Synthesis of stable hydride isocyanide derivatives Nb(η5-C5H4SiMe3)2(H)(CNR) has been achieved through the formation of coordinatively unsaturated 16-electron species Nb(η5-C5H4SiMe3)2H by thermolytic loss of H2 followed by the coordination of an isocyanide ligand. Low-temperature protonation with a slight excess of CF3COOH leads to the η2-dihydrogen complexes [Nb(η5-C5H4SiMe3)2(η2-H2)(CNR)]+. NMR spectra of these H−H complexes and their monodeuterated H−D isotopomers present a single high-field resonance at room temperature. By lowering the temperature to 178 K, decoalescence of the signal was observed for the H−D complexes but not for the H−H ones. By combining DFT electronic structure calculations with a monodimensional rotational tunneling model, it has been shown that the absence of decoalescence of the H−H signal is due to the existence of a very large exchange coupling. Conversely, for the H−D isotopomer, the difference in zero point energy corresponding to two nonequivalent (H−D and D-H) positions leads to a slight asymmetry which dramatically reduces the exchange coupling, allowing decoalescence to be observed. Therefore, the H−D classical rotation and the quantum exchange processes will not be practically observed for this complex, whereas only the classical process for the H−H species is quenched out on the NMR time scale.

The hydride isocyanide complexes, Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (H)(CNR), R ) xylyl ) 2,6-dimethylphenyl; Cy (cyclohexyl); t Bu; as well as the hydride carbonyl complex Nb(η 5 -C 5 H 4 SiMe 3 ) 2 -(H)(CO) react with several activated alkynes to afford the corresponding alkenyl isocyanide complexes Nb(ηThe reaction of hydride isocyanide complexes with activated alkynes gives cis-insertion resulting in the formation of (E)-alkenyl products. However, the hydride carbonyl derivative undergoes insertion to give, under kinetic control, a mixture of (E)-and (Z)-alkenyl isomers. Finally, the trihydride niobocene complex Nb(η 5 -C 5 H 4 -SiMe 3 ) 2 (H) 3 , reacts with esters RO 2 CCtCCO 2 R′ or MeO 2 CCtCH to give the hydride olefin derivatives, Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (H)[η 2 -RO 2 C(H)dC(H)CO 2 R′], R ) R′ ) Me 13; R ) R′ ) t Bu 14; and Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (H)[η 2 -MeO 2 C(H)dCH 2 ] 15, probably as the result of a stereospecific trans-insertion. The different complexes have been characterized by spectroscopic methods. In addition, the structure of 13 and 14 were determined by single crystal X-ray diffraction.

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The reaction of the hydride niobocene complexes [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 H(L)] [L ) CNBu n (1), CNCy (2), and CNXylyl (3)] with ClPPh 2 yielded the cationic niobocene complexes [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (PHPh 2 )-(L)]Cl [L ) CNBu n (4), CNCy (5), and CNXylyl ( 6)]. Treatment of these complexes with NaOH yielded a new family of phosphidoniobocene derivatives [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (PPh 2 )(L)] [L ) CNBu n (7), CNCy (8), and CNXylyl ( 9)] by elimination of the hydrogen atom directly bonded to the phosphorus. The cationic d 2 species [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (PRPh 2 )(L)]X [R ) Me, X ) I, L ) CNBu n (10), CNCy (11), and CNXylyl (12); R ) CH 2 Ph, X ) Br, CNBu n (13), CNCy ( 14), and CNXylyl (15); R ) CH 2 CH 2 Ph, X ) Br, CNBu n (16), CNCy (17), and CNXylyl (18)] were prepared by the reaction of alkyl halides RX (R ) Me, X ) I; CH 2 Ph, X ) Br; CH 2 CH 2 Ph, X ) Br) with 7, 8, or 9 by electrophilic attack on the phosphorus atom present in the phosphido terminal ligand. In the same way, the reaction of 9 with ICH 2 CH 2 I or iodine yielded the complex [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (P(I)Ph 2 )(CNXylyl)]I 3 (19). The insertion reaction of carbon disulfide into the Nb-P bond of 7, 8, or 9 yielded a new family of diphenylphosphidodithioformato complexes with the niobocene system [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (κ 1 -S-SC(S)(PPh 2 ))(L)] [L ) CNBu n (20), CNCy (21), and CNXylyl ( 22)]. The molecular structures of 11 and 12 were determined by single-crystal X-ray diffraction studies.

