Reaction of ferricytochrome c with the iron nitrosyl complex {Fe2[S(CH2)2NH3]2(NO)4}SO4 • 2.5H2O

By an example of cysteamine iron nitrosyl complex {Fe₂[S(CH₂)₂NH₃]₂(NO)₄}SO₄ ? 2.5H₂O (CAC) it was shown for the first time that the NO donor hydrolysis in the presence of ferricytochrome c (cyt c³⁺) affords the iron nitrosyl complex NO—cyt c³⁺. It was found that cyt c³⁺ can serve as a depot for NO evolved during the hydrolysis of CAC. In the presence of CAC, the rate of NO—cyt c³⁺ complex decomposition to NO and cyt c³⁺ depends on the molar ratio [cyt c³⁺]: [CAC] and at [cyt c³⁺]: [CAC] = 0.3 it was found to be lower than that in decomposition of CAC in the absence of cyt c³⁺. As a result, the total NO evolving process becomes 5.6 times more prolonged. The number of NO groups evolved from CAC can be determined by the reaction of CAC with cyt c³⁺ in the presence of ferricyanide: at most one NO group is evolved to a solution in the spontaneous hydrolysis of CAC (pH 7.0), and no less than three of them are evolved from oxidized CAC.     

Mononitrosyl tris(thiolate) iron complex [Fe(NO)(SPh)3]- and dinitrosyl iron complex [(EtS)2Fe(NO)2]-: formation pathway of dinitrosyl iron complexes (DNICs) from nitrosylation of biomimetic rubredoxin [Fe(SR)4]2-/1- (R = Ph, Et)

Nitrosylation of the biomimetic reduced- and oxidized-form rubredoxin [Fe(SR)4]2-/1- (R = Ph, Et) in a 1:1 stoichiometry led to the formation of the extremely air- and light-sensitive mononitrosyl tris(thiolate) iron complexes (MNICs) [Fe(NO)(SR)3]- along with byproducts [SR]- or (RS)2. Transformation of [Fe(NO)(SR)3]- into dinitrosyl iron complexes (DNICs) [(RS)2Fe(NO)2]- and Roussin's red ester [Fe2(mu-SR)2(NO)4] occurs rapidly under addition of 1 equiv of NO(g) and [NO]+, respectively. Obviously, the mononitrosyl tris(thiolate) complex [Fe(NO)(SR)3]- acts as an intermediate when the biomimetic oxidized- and reduced-form rubredoxin [Fe(SR)4]2-/1- exposed to NO(g) were modified to form dinitrosyl iron complexes [(RS)2Fe(NO)2]-. Presumably, NO binding to the electron-deficient [Fe(III)(SR)4]- and [Fe(III)(NO)(SR)3]- complexes triggers reductive elimination of dialkyl/diphenyl disulfide, while binding of NO radical to the reduced-form [Fe(II)(SR)4]2- induces the thiolate-ligand elimination. Protonation of [Fe(NO)(SEt)3]- yielding [Fe(NO)(SPh)3]- by adding 3 equiv of thiophenol and transformation of [Fe(NO)(SPh)3]- to [Fe(NO)(SEt)3]- in the presence of 3 equiv of [SEt]-, respectively, demonstrated that complexes [Fe(NO)(SPh)3]- and [Fe(NO)(SEt)3]- are chemically interconvertible. Mononitrosyl tris(thiolate) iron complex [Fe(NO)(SPh)3]- and dinitrosyl iron complex [(EtS)2Fe(NO)2]- were isolated and characterized by X-ray diffraction. The mean NO bond distances of 1.181(7) A (or 1.191(7) A) in complex [(EtS)2Fe(NO)2]- are nearly at the upper end of the 1.178(3)-1.160(6) A for the anionic {Fe(NO)2}9 DNICs, while the mean FeN(O) distances of 1.674(6) A (or 1.679(6) A) exactly fall in the range of 1.695(3)-1.661(4) A for the anionic {Fe(NO)2}9 DNICs.     

Charge density studies on [(NO)Fe(S2C6H4)2][PPN] and [(NO)3Fe(S2C6H4)3] complexes

Charge density studies of chemical bonds for two iron complexes, [(NO)Fe(S,S-C₆H₄)₂] [PPN] (1), where PPN = N(Pph₃)₂ and Fe₃(NO)₃(S,S-C₆H₄)₃ (2) are investigated in terms of the topological properties at bond critical points based on the ‘atoms in molecule’ theory. The one electron reduction form (1R) of complex 1 and the one electron oxidation form (2O) of complex 2 are also included for comparison. The X-ray absorption spectroscopy of Fe K- and L_III,II-edges, as well as the N/S K-edge are applied to verify the illustration in the variation of the electronic structures. Based on the ρ_c, ?²ρ_c, and H_b values among the compound studied, Fe-S/N can be regarded as polarized covalent bond, and Fe-N bonds show stronger covalent character than that of the Fe–S bond, which is believed to be a highly polarized covalent bond.     

