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), 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). Derived (D) over bar(M-arene) values by iodinolytic batch titration calorimetry in toluene for the process M(arene)(2(solution)) --> M degrees + 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) delta back-bonding from filled metal d(xy) and d(x2-y2) orbitals to unoccupied arene pi orbitals, which decreases in the order Hf > Zr > Ti > W > Mo > Cr. Calculated geometries and (D) over bar(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 (D) over bar(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.