More than 70 equilibrium constants K between acids and bases, mainly phosphine derivatives, have been measured in tetrahydrofuran (THF) at 20 °C by 1 H and/or 31 P NMR. The acids were chosen or newly synthesized in order to cover the wide pK R THF range of 5-41 versus the anchor compound [HPCy 3 ]BPh 4 at 9.7. These pK R THF values are approximations to absolute, free ion pK a THF and are obtained by crudely correcting the observed K for 1:1 ion-pairing effects by use of the Fuoss equation. The acid/base compounds include 14 phosphonium/phosphine couples, 17 cationic hydride/neutral hydride couples, 9 neutral polyhydride/anionic hydride couples, 14 dihydrogen/hydride couples, and 4 other nitrogen-and phosphorus-based acids. The effects on pK R of the counterions BAr′ 4and BF 4vs BPh 4and [K(2,2,2-crypt)] + versus [K(18-crown-6)] + are found to be minor after correcting for differences in inter-ion distances in the ion-pairs involved. Correlations with ν(M-H) noted here for the first time suggest that destabilization of M-H bonding in the conjugate base hydride is an important contributor to hydride acidity. It appears that Re-H bonding in the anions [ReH 6 (PR 3 ) 2 ]is greatly weakened by small increases in the basicity of PR 3 , resulting in a large increase in the pK R of the conjugate acid ReH 7 (PR 3 ) 2 . Correlations with other scales allow an estimate of the pK R THF values of more than 1000 inorganic and organic acids, 20 carbonyl hydride complexes, 46 cationic hydrides complexes, and dihydrogen gas. Therefore, many new acid-base reactions can be predicted and known reactions explained. THF, with its low dielectric constant, disfavors the ionization of neutral acids HA over HB + , and therefore separate lines are found for pK R THF (HA) and pK R THF (HB + ) when plotted against pK a DMSO or pK a MeCN . The crystal structure of [Re(H) 2 (PMe 3 ) 5 ]BPh 4 is reported.

We report electron-donor and steric properties of a diverse group of representative N-heterocyclic carbene (NHC) ligands quantified with the help of DFT calculations. This study afforded the conventional TEP data (Tolman electronic parameter = ν CO (A 1 ) of Ni(CO) 3 (NHC)), which allowed ranking 76 NHC ligands in order of increasing donor power. The TEP data reveal several general trends concerning the influence of NHC ring size, substitution, and annulation. The calculations also provided reaction enthalpies for CO elimination from the Ni(CO) 3 (NHC) complexes and formation of the 16-electron Ni(CO) 2 (NHC) species. This reaction is largely under steric control, which allowed defining a new steric descriptor for NHC ligands, r ("repulsiveness") = 10 Â (7.568 -0.003172TEP -0.0446ΔH), ranging from 0.0 for the smallest (ImNH 2 ) to 8.0 for the most repulsive carbene (ImNAd 2 ) of this work. Ni(CO) 3 L and Os(H 2 )Cl 2 (CO)L 2 complexes eliminate CO and H 2 , respectively, more readily with L = NHC vs PR 3 ligands. Apparently, even relatively small NHC ligands are very sterically demanding, and this property of N-heterocyclic carbenes may play a major role in coordination chemistry and catalysis.

Experimental and synthetic details p. 2-Details of catalytic studies p. 8-Computational details p. 9-10 Basis set used in the DFT calculations p. 10-Calculated energies of the Os and Ru complexes p. 13-Optimized geometries of the Os and Ru complexes at the PBE0 level p. 14-Optimized geometries of the Os and Ru complexes at the mPW1k level p. 19-References p. 31-Crystallographic Data CCDC 820331 (complex 1), CCDC 820332 (complex 2), and CCDC 820333 (complex 7)

Catalyst tune-up: A readily available, air-stable amino-sulfide catalyst, [RuCl(2)(PPh(3)){HN(C(2)H(4)SEt)(2)}], has been developed. This complex displays outstanding efficiency for the hydrogenation of a broad range of substrates with C=X bonds (esters, ketones, imines), as well as for the acceptorless dehydrogenative coupling of ethanol to ethyl acetate.

A large group of two-electron donor ligands were compared and ranked with the help of DFT calculations. The following characteristics were considered: (a) CtO stretching frequencies and distances of Ni(CO) 3 L, IrCl(CO) 2 L, and IrCp(CO)L, (b) Ir-(η 2 -C 2 H 4 ) bond enthalpies, stretching frequencies, and distances of IrCp(η 2 -H 2 CdCH 2 )L, (c) enthalpies of the ligand exchange reactions IrCp(CO)L + IrCpXe(PF 3 ) f IrCp(CO)(PF 3 ) + IrCpXeL and Ir (III) H 2 CpL + Ir (I) CpXe(PF 3 ) f Ir (III) H 2 Cp(PF 3 ) + Ir (I) CpXeL, (d) Ir-(η 2 -H 2 ) bond enthalpies and H-H distances of Os(η 2 -H 2 )Cl 2 (CO)L 2 . Useful trends and correlations have been established for several common ligand types.