Dinitrosyl iron complexes (DNICs) [L(2)Fe(NO)2]- (L = thiolate): interconversion among {Fe(NO)2}9 DNICs, {Fe(NO)2}10 DNICs, and [2Fe-2S] clusters, and the critical role of the thiolate ligands in regulating NO release of DNICs

Dinitrosyl iron complex [(-SC(7)H(4)SN)(2)Fe(NO)(2)](-) (1) was prepared by reaction of [S(5)Fe(NO)(2)](-) and bis(2-benzothiozolyl) disulfide. In synthesis of the analogous dinitrosyl iron compounds (DNICs), the stronger electron-donating thiolates [RS](-) (R = C(6)H(4)-o-NHCOCH(3), C(4)H(3)S, C(6)H(4)NH(2), Ph), compared to [-SC(7)H(4)SN](-) of complex 1, trigger thiolate-ligand substitution to yield [(-SC(6)H(4)-o-NHCOCH(3))(2)Fe(NO)(2)](-) (2), [(-SC(4)H(3)S)(2)Fe(NO)(2)](-) (3), and [(SPh)(2)Fe(NO)(2)](-) (4), respectively. At 298 K, complexes 2 and 3 exhibit a well-resolved five-line EPR signal at g = 2.038 and 2.027, respectively, the characteristic g value of DNICs. The magnetic susceptibility fit indicates that the resonance hybrid of {Fe(+)((*)NO)(2)}(9) and {Fe(-)((+)NO)(2)}(9) in 2 is dynamic by temperature. The IR nu(NO) stretching frequencies (ranging from (1766, 1716) to (1737, 1693) cm(-)(1) (THF)) of complexes 1-4 signal the entire window of possible electronic configurations for such stable and isolable {Fe(NO)(2)}(9) [(RS)(2)Fe(NO)(2)](-). The NO-releasing ability of {Fe(NO)(2)}(9) [(RS)(2)Fe(NO)(2)](-) is finely tuned by the coordinated thiolate ligands. The less electron-donating thiolate ligands coordinated to {Fe(NO)(2)}(9) motif act as better NO-donor DNICs in the presence of NO-trapping agent [Fe(S,S-C(6)H(4))(2)](2)(2-). Interconversion between {Fe(NO)(2)}(9) [(RS)(2)Fe(NO)(2)](-) and {Fe(NO)(2)}(10) [(Ph(3)P)(2)Fe(NO)(2)] was verified in the reaction of (a) [(RS)(2)Fe(NO)(2)](-), 10 equiv of PPh(3) and sodium-biphenyl, and (b) 2 equiv of thiol, [RS](-), and [(Ph(3)P)(2)Fe(NO)(2)], respectively. The biomimetic reaction cycle, transformation between {Fe(NO)(2)}(9) [(RS)(2)Fe(NO)(2)](-) and {Fe(NO)(2)}(9) [(R'S)(2)Fe(NO)(2)](-), reversible interconversion of {Fe(NO)(2)}(9) and {Fe(NO)(2)}(10) DNICs, and degradation/reassembly of [2Fe-2S] clusters may decipher and predict the biological cycle of interconversion of {Fe(NO)(2)}(9) DNICs, {Fe(NO)(2)}(10) DNICs, and the [Fe-S] clusters in proteins.     

Conversion of {Fe(NO)2}10 dinitrosyl iron to nitrato iron(III) species by molecular oxygen

A new {Fe(NO)(2)}(10) dinitrosyl iron complex possessing a 2,9-dimethyl-1,10-phenanthroline ligand has been prepared. This complex exhibits dioxygenase activity, converting NO to nitrate (NO(3)(-)) anions. During the oxygenation reaction, formation of reactive nitrating species is implicated, as shown in the effective o-nitration with a phenolic substrate.     

