Cyclotrimerization of alkynes catalyzed by a self-supported cyclic tri-nuclear nickel(0) complex with α-diimine ligands.

A cyclic tri-nuclear α-diimine nickel(0) complex [{Ni(µ-LMe-2,4)}3] (2) was synthesized from a "pre-organized", trimerized trigonal LNiBr2-type precursor [Ni3(µ2-Br)3(µ3-Br)2(LMe-2,4)3]·Br (1; LMe-2,4 = [(2,4-Me2C6H3)NC(Me)]2). In complex 2, the α-diimine ligands not only exhibit the normal N,N'-chelating mode, but they also act as bridges between the Ni atoms through an unusual π-coordination of a C[double bond, length as m-dash]N bond to Ni. Complex 2 is able to catalyze the cyclotrimerization of alkynes to form substituted benzenes in good yield and regio-selectivity for the 1,3,5-isomers, which is found to vary with the nature of the alkyne employed. This complex represents a convenient self-supported nickel(0) catalyst with no need for additional ligands and reducing agent.

Binuclear Cyclopentadienylmetal Methylene Sulfur Dioxide Complexes of Rhodium and Iridium Related to a Photochromic Metal Dithionite Complex.

The photochromic dithionite complex Cp*2Rh2(µ-CH2)2(µ-O2SSO2) (Cp* = η5-Me5C5) is of interest because it undergoes an unusual fully reversible unimolecular photochemical rearrangement to the isodithionite complex Cp*2Rh2(µ-CH2)2(µ-O2SOSO). In order to obtain more insight into these systems, a comprehensive density functional theory study has been carried out on isomeric Cp2M2(CH2)2(SO2)2 (M = Rh, Ir) derivatives. The experimentally observed rhodium complexes with coupled sulfur dioxide (SO2) units to give dithionite or isodithionite ligands are surprisingly high-energy kinetic isomers in our analysis, reflecting the need for dithionite rather than SO2 for their synthesis. Many isomeric structures containing two separate SO2 ligands are found to lie at lower energies than these dithionite and isodithionite complexes. In the lowest-energy Cp2M2(CH2)2(SO2)2 isomers, the two methylene groups couple to form an ethylene ligand that can be either terminal or bridging. In slightly higher energy structures, a formal hydrogen shift is predicted to occur within the ethylene ligand to give a methylcarbene CH3CH ligand. Isomers with a bridging methylcarbene ligand are energetically preferred over isomers with a terminal methylcarbene ligand. Generation of the lower-energy Cp2Rh2(CH2)2(SO2)2 isomers containing separate SO2 ligands should be achievable through reactions of SO2 with more highly reduced cyclopentadienylrhodium methylene complexes such as Cp*2Rh2(µ-CH2)2.

Unsaturated trinuclear iron fluoroborylene complexes.

The unsaturated trinuclear iron fluoroborylene complexes Fe3(BF)3(CO) n (n = 7, 6) have been studied using density functional theory (DFT). Relatively complicated potential energy surfaces are found with nine and eight structures within 15 kcal mol-1 of the lowest energy structures for the Fe3(BF)3(CO)7 and Fe3(BF)3(CO)6 systems, respectively. In each of these low-energy structures all three BF groups are either edge-bridging or face-bridging but never terminal groups. Some, but not all, of the low-energy structures also have edge-bridging and/or face-bridging CO groups leading to some structures with as many as five bridging groups. The relatively narrow range of Fe-Fe distances in the central Fe3 triangles of the Fe3(BF)3(CO) n (n = 7, 6) structures, mainly between 2.37 and 2.55 Å, suggests considerable delocalization in these unsaturated systems. Graphical Abstract The lowest energy Fe3(BF)3(CO)7 and Fe3(BF)3(CO)6 structures have a face-bridging µ3-BF group with the two remaining BF groups bridging edges. The lowest energy Fe3(BF)3(CO)6 structure also has one four-electron donor bridging η2-µ-CO group.

Binuclear rhenium carbonyl nitrosyls related to dicobalt octacarbonyl and their decarbonylation products.

