Thermodynamic Hydride Donor Abilities of [HW(CO)₄L]^-Complexes (L = PPh₃, P(OMe)₃, CO) and Their Reactions with [C₅Me₅Re(PMe₃)(NO)(CO)]⁺

Abstract

The thermodynamic hydride donor abilities of [HW(CO)(5)](-) (40 kcal/mol), [HW(CO)(4)P(OMe(3))](-) (37 kcal/mol), and [HW(CO)(4)(PPh(3))](-) (36 kcal/mol) have been measured in acetonitrile by either equilibrium or calorimetric methods. The hydride donor abilities of these complexes are compared with other complexes for which similar thermodynamic measurements have been made. [HW(CO)(5)](-), [HW(CO)(4)P(OMe(3))](-), and [HW(CO)(4)(PPh(3))](-) all react rapidly with [CpRe(PMe(3))(NO)(CO)](+) to form dinuclear intermediates with bridging formyl ligands. These intermediates slowly form [CpRe(PMe(3))(NO)(CHO)] and [W(CO)(4)(L)(CH(3)CN)]. The structure of cis-[HW(CO)(4)(PPh(3))](-) has been determined and has the expected octahedral structure. The hydride ligand bends away from the CO ligand trans to PPh(3) and toward PPh(3).

Ionic Hydrogenations

“…Metal hydrides span a wide range of pK a values; considering only metal carbonyl hydrides shown in Table 7.1, the range exceeds 20 pK a units. As expected, a substantial decrease in acidity is 7.2 Stoichiometric Ionic Hydrogenations 157 [13][14][15][16][17][18][19]. Measurements of pK a values for a series of cobalt hydrides ( 2 ] + , which is much more acidic (pK a = 23.6).…”

“…DuBois et al carried out extensive studies on the thermodynamic hydricity of a series of metal hydrides [13,[15][16][17][18][19]. The determination of thermodynamic hydricity generally requires several measurements (coupled with known thermochemical data) to constitute a complete thermochemical cycle.…”

“…The same conclusion is reached for comparisons of Ni complexes with different diphosphine ligands. Replacement of one of the electron-withdrawing CO ligands of [HW(CO) 5 ] -with PPh 3 leads to an increase of 4 kcal mol -1 in the hydricity of [HW(CO) 4 (PPh 3 )] - [18]. DuBois et al found a dramatic dependence of hydricity on the bite angle of a series of Pd hydrides [19].…”

The roles of the first and second coordination spheres in the design of molecular catalysts for H2production and oxidation

“…This led to the development of several different methods for measuring the free energy associated with the reverse reaction, MH -M + + H À , using a variety of thermodynamic cycles. [11][12][13][14] One of these cycles is shown in Scheme 1. Reaction (2) in this cycle is the deprotonation of the metal hydride represented by [HML n ] + .…”

Catalytic Ionic Hydrogenations

“…[12,13] Extensive reactivity studies [14] by Darensbourg and co-workers documented the versatile hydridic reactions of anionic tungsten hydrides like [W(CO) 5 (PPh 3 )H] À , and recent measurements by DuBois provided a quantitative measure of the high hydricity. [13] The thermodynamic hydricity of this anionic hydride is 29 kcal mol À1 greater than than that of the cationic cobalt dihydride [CoH 2 (dppe) 2 ]…”