Monoanionic {Mn(NO)}5 and dianionic {Mn(NO)}6 thiolatonitrosylmanganese complexes: [(NO)Mn(L)2]- and [(NO)Mn(L)2]2- (LH2 = 1,2-benzenedithiol and toluene-3,4-dithiol)

The reaction of MnBr(2) and [PPN](2)[S,S-C(6)H(3)-R] (1:2 molar ratio) in THF yielded [(THF)Mn(S,S-C(6)H(3)-R)(2)](-) [R = H (1a), Me (1b); THF = tetrahydrofuran]. Formation of the dimeric [Mn(S,S-C(6)H(3)-R)(2)](2)(2-) [R = H (2a), Me (2b)] was presumed to compensate for the electron-deficient Mn(III) core via two thiolate bridges upon dissolution of complexes 1a and 1b in CH(2)Cl(2). Complex 2a displays antiferromagnetic coupling interaction between two Mn(III) centers (J = -52 cm(-1)), with the effective magnetic moment (mu(eff)) increasing from 0.85 mu(B) at 2.0 K to 4.86 mu(B) at 300 K. The dianionic manganese(II) thiolate complexes [Mn(S,S-C(6)H(3)-R)(2)](2-) [R = H (3a), Me (3b)] were isolated upon the addition of [BH(4)](-) into complexes 1a and 1b or complexes 2a and 2b, respectively. The anionic mononuclear {Mn(NO)}(5) thiolatonitrosylmanganese complexes [(NO)Mn(S,S-C(6)H(3)-R)(2)](-) [R = H (4a), Me (4b)] were obtained from the reaction of NO(g) with the anionic complexes 1a and 1b, respectively, and the subsequent reduction of complexes 4a and 4b yielded the mononuclear {Mn(NO)}(6) [(NO)Mn(S,S-C(6)H(3)-R)(2)](2-) [R = H (5a), Me (5b)]. X-ray structural data, magnetic susceptibility measurement, and magnetic fitting results imply that the electronic structure of complex 4a is best described as a resonance hybrid of [(L)(L)Mn(III)(NO(*))](-) and [(L)(L(*))Mn(III)(NO(-))](-) (L = 1,2-benzenedithiolate) electronic arrangements in a square-pyramidal ligand field. The lower IR v(NO) stretching frequency of complex 5a, compared to that of complex 4a (shifting from 1729 cm(-1) in 4a to 1651 cm(-1) in 5a), supports that one-electron reduction occurs in the {(L)(L(*))Mn(III)} core upon reduction of complex 4a.     

Neutral {Fe(NO)2}9 dinitrosyliron complex (DNIC) [(SC6H4-o-NHCOPh)(Im)Fe(NO)2] (Im = imidazole): interconversion among the anionic/neutral {Fe(NO)2}9 DNICs and Roussin's red ester

A neutral {Fe(NO)2}9 dinitrosyliron complex (DNIC) [(SC6H4-o-NHCOPh)(Im)Fe(NO)2] (Im = imidazole; 2) was prepared by the reaction of [Fe(mu-SC6H4-o-NHCOPh)(NO)2]2 (1) and 2 equiv of imidazole. In the synthesis of the anionic {Fe(NO)2}9 DNIC [(SC6H4-o-NHCOPh)2Fe(NO)2]- (3), thiolate [SC6H4-o-NHCOPh]- triggers ligand substitution of DNIC 2 to yield DNIC 3. At 298 K, complexes 2 and 3 exhibit well-resolved nine- and five-line electron paramagnetic resonance (EPR) signals at g = 2.031 and 2.029, respectively, the characteristic g value of DNICs. The facile interconversions among the neutral {Fe(NO)2}9 complex 2, the anionic {Fe(NO)2}9 complex 3, and Roussin's red ester 1 were demonstrated. The EPR spectrum (the pattern of hyperfine splitting) in combination with the IR nu(NO) (the relative position of the nu(NO) stretching frequencies) spectrum may serve as an efficient tool for the discrimination of the anionic {Fe(NO)2}9 DNICs, the neutral {Fe(NO)2}9 DNICs, and Roussin's red ester.     