The geometries and thermochemistry of Re2(NO)4(CO) n (n = 4, 3, 2, 1, 0) structures isovalent with the binuclear cobalt carbonyls Co2(CO) n+4 have been examined using density functional theory. Eight low-energy Re2(NO)4(CO)4 structures, all with formal Re-Re single bonds, lie within 6 kcal mol(-1) of the global minimum. These eight structures include unbridged structures as well as structures with two bridging NO groups but no structures with bridging CO groups. Similarly, five low-energy Re2(NO)4(CO)3 structures, all with formal Re=Re double bonds, lie within 6 kcal mol(-1) of the global minimum. Again these five structures include unbridged structures as well as structures with one or two bridging NO groups but no structures with bridging CO groups. The Re2(NO)4(CO) n (n = 4, 3) appear to be fluxional systems similar to the well-known Co2(CO)8 for which doubly bridged and unbridged structures have approximately the same energies. The lowest energy Re2(NO)4(CO)2 structures have formal Re=Re double bonds including a structure with a five-electron donor bridging η(2)-µ-NO group. Isomeric Re2(NO)4(CO)2 structures with formal Re≡Re triple bonds lie approximately ∼10 kcal mol(-1) above the global minimum. For the more highly unsaturated Re2(NO)4(CO) and Re2(NO)4 systems, the lowest energy structures have formal Re≡Re triple bonds of length ∼2.6 Å. Higher energy Re2(NO)4(CO) structures have shorter Re-Re distances of length ∼2.5 Å suggesting formal quadruple bonds. Graphical Abstract The geometries and thermochemistry of Re2(NO)4(CO) n (n = 4, 3, 2, 1, 0) structures isovalent with the binuclear cobalt carbonyls Co2(CO) n+4 have been examined using density functional theory. A number of energetically closely spaced Re2(NO)4(CO)4 and Re2(NO)4(CO)3 structures are found, including unbridged and NO-bridged structures but no CO-bridged structures. The Re2(NO)4(CO) n (n = 2, 1, 0) systems provide examples of Re-Re multiple bonds of orders ranging from 2 to 4.

I + (H2O)2 → HI + (H2O)OH Forward and Reverse Reactions. CCSD(T) Studies Including Spin-Orbit Coupling.

The potential energy profile for the atomic iodine plus water dimer reaction I + (H2O)2 → HI + (H2O)OH has been explored using the "Gold Standard" CCSD(T) method with quadruple-ζ correlation-consistent basis sets. The corresponding information for the reverse reaction HI + (H2O)OH → I + (H2O)2 is also derived. Both zero-point vibrational energies (ZPVEs) and spin-orbit (SO) coupling are considered, and these notably alter the classical energetics. On the basis of the CCSD(T)/cc-pVQZ-PP results, including ZPVE and SO coupling, the forward reaction is found to be endothermic by 47.4 kcal/mol, implying a significant exothermicity for the reverse reaction. The entrance complex I···(H2O)2 is bound by 1.8 kcal/mol, and this dissociation energy is significantly affected by SO coupling. The reaction barrier lies 45.1 kcal/mol higher than the reactants. The exit complex HI···(H2O)OH is bound by 3.0 kcal/mol relative to the asymptotic limit. At every level of theory, the reverse reaction HI + (H2O)OH → I + (H2O)2 proceeds without a barrier. Compared with the analogous water monomer reaction I + H2O → HI + OH, the additional water molecule reduces the relative energies of the entrance stationary point, transition state, and exit complex by 3-5 kcal/mol. The I + (H2O)2 reaction is related to the valence isoelectronic bromine and chlorine reactions but is distinctly different from the F + (H2O)2 system.

Synthesis and structures of mononuclear and dinuclear gallium complexes with α-diimine ligands: reduction of the metal or ligand?

