From unsuccessful H2-activation with FLPs containing B(Ohfip)3 to a systematic evaluation of the Lewis acidity of 33 Lewis acids based on fluoride, chloride, hydride and methyl ion affinitiesElectronic supplementary information (ESI) available: Experimental procedures and spectroscopic data are included in the supporting information, as well as detailed energies, vibrational frequencies, xyz-coordinates, solvation energy calculations, HOMO-LUMO-gaps and other data on the computations (>700 pages

The possibility of obtaining frustrated Lewis pairs (FLPs) suitable for H 2 -activation based on the Lewis acid B(Ohfip) 3 1 (Ohfip = OC(H)(CF 3 ) 2 ) was investigated. In this context, the crystal structure of 1 as well as the crystal structure of the very weak adduct 1 · NCMe was determined. When reacting solutions of 1 with H 2 (1 bar) and selected phosphanes, amines, pyridines and N-heterocyclic carbenes, dihydrogen activation was never observed. Without H 2 , adduct formation with 1 was observed to be an equilibrium process, regardless of the Lewis base adduct. Thus, the thermodynamics of H 2 activation of 1 in comparison with the well-known B(C 6 F 5 ) 3 was analyzed using DFT calculations in the gas phase and different solvents (CH 2 Cl 2 , ortho -difluorobenzene and acetonitrile). These investigations indicated that FLP chemistry based on 1 is considerably less favored than that with B(C 6 F 5 ) 3 . This is in agreement with control NMR experiments indicating hydride transfer from [H-B(Ohfip) 3 ] − upon reaction with B(C 6 F 5 ) 3 , giving [H-B(C 6 F 5 ) 3 ] − and B(Ohfip) 3 in toluene and also MeCN. Induced by these unsuccessful reactions, the Lewis acidity towards HSAB hard and soft ions was investigated for gaining a deeper insight. A unified reference system based on the trimethylsilyl compounds Me 3 Si-Y (Y = F, Cl, H, Me) and their respective ions Me 3 Si + /Y − calculated at the G3 level was chosen as the anchor point. The individual ion affinities were then assessed based on subsequent isodesmic reactions calculated at a much less expensive level (RI-)BP86/SV(P). This method was validated by systematic calculations of smaller reference systems at the frozen core CCSD(T) level with correlation effects extrapolated to a full quadruple-ζ basis. Overall, 33 common and frequently used Lewis acids were ranked with respect to their FIA, CIA, HIA and MIA (fluoride/chloride/hydride/methyl ion affinity). From hard to soft : The ion affinities of a large set of 33 Lewis acids towards hard and soft bases were examined with a unified isodesmic approach.