Photolysis of [M(CO)6] (M = Cr, W) in the presence of BH3·L (L = NMe3, PMe3, PPh3) gave isolable borane complexes [M(CO)5(η1-BH3·L)] (1a, M = Cr, L = PMe3; 1b, M = Cr, L = PPh3; 1c, M = Cr, L = NMe3; 2a, M = W, L = PMe3; 2b, M = W, L = PPh3; 2c, M = W, L = NMe3). In products 1 and 2, the monoborane−Lewis base adduct coordinates to the metal center through a B−H−M three-center two-electron bond, which was confirmed by X-ray structural analyses of 1a, 2a, and 2b at low temperature. The X-ray crystal structural analysis of 1c at ambient temperature also showed the same coordination mode, although the positions of hydrogen atoms on the boron were not determined. The 1H NMR spectra of 1 and 2 exhibit only one BH signal at −2 to −3 ppm with an intensity of 3H in the temperature range of −80 °C to room temperature. This indicates that the coordinated BH and terminal BH's are rapidly exchanging in solution even at low temperature. When [Mo(CO)6] was used as a precursor, the formation of the corresponding molybdenum−borane complexes, [Mo(CO)5(η1-BH3·L)] (3a, L = PMe3; 3b, L = PPh3; 3c, L = NMe3), was observed by NMR spectroscopy, but the complexes could not be isolated because of their thermal instability. Complexes of pyridineborane [M(CO)5(η1-BH3·NC5H5)] (1d, M = Cr; 2d, M = W) were also observable by NMR spectroscopy. Fenske−Hall MO calculations for the model compound [Cr(CO)5(η1-BH3·PH3)] (1e) demonstrated that the bonding between the borane and metal can be described as donation of the bonding electron pair of BH to the a1 orbital of [Cr(CO)5], and that π back-donation from the metal d orbital to the antibonding σ* orbital of BH is negligible. Compounds 1−3 can be regarded as model compounds of the methane complex [M(CO)5(CH4)], which is observed in the photolyses of [M(CO)6] in methane matrixes. Structural and spectroscopic features of the ligated borane are discussed and compared with those of related compounds.

Photoirradiation of a solution of BH(3).NHR(2) (1a: R = Me, 1b: R = 1/2C(4)H(8), 1c: R = 1/2C(5)H(10), 1f: R = Et) containing a catalytic amount of a group-6 metal carbonyl complex, [M(CO)(6)] (M = Cr, Mo, W), led to dehydrogenative B-N covalent bond formation to produce aminoborane dimers, [BH(2)NR(2)](2) (2a-c, f), in high yield. During these reactions a borane sigma complex, [M(CO)(5)(eta(1)-BH(3).NHR(2))] (3), was detected by NMR spectroscopy. Similar catalytic dehydrogenation of bulkier amineboranes, BH(3).NH(i)Pr(2) (1d) and BH(3).NHCy(2) (1e, Cy = cyclo-C(6)H(11)), afforded monomeric products BH(2) horizontal lineNR(2) (4d, e). The reaction mechanism of the dehydrocoupling was investigated by DFT calculations. On the basis of the computational study, we propose that the catalytic dehydrogenation reactions proceed via an intramolecular pathway and that the active catalyst is [Cr(CO)(4)]. The reaction follows a stepwise mechanism involving NH and BH activation. Dehydrocoupling of borane-primary amine adducts BH(3).NH(2)R (1g: R = Me, 1h: R = Et, 1i: R = (t)Bu) gave borazine derivatives [BHNR](3) (5g-i).

Photolysis of [CpMn(CO)3] in the presence of BH3·L (L = NMe3, PMe3) produced borane σ complexes, 1a and 1b. These compounds were characterized by X-ray crystallography. The bonding between the borane ligand and metal is considerably different from those of other σ complexes. It consists of electron donation from the BH σ orbital to metal exclusively, and back-donation into the BH σ* orbital is negligible.

We investigated the influence of a substituent and a Lewis base on boron upon the thermodynamic stability of metal complexes of borane-Lewis base adducts, [M(CO)5(eta1-BH(2)R.L)] (M=Cr, W) and [CpMn(CO)2(eta1-BH2R.L)], where R=Cl, I, m-C6H4F, Ph, H, Me, Et; L=PMe3, PPh3, NMe3, quinuclidine. In these compounds, the stability of the metal-borane linkage was enhanced by increasing the electron-releasing ability of the substituent on boron. A stronger base L additionally stabilized the complexes. The strength of the borane-metal interaction is thus mainly ascribed to the electron donation from the BH sigma orbital to metal rather than the back-donation into the BH sigma* orbital. This result supports the bonding model for the B-H-M linkage in the borane complexes suggested by MO calculations, where the borane-to-metal electron donation is predominant while the metal back-donation into the BH sigma* orbital is negligible. Such a stability trend of the borane complexes makes a sharp contrast to that of many silane and dihydrogen complexes.

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Reaction of nido- [(Cp*Ru) 2 (µ-H) 2 (B 3 H 7 )] (1) with tertiary phosphines led to the degradation of the cluster framework and yielded [(Cp*Ru) 3). The solid-state structures of phosphine complex 2a and the phosphido-bridged 3 were determined by single-crystal X-ray diffraction. The solid-state structures of these complexes revealed two new modes of coordination for the B 2 H 5 fragment. Compound 3 exhibits fluxional behavior in solution.

Photolysis of methylmolybdenum and tungsten complexes, Cp*M(CO)3Me (M = Mo, W), in the presence of BH3·PMe3 (1) afforded nonsubstituted boryl complexes Cp*M(CO)3(BH2·PMe3) (2a:  M = Mo, 3a: M = W) with concomitant evolution of methane. The structures of 2a and 3a were determined by single-crystal X-ray crystallography. The boryl group in these complexes adopts a tetra-coordinate geometry with coordination of trimethylphosphine to the boron atom. The tungsten−boron bond distance in 3a is significantly longer than those of boryltungsten complexes with a tri-coordinate boryl group. The CO stretching bands in the IR spectra of 2a and 3a were observed at lower wavenumbers than those of the corresponding methyl complexes, indicating the pronounced polarization of the M−B bond, M(−)−B(+). Reactivity of the boryl complexes also featured the polar M−B bond. Boryl complexes having a nonsubstituted cyclopentadienyl ligand, CpM(CO)3(BH2·PMe3) (2b:  M = Mo, 3b:  M = W) were generated by the similar photoreaction although they were thermally unstable.

Koordinationswechsel bei Protonierung: Die Borylmangankomplexe 1 reagieren unter Protonierung zu den kationischen Boran‐σ‐Komplexen 2. Die Zersetzung von 2 verläuft unter heterolytischer Spaltung der Metall‐koordinierten B‐H‐Bindung.