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

• Published in: Journal of the American Chemical Society 1996-01, Vol.118 (3), p.627-635
• Main Authors: King, Wayne A, Di Bella, Santo, Lanza, Giuseppe, Khan, Karl, Duncalf, David J, Cloke, F. Geoffrey N, Fragala, Ignazio L, Marks, Tobin J
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja9529697

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.