Reduction of the dichloro gallium(III) α-diimine complex [(L(ipr))˙(-)GaCl2] (1, L(ipr) = [(2,6-iPr2C6H3)NC(Me)]2) by different equivalents of sodium metal afforded the gallium complexes [(L(ipr))(2-)Ga(III)(µ2-Cl)2Na(THF)4] (2) and [(Na(THF)6)(+)·((L(ipr))(2-)Ga-Ga(L(ipr))(2-))˙(-)] (3). Interestingly, in complex 2 a Na(+)Cl(-) ion pair is incorporated, while compound 3 is an anionic digallium complex. Moreover, a cationic gallium complex with a tetrachlorogallium(III) counter anion, [(LGaCl2)(+)·(GaCl4)(-)] (4), was accessed from the reaction of GaCl3 with 0.5 equiv. of ligand L(ipr). In contrast, the reaction of GaCl3 with the doubly reduced anion (Na2L(2-)) of the smaller α-diimine ligands L(Me) ([(2,6-Me2C6H3)NC(Me)]2) or L(Et) ([(2,6-Et2C6H3)NC(Me)]2) yielded the Ga-Ga-bonded complexes [(L(Et))˙(-)ClGa(II)-Ga(II)Cl(L(Et))˙(-)] (5) and [(L(Me))˙(-)ClGa(II)-Ga(II)Cl(L(Me))˙(-)] (6). Here L is the neutral α-diimine ligand, L˙(-) represents the monoanion, and L(2-) is the dianionic form of the ligand. The complexes were characterized by X-ray diffraction and their electronic structures were studied by DFT computations.

The Reaction between Bromine and the Water Dimer and the Highly Exothermic Reverse Reaction.

The entrance complex, transition state, and exit complex for the bromine atom plus water dimer reaction Br + (H2O)2 → HBr + (H2O)OH and its reverse reaction have been investigated using the CCSD(T) method with correlation consistent basis sets up to cc-pVQZ-PP. Based on the CCSD(T)/cc-pVQZ-PP results, the reaction is endothermic by 31.7 kcal/mol. The entrance complex Br⋯(H2O)2 is found to lie 6.5 kcal/mol below the separated reactants. The classical barrier lies 28.3 kcal/mol above the reactants. The exit complex HBr⋯(H2O)OH is bound by 6.0 kcal/mol relative to the separated products. Compared with the corresponding water monomer reaction Br + H2 O → HBr + OH, the second water molecule lowers the relative energies of the entrance complex, transition state, and exit complex by 3.0, 3.8, and 3.7 kcal/mol, respectively. Both zero-point vibrational energies and spin-orbit coupling effects make significant changes to the above classical energetics. Including both effects, the predicted energies relation to separated Br + (H2O)2 are -3.0 kcal/mol [Br···(H2O)2 ], 28.2 kcal/mol [transition state], 26.4 kcal/mol [HBr···(H2O)OH], and 30.5 kcal/mol [separated HBr + (H2O)OH]. The potential energy surface for the Br + (H2O)2 reaction is related to that for the valence isoelectronic Cl + (H2O)2 system but radically different from the F + (H2O)2 system.

Controlling the Reactivity of the Boronyl Group in Platinum Complexes toward Cyclodimerization: A Theoretical Survey.

A theoretical study of the cyclodimerization of (Cy3P)2Pt(BO)Br (1Br) and [(Cy3P)2Pt(BO)](+) (1) (Cy = cyclohexyl) suggests that the reactivity of the BO ligand is primarily controlled by M←BO σ donation. Therefore, increasing the electron density at the metal center through strong σ-donor and weak π-acceptor ancillary ligands and a low formal metal oxidation state are suggested to reduce the polarity of the boronyl ligand and thus lower its reactivity toward cyclodimerization. The stable 1Br has lower Pt←BO σ donation and thus a less electrophilic boron atom, leading to a less polarized BO ligand. However, 1 is unstable in dichloromethane, since the dicationic dimer and transition state are highly stabilized by strong electrostatic interactions.

Mono- and Dinuclear Heteroleptic Cobalt Complexes with α-Diimine and Polyarene Ligands.