Insight into the dinuclear {Fe(NO)2}10{Fe(NO)2}10 and mononuclear {Fe(NO)2}10 dinitrosyliron complexes

The reversible redox transformations [(NO)(2)Fe(S(t)Bu)(2)](-) ? [Fe(μ-S(t)Bu)(NO)(2)](2)(2-) ? [Fe(μ-S(t)Bu)(NO)(2)](2)(-) ? [Fe(μ-S(t)Bu)(NO)(2)](2) and [cation][(NO)(2)Fe(SEt)(2)] ? [cation](2)[(NO)(2)Fe(SEt)(2)] (cation = K(+)-18-crown-6 ether) are demonstrated. The countercation of the {Fe(NO)(2)}(9) dinitrosyliron complexes (DNICs) functions to control the formation of the {Fe(NO)(2)}(10){Fe(NO)(2)}(10) dianionic reduced Roussin's red ester (RRE) [PPN](2)[Fe(μ-SR)(NO)(2)](2) or the {Fe(NO)(2)}(10) dianionic reduced monomeric DNIC [K(+)-18-crown-6 ether](2)[(NO)(2)Fe(SR)(2)] upon reduction of the {Fe(NO)(2)}(9) DNICs [cation][(NO)(2)Fe(SR)(2)] (cation = PPN(+), K(+)-18-crown-6 ether; R = alkyl). The binding preference of ligands [OPh](-)/[SR](-) toward the {Fe(NO)(2)}(10){Fe(NO)(2)}(10) motif of dianionic reduced RRE follows the ligand-displacement series [SR](-) > [OPh](-). Compared to the Fe K-edge preedge energy falling within the range of 7113.6-7113.8 eV for the dinuclear {Fe(NO)(2)}(9){Fe(NO)(2)}(9) DNICs and 7113.4-7113.8 eV for the mononuclear {Fe(NO)(2)}(9) DNICs, the {Fe(NO)(2)}(10) dianionic reduced monomeric DNICs and the {Fe(NO)(2)}(10){Fe(NO)(2)}(10) dianionic reduced RREs containing S/O/N-ligation modes display the characteristic preedge energy 7113.1-7113.3 eV, which may be adopted to probe the formation of the EPR-silent {Fe(NO)(2)}(10)-{Fe(NO)(2)}(10) dianionic reduced RREs and {Fe(NO)(2)}(10) dianionic reduced monomeric DNICs in biology. In addition to the characteristic Fe/S K-edge preedge energy, the IR ν(NO) spectra may also be adopted to characterize and discriminate [(NO)(2)Fe(μ-S(t)Bu)](2) [IR ν(NO) 1809 vw, 1778 s, 1753 s cm(-1) (KBr)], [Fe(μ-S(t)Bu)(NO)(2)](2)(-) [IR ν(NO) 1674 s, 1651 s cm(-1) (KBr)], [Fe(μ-S(t)Bu)(NO)(2)](2)(2-) [IR ν(NO) 1637 m, 1613 s, 1578 s, 1567 s cm(-1) (KBr)], and [K-18-crown-6 ether](2)[(NO)(2)Fe(SEt)(2)] [IR ν(NO) 1604 s, 1560 s cm(-1) (KBr)].     

Synthesis and characterization of {Ni(NO)}10 and {Co(NO)2}10 complexes supported by thiolate ligands

Nitric oxide is an important molecule in biology and modulates a variety of physiological and pathophysiological processes. Some of its regulatory functions are exerted through interactions with redox-active elements, including iron, nickel, cobalt, and sulfur. Metalloenzymes containing [ nFe- nS] ( n = 2 or 4) clusters can be activated or inactivated by reaction with NO, affording dinitrosyl iron complexes. Studies of the NO chemistry of small-molecule iron thiolate complexes have provided insight into these biological processes and suggested probable intermediates. To explore this chemistry from a different perspective, we prepared nickel and cobalt thiolate complexes and investigated their reactions with NO and related compounds. We report here the first examples of anionic complexes containing {Ni(NO)} (10) and {Co(NO) 2} (10) units, the reactivity of which suggests possible intermediates in the interconversion of iron thiolate nitrosyl compounds. Our results demonstrate new chemistry involving NO and simple complexes of nickel and cobalt supported by thiolates, which have been known for more than 30 years. The use of mass balance methodology was key to their discovery. Among the novel complexes reported are (Et 4N) 2[Ni(NO)(SPh) 3] ( 2), from (Et 4N) 2[Ni(SPh) 4] ( 1) and NO, (Et 4N) 2[Ni 2(NO) 2(mu-SPh) 2(SPh) 2] ( 3), from 1 and NO (+) or 2 and Me 3O (+), (Et 4N)[Co(NO) 2(SPh) 2] ( 5), from (Et 4N) 2[Co(SPh) 4] ( 4) and NO, and [Co 3(NO) 6(mu-SPh) 3] ( 6), from 5 and Me 3O (+). In the syntheses of 2 and 5, NO could be replaced by the convenient solid Ph 3CSNO.     

