), Lu(TTB)2 (7), Ti(TTB)2 (8), Zr(TTB)2 (9), Hf(TTB)2 (10), Ti(toluene)2 (11), and Nb(mesitylene)2 (12) (TTB = η6-(1,3,5-tBu)3C6H3). Derived D̄(M−arene) values by iodinolytic batch titration calorimetry in toluene for the process M(arene)2(solution) → M° + 2arene(solution) are rather large (kcal/mol):  45(3) (1), 72(2) (2), 68(2) (3), 47(2) (4), 56(2) (5), 57(2) (6), 62(2) (7), 49(1) (8), 55(2) (11), 64(3) (9), 67(4) (10), and 73(3) (12). Ab initio relativistic core potential calculations on M(C6H6)2, M = Ti, Zr, Hf, Cr, Mo, W, reveal that the metal−ligand bonding is dominated by strong (greater in group 4 than in the group 6 congeners) δ back-bonding from filled metal d x y and d x 2 -y 2 orbitals to unoccupied arene π orbitals, which decreases in the order Hf > Zr > Ti > W > Mo > Cr. Calculated geometries and D̄(M−C6H6) values (at the MP2 level) yield parameters in favorable agreement with experiment. The latter analyses evidence a great sensitivity to electron correlation effects. Marked, group-centered dependences of the measured D̄(M−arene) values on the sublimation enthalpies of the corresponding bulk metals, on the metal atomic volumes, and, for the lanthanides and Y, on the corresponding free atom f → d promotion energies are also evident.

Absolute metal-ligand σ 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 Cp tt 2 TiI, (Me 5 C 5 ) 2 -TiI, Cp tt 2 ZrI, and Cp tt 2 HfI (Cp tt ) η 5 -1,3-di-tert-butylcyclopentadienyl). Iodinolysis of Cp tt 2 ZrMe 2 and Cp tt 2 HfMe 2 in turn yields absolute Zr-Me and Hf-Me bond enthalpies. Derived values (kcal/mol) are D[Cp tt 2 Ti(I)-I] ) 40.6(5); D[(Me 5 C 5 ) 2 Ti(I)-I] ) 52.3(6); D[Cp tt 2 Zr(I)-I] ) 58.0(5); D[Cp tt 2 Hf(I)-I] ) 61.2(4); D h [Cp tt 2 Zr-Me 2 ] ) 43(1); and D h [Cp tt 2 Hf-Me 2 ] ) 47.6(9). That D[Cp tt 2 Zr(I)-(I)] ≈ D(I 3 Zr-I) and D[(Me 5 C 5 ) 2 -Ti(I)-I] ≈ D(I 3 Ti-I), while D[Cp tt 2 Ti(I)-I] ≈ D(I 3 Ti-I) -12 kcal/mol, argues for more reliable transferability of D(M IV -I) in sterically less congested metallocenes. The molecular structures of Cp 2 tt ZrI 2 , Cp 2 tt ZrI, andCp 2 ttHfI were determined by X-ray diffraction. In Cp tt 2 ZrI 2 , 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 Cp tt 2 ZrI is pseudotrigonal, with contracted Zr-ring centroid and Zr-I distances versus Cp tt 2 ZrI 2 , primarily reflecting substantially diminished ligand-ligand repulsive nonbonded interactions in the latter. Cp tt 2 HfI is isomorphous with Cp tt 2 ZrI, 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 σ and π donor. Trends in the large Cp 2 MX n 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 σ vs π M-X bonding.

No abstract

No abstract