Reactions of the dimeric cobalt complex [(L(-) Co)2 ] (1, L=[(2,6-iPr2 C6 H3 )NC(Me)]2 ) with polyarenes afforded a series of mononuclear and dinuclear complexes: [LCo(η(4) -anthracene)] (2), [LCo(µ-η(4) :η(4) -naphthalene)CoL] (3), and [LCo(µ-η(4) :η(4) -phenanthrene)CoL] (4). The pyrene complexes [{Na2 (Et2 O)2 }{LCo(µ-η(3) :η(3) -pyrene)CoL}] (5) and [{Na2 (Et2 O)3 }{LCo(η(3) -pyrene)}] (6) were obtained by treating precursor 1 with pyrene followed by reduction with Na metal. These complexes contain three potential redox active centers: the cobalt metal and both α-diimine and polyarene ligands. Through a combination of X-ray crystallography, EPR spectroscopy, magnetic susceptibility measurement, and DFT computations, the electronic configurations of these complexes were studied. It was determined that complexes 2-4 have a high-spin Co(I) center coupled with a radical α-diimine ligand and a neutral polyarene ligand. Whereas, the ligand L in complexes 5 and 6 has been further reduced to the dianion, the cobalt remains in a formal (I) oxidation state, and the pyrene molecule is either neutral or monoanionic.

α-Diimine nickel complexes of ethylene and related alkenes.

A series of nickel mono(alkene) complexes, [LNi(alkene)], which consist of nickel(0) and neutral α-diimine ligand L (L = [(2,6-iPr2C6H3)NC(Me)]2), have been synthesized. The bonding and structures of the complexes were studied by X-ray diffraction, spectroscopic methods, and DFT computations.

From gas-phase to liquid-water chemical reactions: the fluorine atom plus water trimer system.

The potential energy profile for the F+(H2 O)3 →HF+(H2 O)2 OH reaction has been investigated using the "gold standard" CCSD(T) method with correlation-consistent basis sets up to cc-pVQZ. Four different reaction pathways have been found and these are related, both geometrically and energetically. The entrance complexes F⋅⋅⋅(H2 O)3 for all four reaction pathways are found lying ca. 7 kcal mol(-1) below the separated reactants F+(H2 O)3 . The four reaction barriers on their respective reaction coordinates lie ca. 4 kcal mol(-1) below the reactants. There are also corresponding exit complexes HF⋅⋅⋅(H2 O)2 OH, lying about 13 kcal mol(-1) below the separated products HF+(H2 O)2 OH. Compared with analogous F+(H2 O)2 and F+H2 O reactions, the F+(H2 O)3 reaction is somewhat similar to the former but qualitatively different from the latter. It may be reasonable to predict that the reactions between atomic fluorine and water tetramer (or even larger water clusters) may be similar to the F+(H2 O)3 reaction.

Triple decker sandwiches and related compounds of the first row transition metals with cyclopentadienyl and hexafluorobenzene rings: remarkable effects of fluorine substitution.

The complete series of Cp2M2(µ-C6F6) (M = Ti, V, Cr, Mn, Fe, Co, Ni) structures have been examined theoretically for comparison with their unsubstituted Cp2M2(µ-C6H6) analogues. The singlet triple decker sandwich titanium complex Cp2Ti2(η(6),η(6)-C6F6) with a closed shell electronic structure and a non-planar C6F6 ring is preferred energetically by a wide margin (>20 kcal mol(-1)) over other isomers and spin states. This is in contrast to the hydrogen analogue for which related triplet spin state structures are clearly preferred. A similar low-energy triple-decker sandwich Cp2V2(η(6),η(6)-C6F6) structure is found for vanadium but with a quintet spin state. The later transition metals from Cr to Ni energetically prefer the so-called "rice-ball" cis-Cp2M2(µ-C6F6) structures with varying hapticities of metal-ring bonding, a range of formal orders of metal-metal bonding, and varying spin states depending on the metal atom. Thus the lowest energy Cp2Cr2(µ-C6F6) structures are triplet and quintet structures with pentahapto-trihapto η(5),η(3)-µ-C6F6 rings and formal Cr=Cr double bonds. This contrasts with the structure of Cp2Cr2(µ-C6H6) having a bis(tetrahapto) η(4),η(4)-C6H6 ring and a formal Cr-Cr quadruple bond. The lowest energy Cp2Mn2(µ-C6F6) structures are trans and cis quintet spin state structures. This contrasts with Cp2Mn2(µ-C6H6) for which a closed-shell singlet triple decker sandwich structure is preferred. The lowest energy Cp2Fe2(µ-C6F6) structure is a triplet cis structure with a tetrahapto-dihapto η(4),η(2)-µ-C6F6 ring and a formal Fe-Fe single bond. The lowest energy Cp2Co2(µ-C6F6) structures are singlet spin state structures with formal M-M single bonds and either bridging bis(trihapto) η(3),η(3)-C6F6 or tetrahapto-dihapto η(4),η(2)-C6F6 rings. For Cp2Ni2(µ-C6F6) low energy singlet cis and trans structures are both found. The singlet cis-Cp2Ni2(µ-C6F6) structure has a Ni-Ni single bond of length ∼2.5 Å and a bridging bis(dihapto) η(2),η(2)-C6F6 ligand with an uncomplexed C=C double bond. The singlet trans-Cp2Ni2(µ-C6F6) structure has a bis(trihapto) η(3),η(3)-C6F6 ligand.