Phenol nitration induced by an {Fe(NO)2}(10) dinitrosyl iron complex

Cellular dinitrosyl iron complexes (DNICs) have long been considered NO carriers. Although other physiological roles of DNICs have been postulated, their chemical functionality outside of NO transfer has not been demonstrated thus far. Here we report the unprecedented dioxygen reactivity of a N-bound {Fe(NO)(2)}(10) DNIC, [Fe(TMEDA)(NO)(2)] (1). In the presence of O(2), 1 becomes a nitrating agent that converts 2,4,-di-tert-butylphenol to 2,4-di-tert-butyl-6-nitrophenol via formation of a putative iron-peroxynitrite [Fe(TMEDA)(NO)(ONOO)] (2) that is stable below -80 °C. Iron K-edge X-ray absorption spectroscopy on 2 supports a five-coordinated metal center with a bound peroxynitrite in a cyclic bidentate fashion. The peroxynitrite ligand of 2 readily decays at increased temperature or under illumination. These results suggest that DNICs could have multiple physiological or deleterious roles, including that of cellular nitrating agents.     

Transformation of the {Fe(NO)2}9 dinitrosyl iron complexes (DNICs) into S-nitrosothiols (RSNOs) triggered by acid-base pairs

S‐Nitrosation of the coordinated thiolate of dinitrosyl iron complexes (DNICs) to generate S‐nitrosothiols (RSNOs) was demonstrated. Transformation of [{(NO)₂Fe(μ‐StBu)}₂] ( 1‐tBuS ) into the {Fe(NO)₂}⁹ DNIC [(NO)₂Fe(StBu)(MeIm)] ( 2‐MeIm ) occurs under addition of 20 equiv of 1‐methylimidazole (MeIm) into a solution of 1‐tBuS in THF. The dynamic interconversion between {Fe(NO)₂}⁹ [(NO)₂Fe(S‐NAP)(dmso)] ( 2‐dmso ) (NAP=N‐acetyl‐D ‐penicillamine) and [{(NO)₂Fe(μ‐S‐NAP)}₂] ( 1‐NAP ) was also observed in a solution of complex 1‐NAP in DMSO. In contrast to the reaction of complex 2‐MeIm and bis(dimethylthiocarbamoyl) disulfide ((DTC)₂) to yield {Fe(NO)}⁷ [(NO)Fe(DTC)₂] ( 3 ) (DTC=S₂CNMe₂) accompanied by (tBuS)₂ and NO_(g), transformation of {Fe(NO)₂}⁹ 2‐MeIm ( 2‐dmso ) into RSNOs (RS=tBuS, NAP‐S) along with complex 3 induced by the Brønsted acid solution of (DTC)₂ demonstrated that Brønsted acid may play a critical role in triggering S‐nitrosation of the coordinated thiolate of DNICs 2‐MeIm (or 2‐dmso ) to produce RSNOs. That is, DNIC‐mediated S‐nitrosation requires a Brønsted acid–Lewis base pair to produce RSNO. Transformation of DNICs into RSNOs may only occur on the one‐thiolate‐containing {Fe(NO)₂}⁹ DNICs, in contrast to protonation of the two‐thiolate‐containing DNICs [(NO)₂Fe(SR)₂]^? by Brønsted acid to yield [{(NO)₂Fe(μ‐SR)}₂]. These results might rationalize that the known protein‐Cys‐SNO sites derived from DNICs were located adjacent to acid and base motifs, and no protein‐bound SNO characterized to date has been directly derived from [protein–(cysteine)₂Fe(NO)₂] in biology.     

New members of a class of dinitrosyliron complexes (DNICs): interconversion and spectroscopic discrimination of the anionic {Fe(NO)2}9 [(NO)2Fe(C3H3N2)2]- and [(NO)2Fe(C3H3N2)(SR)]- (C3H3N2 = deprotonated imidazole; R = tBu, Et, Ph)

