[reaction: see text] Computational studies of three different reaction types involving hydrocarbons (homolytic C-C bond breaking of alkanes, progressive insertions of triplet methylene into C-H bonds of ethane, and [2+2] cyclizations of methyl-substituted alkenes to form polymethylcyclobutanes) show that the B3LYP model consistently underestimates the reaction energy, even when extremely large basis sets are employed. The error is systematic and cumulative, such that the reaction energies of reactions involving hydrocarbons with more than 4-6 C-C bonds are predicted quite poorly. Energies are underestimated for slightly and highly methyl-substituted cyclic and acyclic hydrocarbons, so the errors do not arise from structural issues such as steric repulsion or ring strain energy. We trace the error associated with the B3LYP approach to its consistent underestimation of the C-C bond energy. Other DFT models show this problem to lesser extents, while the MP2 method avoids it. As a consequence, we discourage the use of the B3LYP model for reaction energy calculations for organic compounds containing more than four carbon atoms. We advocate use of a collection of pure and hybrid DFT models (and ab initio models where possible) to provide computational "error bars".

Catechol borane reacts with the frustrated Lewis pairs tBu2RP (R = tBu, 2-C6H4(C6H5)) and B(C6F5)3 to give the species [(C6H4O2)BPtBu2R][HB(C6F5)3] that can formally be described as either borenium cation or boryl-phosphonium salts; the nature of these species was probed with DFT calculations.

Phosphinoboranes that combine bulky electron-rich phosphides and electron deficient B(C6F5)2 fragments produce monomeric phosphinoboranes that undergo facile addition of H2 to give the phosphine-borane adducts (R2PH)(HBR'2). This finding in combination with DFT calculations shed light on the uptake of H2 across a group 13-group 15 bond, a critical requirement for the development of recyclable H2 storage materials.

Computational studies of amine-boranes (X 3 C) m H 3-m B-N(CH 3 ) n H 3-n (X ) H, F; m ) 0-3; n ) 0-3) show that the B3LYP model performs poorly in predicting the structures and B-N bond dissociation energies of such species. A survey of several models shows that the MP2 approach gives the best agreement, but is too computationally intensive for general use. Among several hybrid and pure DFT approaches, the MPW1K model gives the best agreement with experiment and/or with the MP2 model. Scans of the potential surface for rotation around the B-N bond in several molecules and examinations of other amine-boranes suggest that the difficulty with the B3LYP method does not arise from its inability to incorporate nonbonded intramolecular interactions, but from an inherent inability to model the dative bond. The MPW1K approach evidently does this better because it was designed to model "incompletely bound" transition states, which mimic datively bonded systems.

Reaction of LiBH, with [(C,(CH3),)Ir]2(w-H)3PF6 (1) results in the formation of the borohydride-bound dimer [(C5(CH3),)IrI2H3BH4 (2, R = H), for which a single-crystal X-ray diffraction study shows the borohydride moiety bridging the two metal centers in a unique fashion. Hydrolysis of this complex yields the formally iridium(1V) dimer [(C5(CH3)5)IrH3]2. Use of the more nucleophilic reducing agent LiEt3BH in this reaction leads to the mononuclear salt (C,(CH3),)IrH3[Li(THF),1 (5), the anion of which may by hydrolyzed to the iridium(V) complex (C5(CH3),)IrH4 ( 6 ) . The overall mechanism of the formation of tetrahydride 6 is discussed on the basis of these results. Deprotonation of 6 with t-BuLi in the presence of pmdeta leads to the sequestered salt (C5(CH3),)IrH3[Li(pmdeta)] (8). Silyl-or stannylation of the anion occurs with Me3Si03SCF3, Me3SnC1, and Ph3SnBr to yield the new iridium(V) polyhydrides (C5(CH3),)1rH3SiMe3, (C5(CH3),)IrH3SnMe3, and (C5-