Pathways to the polymerization of boron monoxide dimer to give low-density porous materials containing six-membered boroxine rings.

Density functional theory has been used to examine the key mechanistic details of the polymerization of boron monoxide (BO) via its O≡B-B≡O dimer to give ultimately low-density porous polymeric (BO)n materials. The structures of such materials consist of planar layers of six-membered boroxine (B3O3) rings linked by boron-boron bonds. Initial cyclooligomerization of B2O2 leads to a B4O4 dimer with a four-membered B2O2 ring, a B6O6 trimer containing a six-membered B3O3 (boroxine) ring, a B8O8 tetramer containing an eight-membered B4O4 ring, and even a B10O10 pentamer containing a ten-membered B5O5 ring. However, an isomeric B10O10 structure containing two boroxine rings linked by a B-B bond is a much lower energy structure by ∼31 kcal/mol owing to the special stability of the aromatic boroxine rings. Rotation of the boroxine rings around the central B-B bond in this B10O10 structure has a low rotation barrier suggesting that further oligomerization to give products containing either perpendicular or planar orientations of the B3O3 rings is possible. However, the planar oligomers are energetically more favorable since they have fewer high-energy external BO groups bonded to the network of boroxine rings. The pendant boronyl groups are reactive sites that can be used for further polymerization. Mechanistic aspects of the further oligomerization of (BO)x systems to give a B24O24 oligomer with a naphthalene-like arrangement of boroxine rings and a B84O84 structure with a coronene-like arrangement of boroxine rings have been examined. Further polymerization of these intermediates by similar processes is predicted to lead ultimately to polymers consisting of planar networks of boroxine rings. The holes between the boroxine rings in such polymers suggests that they will be porous low-density materials. Applications of such materials as absorbents for small molecules are anticipated.

Cyclization of thiocarbonyl groups in binuclear homoleptic nickel thiocarbonyls to give ligands derived from sulfur analogues of croconic and rhodizonic acids.

The sulfur analogue of the well-known Ni(CO)4, namely, Ni(CS)4, has been observed spectroscopically in low temperature matrices but is not known as a stable species under ambient conditions. Theoretical studies show that Ni(CS)4 with monomeric CS ligands and tetrahedrally coordinated nickel is disfavored by ∼17 kcal/mol relative to unusual isomeric Ni(C2S2)2 structures. In the latter structures the CS ligands couple pairwise through C-C bond formation to give dimeric S═C═C═S ligands, which bond preferentially to the nickel atom through their C═S bonds rather than their C═C bonds. Coupling of CS ligands in the lowest energy binuclear Ni2(CS)n (n = 7, 6, 5) structures results in cyclization to give remarkable CnSn (n = 5, 6) ligands containing five- and six-membered carbocyclic rings. Such ligands, which are the sulfur analogues of the well-known croconate (n = 5) and rhodizonate (n = 6) oxocarbon ligands, function as bidentate ligands to the central Ni2 unit. Higher energy Ni2(CS)n (n = 7, 6, 5) structures contain dimeric C2S2 ligands, which can bridge the central Ni2 unit. Dimeric C2S2 ligands rather than tetrathiosquare C4S4 ligands are found in the lowest energy Ni2(CS)4 structures.