The anionic {Fe(NO)2}(9) DNIC[(NO)2Fe(C3H3N2)2](-) (2) (C3H3N2 = deprotonated imidazole) containing the deprotonated imidazole-coordinated ligands and DNICs [(NO)2Fe(C3H3N2)(SR)](-) (R = (t)Bu(3), Et(4), Ph(5)) containing the mixed deprotonated imidazole-thiolate coordinated ligands, respectively, were synthesized by thiol protonation or thiolate(s) ligand-exchange reaction. The anionic {Fe(NO)2}(9) DNICs 2- 5 were characterized by IR, UV-vis, EPR, and single-crystal X-ray diffraction. The facile transformation among the anionic {Fe(NO)2}(9) DNICs 2- 5 and [(NO)2Fe(S(t)Bu)2](-)/[(NO)2Fe(SEt)2](-)/[(NO)2Fe(SPh)2](-) was demonstrated in this systematic study. Of importance, the distinct electron-donating ability of thiolates serve to regulate the stability of the anionic {Fe(NO)2}(9) DNICs and the ligand-substitution reactions of DNICs. At 298 K, DNIC 2 exhibits the nine-line EPR signal with g = 2.027 (aN(NO) = 2.20 and aN(Im-H) = 3.15 G; Im-H = deprotonated imidazole) and DNIC 3 displays the nine-line signals with g = 2.027 (aN(NO) = 2.35 and aN(Im-H) = 4.10 G). Interestingly, the EPR spectrum of complex 4 exhibits a well-resolved 11-line pattern with g = 2.027 (aN(NO) = 2.50, aN(Im-H) = 4.10 G, and aH = 1.55 G) at 298 K. The EPR spectra (the pattern of hyperfine splitting) in combination with IR nu NO spectra (DeltanuNO = the separation of NO stretching frequencies, DeltanuNO = approximately 62 cm (-1) for 2 vs approximately 50 cm(-1) for 3- 5 vs approximately 43 cm(-1) for [(NO)2Fe(S(t)Bu)2](-)/[(NO)2Fe(SEt)2](-)/[(NO)2Fe(SPh)2](-)) may serve as an efficient tool for the discrimination of the existence of the anionic {Fe(NO)2}(9) DNICs containing the different ligations [N,N]/[N,S]/[S,S].     

Crystal structures of nitrato-{4-bromo-2-[(2-hydroxyethylimion)methyl]phenolo}-(3,5-Dibromopyridine)copper and nitrato-{2,4-dibromo-6-[(2-hydroxyethylimino)methyl]phenolo}-(3,5-dibromopyridine)copper

The crystal structures of nitrato-{4-bromo-2-[2-hydroxyethylimino)methyl]phenolo}-(3,5-dibromopyridine)copper (I) and nitrato-{2,4-dibromo-6-[(2-hydroxyethylimino)methyl]phenolo}-(3,5-dibromopyridine)copper (II) are determined. The crystals of compound I are orthorhombic: a = 14.157(3) Å, b = 15.420(3) Å, c = 17.494(4) Å, space group Pbca, Z = 8, R = 0.067. The crystals of compound II are monoclinic: a = 10.675 Å, b = 13.973 Å, c = 14.007 Å, β = 111.92°, space group P2₁/n, Z = 4, R = 0.0464. In the structures of compounds I and II, the copper atom coordinates, correspondingly, singly deprotonated 4-bromo-2-[(2-hydroxyethylimino)methyl]phenol and 2,4-dibromo-6-[(2-hydroxyethylimino)methyl]phenol molecules, and 3,5-dibromopyridine, and the nitrate ion. The coordination polyhedron of the copper ion in complexes I and II is a slightly distorted tetragonal pyramid. The bases of the pyramids are formed by the imine and pyridine nitrogen atoms and the phenolic and alcoholic oxygen atoms, and the axial vertices are occupied by the oxygen atoms of the monodentate nitrato groups. In the complexes under study, the six-membered metallocycles have asymmetric gauche conformation. In crystal, complexes I are united, due to the slip plane a, through bifurcate hydrogen bonds into infinite chains along the direction [100]. Complexes II in crystal form two-dimensional networks by means of hydrogen bonds.     

Condensed tantalaborane clusters: synthesis and structures of [(Cp*Ta)2B5H7{Fe(CO)3}2] and [(Cp*Ta)2B5H9{Fe(CO)3}4

The reaction of [(Cp*Ta)(2)B(4)H(9)(μ-BH(4))] (1; Cp* = η(5)-C(5)Me(5)) with [Fe(2)(CO)(9)] in hexane yielded [(Cp*Ta)(2)B(5)H(7){Fe(CO)(3)}(2)] (2) and [(Cp*Ta)(2)B(5)H(9){Fe(CO)(3)}(4)] (3) in moderate yield. Cluster 2 represents the first example of a bicapped pentagonal-bipyramidal metallaborane with a deformed equatorial plane, and 3 can be described as a fused cluster in which two pentagonal-bipyramidal units are fused through a common 3-vertex triangular face. Compounds 2 and 3 have been characterized by mass spectrometry and IR, (1)H, (11)B, and (13)C NMR spectroscopy, and the geometric structures were unequivocally established by crystallographic analysis.