METAL-LIGAND BONDING AND BONDING ENERGETICS IN ZEROVALENT LANTHANIDE,GROUP-3, GROUP-4, AND GROUP-6 BIS(ARENE) SANDWICH COMPLEXES - A COMBINED SOLUTION THERMOCHEMICAL AND AB-INITIO QUANTUM-CHEMICAL INVESTIGATION

In this paper we report a comparative experimental thermochemical and ab initio quantum chemical study of metal-ligand bonding and bonding e nergetics in the group 3, lanthanide, group 4, and group 5 zerovalent bis(arene) sandwich complexes Sc(TTB)(2) (1) Y(TTB)(2) (2), GdC(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 = eta(6)-(1,3,5-(t)Bu)(3)C6H3). Deriv ed (D) over bar(M-arene) values by iodinolytic batch titration calorim etry in toluene for the process M(arene)(2(solution)) --> M degrees 2arene((solution)) are rather large(kcal/mol): 45(3) (1), 72(2) (2), 6 8(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 c ore potential calculations on M(C6H6)(2), M = Ti, Zr, Hf, Cr, Mo, W, r eveal that the metal-ligand bonding is dominated by strong (greater in group 4 than in the group 6 congeners) delta back-bonding from filled metal d(xy) and d(x2-y2) orbitals to unoccupied arene pi orbitals, wh ich decreases in the order Hf > Zr > Ti > W > Mo > Cr. Calculated geom etries and (D) over bar(M-C6H6) values (at the MP2 level) yield parame ters in favorable agreement with experiment. The latter analyses evide nce a great sensitivity to electron correlation effects. Marked, group -centered dependences of the measured (D) over bar(M-arene) values on the sublimation enthalpies of the corresponding bulk metals, on the me tal atomic volumes, and, for the lanthanides and Y, on the correspondi ng free atom f --> d promotion energies are also evident.