Construction of the tetrahedral trifluorophosphine platinum cluster Pt4(PF3)8 from smaller building blocks.

The experimentally known but structurally uncharacterized Pt4(PF3)8 is predicted to have an S4 structure with a central distorted Pt4 tetrahedron having four short Pt═Pt distances, two long Pt-Pt distances, and all terminal PF3 groups. The structures of the lower nuclearity species Pt(PF3)n (n = 4, 3, 2), Pt2(PF3)n (n = 7, 6, 5, 4), and Pt3(PF3)6 were investigated by density functional theory to assess their possible roles as intermediates in the formation of Pt4(PF3)8 by the pyrolysis of Pt(PF3)4. The expected tetrahedral, trigonal planar, and linear structures are found for Pt(PF3)4, Pt(PF3)3, and Pt(PF3)2, respectively. However, the dicoordinate Pt(PF3)2 structure is bent from the ideal 180° linear structure to approximately 160°. Most of the low-energy binuclear Pt2(PF3)n (n = 7, 6, 5) structures can be derived from the mononuclear Pt(PF3)n (n = 4, 3, 2) structures by replacing one of the PF3 groups by a Pt(PF3)4 or Pt(PF3)3 ligand. In some of these binuclear structures one of the PF3 groups on the Pt(PF3)n ligand becomes a bridging group. The low-energy binuclear structures also include symmetrical [Pt(PF3)n]2 dimers (n = 2, 3) of the coordinately unsaturated Pt(PF3)n (n = 3, 2). The four low-energy structures for the trinuclear Pt3(PF3)6 include two structures with central equilateral Pt3 triangles and two structures with isosceles Pt3 triangles and various arrangements of terminal and bridging PF3 groups. Among these four structures the lowest-energy Pt3(PF3)6 structure has an unprecedented four-electron donor η(2)-µ3-PF3 group bridging the central Pt3 triangle through three Pt-P bonds and one Pt-F bond. Thermochemical studies on the aggregation of these Pt-PF3 complexes suggest the tetramerization of Pt(PF3)2 to Pt4(PF3)8 to be highly exothermic regardless of the mechanistic details.

The exothermic HCl + OH·(H2O) reaction: removal of the HCl + OH barrier by a single water molecule.

The entrance complex, transition state, and exit complex for the title reaction have been investigated using the CCSD(T) method with correlation consistent basis sets up to cc-pVQZ. The stationary point geometries for the reaction are related to but different from those for the water monomer reaction HCl + OH → Cl + H2O. Our most important conclusion is that the hydrogen-bonded water molecule removes the classical barrier entirely. For the endothermic reverse reaction Cl + (H2O)2, the second water molecule lowers the relative energies of the entrance complex, transition state, and exit complex by about 4 kcal/mol. The title reaction is exothermic by 17.7 kcal/mol. The entrance complex HCl⋯OH·(H2O) is bound by 6.9 kcal/mol relative to the separated reactants. The classical barrier height for the reverse reaction is predicted to be 16.5 kcal/mol. The exit complex Cl⋯(H2O)2 is found to lie 6.8 kcal/mol below the separated products. The potential energy surface for the Cl + (H2O)2 reaction is radically different from that for the valence isoelectronic F + (H2O)2 system.

A density functional theory study of paramagnetic cyclopentadienylcobalt(III) derivatives: fluoride versus cyanide.

The cobalt(III) complexes Cp2Co2F4 and Cp2Co2(CN)4 have been studied by density functional theory methods as representatives of the experimentally known Cp2Co2X4 species with the weak-field fluoride ligand and the strong-field cyanide ligand. Both complexes were found to have relatively complicated energy surfaces with low-energy triplet and quintet spin state structures as well as the expected singlet-state structures for Co(III) complexes. This existence of singlet-, triplet-, and quintet-state structures of similar energies complicates the study of these complexes by density functional theory. The B3LYP* method of Reiher et al. was chosen in an effort to provide the most reliable estimates of the relative energies of the singlet, triplet, and quintet spin states. The lowest-energy Cp2Co2F4 structure was found to be a doubly bridged quintet spin state structure, with similar triplet and singlet structures lying within ∼4 kcal mol⁻¹ of this quintet structure. The lowest-energy Cp2Co2(CN)4 structure was found to be a triplet spin state structure, with a singlet structure lying within ∼1 kcal mol⁻¹ of this triplet structure. Almost all of the Cp2Co2X4 structures were found to have nonbonding Co···Co distances in excess of 2.9 Å, as expected for Co(III) complexes. In general, structures with trans stereochemistry of the Cp and other terminal ligands were found to be of lower energy than the corresponding structures with cis stereochemistry.

