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.

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The photochemical reaction of Mn(CO)5Br with PCy3 in toluene proceeds with the fast evolution of CO. The complex formed, MnBr(CO)3(PCy3)2, 1, readily reacts with NaA (A = B[C6H3(3,5-CF3)2]4) to form the dark green salt [Mn(CO)3(PCy3)2][A], 2, which has an agostic interaction of a cyclohexyl C−H with manganese. The light yellow η2-H2 complex, [Mn(H2)(CO)3(PCy3)2][A], 3, forms at room temperature by placing solutions of 2 under 1 atm of H2. The H2 ligand is labile and readily dissociates when the H2 atmosphere is removed in vacuo. 31P NMR clearly shows that at 25 °C under 1 atm of H2, 3 exists in equilibrium with 2. At −78 °C and under <1 atm of H2, only 3 is observed. The synthesis of the new tricarbonyl complex [Mn(CO)3(PCy3)2]+ provides an excellent system of comparison for the binding of small molecules to similar known 16e fragments such as M(CO)3(PCy3)2 (M = Cr, Mo, W, Re+) and M(CO)(dppe)2 (M = Mo, Mn+). The cationic manganese complex 2 appears to give somewhat more stable binding of H2 than the isoelectronic neutral chromium congener, which would not have been expected on the basis of relative back-bonding ability of the metal centers. Thus it is clear that increased σ-donation more than compensates for decreased back-bonding in the relative metal−H2 bonding energies. Surprisingly, binding of N2, ethylene, or silanes to 2 was not observed in solution even at low temperature and SO2 binds only weakly. By comparison with other similar systems, it is clear that H2 becomes an increasingly better ligand than N2 as the electrophilicity of the metal increases. Thus nonclassical H2 is a more versatile ligand than most classical ligands in its ability to adjust to a larger range of electronic situations.

Electrophilic 16e [Mn(CO)(R(2)PC(2)H(4)PR(2))(2)](+) complexes (R = Et, Ph) are synthesized by metathesis of MnBr(CO)(R(2)PC(2)H(4)PR(2))(2) with Na or Li salts of low-coordinating boron or gallium anions (e.g., [B{C(6)H(3)(3,5-CF(3))(2)}(4)](-) or [Ga(C(6)F(5))(4)] (-)). They contain weak polyagostic interactions that are reversibly displaced by H(2), N(2), and SO(2) (which is a surprisingly weak ligand here). The agostic and H(2) complexes, as well as the gallium anions including the new species [{Ga(C(6)F(5))(3)}(2)(&mgr;-Cl)](-), have been characterized by NMR, IR, and X-ray crystallography. The agostic Mn-H distances (e.g., 2.9 Å) are much longer than those found for the single agostic interactions in Mo(CO)(diphosphine)(2) and [Mn(CO)(3)(PCy(3))(2)](+). The H-H and also the Mn-H distances have been determined in the H(2) complex by T(1) measurements for both the H(2) and HD isotopomers. IR data and C-O and M-C bond lengths are used to gauge the pi-acceptor strengths of ligands trans to the CO. The agostic C-H bonds are the weakest ligands and also the weakest acceptors, but the H(2) ligand is an excellent acceptor as strong as N(2) and ethylene. The variation of nu(CO) on increasing the basicity of the cis-phosphine (dppe versus depe) in trans-M(CO)(diphosphine)(2)(L) is less than expected and far less than that on increasing the charge on the complex (M = Mn(+) versus Mo). The H-H bond lengths (0.87-0.90 Å) and J(HD) NMR couplings (32-34 Hz) in [Mn(CO)(R(2)PC(2)H(4)PR(2))(2)(H(2))](+) and other cationic H(2) complexes with trans-CO are strikingly similar to their neutral analogues and nearly invariant. Activation of H-H in the more electrophilic cationic systems occurs primarily via increased sigma donation from H(2) as compared to the more electron-rich neutral analogues where back-bonding dominates. The nature of the ligand trans to H(2) (the strong acceptor CO here) controls the H-H distance more so than all of the cis ligands combined, especially for cationic complexes.

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