The addition of diphenyl disulfide (PhSSPh) to tBu(2)P(C(6)F(4))B(C(6)F(5))(2) (1) affords the zwitterionic phosphonium borate [tBu(2)P(SPh)(C(6)F(4))B(SPh)(C(6)F(5))(2)] (2), while the addition of a base or donor solvent to 2 effected the liberation of disulfide and the formation of [tBu(2)P(C(6)F(4))B(donor)(C(6)F(5))(2)]. The reaction of 1 with S(8) gave tBu(2)P(S)(C(6)F(4))B(C(6)F(5))(2) (3). In a similar fashion, the frustrated Lewis pair of tBu(3)P/B(C(6)F(5))(3) reacts with RSSR to give [tBu(3)P(SR)][(RS)B(C(6)F(5))(3)] (R = Ph (4), p-tolyl (5), iPr (6)). In contrast, the corresponding reaction of BnSSBn yields a 1:1:1 mixture of tBu(3)P horizontal lineS, Bn(2)S, and B(C(6)F(5))(3). Species 4 reacts with p-tolylSSp-tolyl to give a mixture of 4, 5, PhSSPh, and p-tolylSS p-tolyl, while treatment of 5 with PhSSPh afforded a similar mixture. To probe this, a crossover experiment between [tBu(3)P(SPh)][B(C(6)F(5))(4)] (7) and [NBu(4)][(p-tolylS)B(C(6)F(5))(3)] (9) was performed. The former species was prepared by a reaction of 4 with [Ph(3)C][B(C(6)F(5)) (4)], while cation exchange of [(Et(2)O)(2)Li( p-tolylS)B(C(6)F(5))(3)] (8) with [NBu(4)]Br gave 9. The reaction of compounds 7 and 9 gave a statistical mixture of the cations [tBu(3)P(SR)](+) and anions [(RS)B(C(6)F(5))(3)](-), R = Ph, Sp-tolyl. The mechanism of this exchange process was probed and is proposed to be an equilibrium involving disulfide and the frustrated Lewis pair. Crystallographic data are reported for compounds 4-8, and the natures of the P-S cations are examined via DFT calculations.

The pyridine adducts of B(C(6)F(5))(3), (4-tBu)C(5)H(4)NB(C(6)F(5))(3) 1, ((2-Me)C(5)H(4)N)B(C(6)F(5))(3) 2, ((2-Et)C(5)H(4)N)B(C(6)F(5))(3) 3, ((2-Ph)C(5)H(4)N)B(C(6)F(5))(3) 4, ((2-C(5)H(4)N)C(5)H(4)N)B(C(6)F(5))(3) 5, (C(9)H(7)N)B(C(6)F(5))(3) 6, and ((2-C(5)H(4)N)NH(2-C(5)H(4)N))B(C(6)F(5))(3) 7, were prepared and characterized. The B-N bond lengths in 2-7 reflect the impact of ortho-substitution, increasing significantly with sterically larger and electron-withdrawing substituents. In the case of 2-amino-6-picoline, reaction with B(C(6)F(5))(3) affords the zwitterionic species (5-Me)C(5)H(3)NH(2-NH)B(C(6)F(5))(3) 8. In contrast, lutidine/B(C(6)F(5))(3) yields an equilibrium mixture containing both the free Lewis acid and base and the adduct (2,6-Me(2)C(5)H(3)N)B(C(6)F(5))(3) 9. This equilibrium has a DeltaH of -42(1) kJ/mol and DeltaS of -131(5) J/mol x K. Addition of H(2) shifts the equilibrium and yields [2,6-Me(2)C(5)H(3)NH][HB(C(6)F(5))(3)] 10. The corresponding reactions of 2,6-diphenylpyridine or 2-tert-butylpyridine with B(C(6)F(5))(3) showed no evidence of adduct formation and upon exposure to H(2) afforded [(2,6-Ph(2))C(5)H(3)NH][HB(C(6)F(5))(3)] 11 and [(2-tBu)C(5)H(4)NH][HB(C(6)F(5))(3)] 12, respectively. The energetics of adduct formation and the reactions with H(2) are probed computationally. Crystallographic data for compounds 1-10 are reported.

The phosphinoboranes [R(2)PB(C(6)F(5))(2)](2) (R = Et 1, Ph 2) and R(2)PB(C(6)F(5))(2) (R = tBu 3, Cy 4, Mes 5) were synthesized from the reaction of (C(6)F(5))(2)BCl and the corresponding lithium phosphide. The relationships between B-P distance, P pyramidality, and the extent of BP multiple bonding were further explored computationally. Natural Bond Order (NBO) analyses of 3 and 4 showed that the π-bonding highest occupied molecular orbitals (HOMOs) were highly polarized. In addition the Lewis acid-base adducts, R(2)(H)P·B(H)(C(6)F(5))(2) (R = Et 6; Ph 7; tBu 8; Cy 9; Mes 10) were prepared via the reaction of the phosphines R(2)PH with the borane HB(C(6)F(5))(2). Compounds 1 and 2 showed no signs of reaction with H(2); however, reaction of compounds 3 and 4 with H(2) was observed to give 8 and 9. In a related set of reactions compounds 3 and 4 were reacted with H(3)NBH(3) or Me(2)(H)NBH(3) also led to the generation of 8 and 9, respectively. The reaction profile of the reaction of (CF(3))(2)BPR(2) with H(2) was examined computationally and shown to be exothermic. Efforts to effect the reverse reaction, that is, dehydrogenation of adducts 6-10 were unsuccessful. Compound 4 was also shown to react with 4-tert-butylpyridine to give Cy(2)PB(C(6)F(5))(2)(4-tBuC(5)H(4)N) 11 while reactions of 3 and 4 with the Lewis acid BCl(3) gave the dimers (R(2)PBCl(2))(2) (R = tBu 12, Cy 13) and the byproduct ClB(C(6)F(5))(2).