Distinct stepwise reduction of a nickel-nickel-bonded compound containing an α-diimine ligand: from perpendicular to coaxial structures.

A nickel-nickel-bonded complex, [{Ni(µ-L(.-))}2] (1; L=[(2,6-iPr2C6H3)NC(Me)]2), was synthesized from reduction of the LNiBr2 precursor by sodium metal. Further controllable reduction of 1 with 1.0, 2.0 and 3.0 equiv of Na, respectively, afforded the singly, doubly, and triply reduced compounds [Na(DME)3]·[{Ni(µ-L(.-))}2] (2; DME=1,2-dimethoxyethane), [Na(Et2O)]Na[(L(.-))Ni-NiL(2-)] (3), and [Na(Et2O)]2Na[L(2-)Ni-NiL(2-)] (4). Here L represents the neutral ligand, L(.-) denotes its radical monoanion, and L(2-) is the dianion. All of the four compounds feature a short Ni-Ni bond from 2.2957(6) to 2.4649(8) Å. Interestingly, they display two different structures: the perpendicular (1 and 2) and the coaxial (3 and 4) structure, in which the metal-metal bond axis is perpendicular to or collinear with the axes of the α-diimine ligands, respectively. The electronic structures, Ni-Ni bonding nature, and energetic comparisons of the two structure types were investigated by DFT computations.

F + (H2O)2 reaction: the second water removes the barrier.

In light of controversy concerning the classical barrier for the F + H2O → HF + OH reaction, higher level theoretical methods are applied. Both aug-cc-pV5Z CCSD(T) and cc-pVQZ full CCSDT methods predict a low classical barrier of about 2 kcal/mol. For the analogous water dimer reaction, the presence of the second water molecule erases this barrier entirely. An entrance complex F···(H2O)2 is found lying 7.3 kcal/mol below separated F + (H2O)2. A barrier follows this complex on the reaction coordinate, but lying 3.1 kcal/mol below the reactants. There is also an exit complex HF···H2O3 lying 11.0 kcal/mol below the separated products HF + H2O3. Thus the reactions of atomic fluorine with the water monomer and dimer are qualitatively different.

Major differences between the binuclear manganese boronyl carbonyl Mn2(BO)2(CO)9 and its isoelectronic chromium carbonyl analogue Cr2(CO)11.

The lowest energy structures of the manganese boronyl carbonyl Mn2(BO)2(CO)9 by more than 8 kcal/mol are found to have a single end-to-end bridging BO group bonding to one manganese atom through its boron atom and to the other manganese atom through its oxygen atom. The long Mn···Mn distances in these structures indicate the lack of direct manganese-manganese bonding as confirmed by essentially zero Wiberg bond indices. These Mn2(BO)2(CO)9 structures are favored thermochemically by more than 25 kcal/mol over dissociation into mononuclear fragments and thus appear to be viable synthetic objectives. This contrasts with the isoelectronic Cr2(CO)11 system, which is predicted to be disfavored relative to the mononuclear fragments Cr(CO)6 + Cr(CO)5. Analogous Mn2(BO)2(CO)9 structures with an end-to-end bridging CO group lie ∼17 kcal/mol in energy above the corresponding structures with end-to-end bridging BO groups. The lowest energy Mn2(BO)2(CO)9 structures without an end-to-end bridging BO group provide unprecedented examples of the coupling of two terminal BO groups to form a terminal dioxodiborene (B2O2) ligand with a B-B distance of ∼1.9 Å. Still higher energy Mn2(BO)2(CO)9 structures include singly bridged and doubly semibridged structures analogous to the previously optimized lowest energy Cr2(CO)11 structures.