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The complex SiMe,),H,] I reacted with CO, to give the formato complex [Nb(q5-C,H,SiMe,),{OC(O) H-O,O'}] 2, which can alternatively be prepared from a two-electron reduction of [Nb(q5-C5H,SiMe,),CI2] 3 in the presence of formic acid. The reaction of 2 with different n-acids or heterocumulene molecules resulted in opening of the bidentate formate ligand giving rise to the monodentate formato-containing complexes [Nb(q5-C5H4SiMe,),{OC(0)H-O}L], L = CS, 4, CO 5 or 2,6-Me,C6H,NC 6. On the other hand, [Nb(q5-C,H,SiMe,),(OC(O) Me-O,O'}] 8 was prepared from either the reaction of [Nb(q5-C,H,SiMe,),CI] 7 with 1 equivalent of TI(0,CMe) or from a twoelectron reduction of 3 in the presence of acetic acid. The complex [Nb(q5-C,H,SiMe,),(MeCOCHCOMe-O,O')] 9 can also be isolated from the reaction of 7 with 1 equivalent of Tl(acac) (acac = acetylacetonate). Complex 8 reacted with oxygen to give [Nb(q5-C,H,SiMe,),O{OC(O) Me-O}] 10, and with n-acids or heterocumulenes to give products with a monodentate OC(0)Me ligand, namely [Nb(q5-C,H,SiMe,),{OC(O)Me-O}L],

The reactions of phosphido complexes [Nb(eta(5)-C(5)H(4)SiMe(3))(2)(L)(PPh(2))] [L = CO (1), CNxylyl (2), CNCy (3)] with alkynes have been carried out. The new diphenylphosphinoalkenyl niobocene complexes [Nb(eta(5)-C(5)H(4)SiMe(3))(2)(eta(1)-C-C(CO(2)CH(3))=C(R)PPh(2))(CO)] [R = H (4), CH(3) (5)] and [Nb(eta(5)-C(5)H(4)SiMe(3))(2)(eta(1)-C-C(CO(2)R)=C(CO(2)R)PPh(2))(CO)] [R = CH(3), (6), R = (t)Bu, (7)] were successfully synthesized by the reaction of with methyl propiolate (HC[triple bond]CCO(2)CH(3)) or methyl 2-butynoate (CH(3)C[triple bond]CCO(2)CH(3)) and dimethyl 2-butynedioate [(CH(3) O(2)C)C[triple bond]C(CO(2)CH(3))] or di(tert-butyl) 2-butynedioate [((t)BuO(2)C)C[triple bond]C(CO(2)(t)Bu)], respectively. However, reaction was not observed with more electron-rich alkynes. Complex reacted with methyl propiolate, methyl 2-butynoate (MeC[triple bond]CCO(2)Me) or di(tert-butyl) 2-butynedioate to give surprising new heteroniobacycle complexes [Nb(eta(5)-C(5)H(4)SiMe(3))(2)(eta(1)-C-C(=NXylyl)C(R(1))=C(R(2))PPh(2)-kappa(1)-P)] [R(1) = H, R(2) = CO(2)Me (8); R(1) = Me, R(2) = CO(2)Me (9); R(1) = CO(2)(t)Bu, R(2) = CO(2)(t)Bu (10)]. Finally, the phosphido complexes and reacted with phenylacetylene (PhC[triple bond]CH) to give new diphenylphosphinoalkenyl niobocene derivatives [Nb(eta(5)-C(5)H(4)SiMe(3))(2)(eta(1)-C-C(C(6)H(5))=C(H)PPh(2))(CNR)] [R = xylyl (11), Cy (12)]. All of these compounds were characterized by NMR spectroscopy and the molecular structure of was determined by single-crystal X-ray diffraction studies. Theoretical studies were also carried out by means of density functional theory (DFT) calculations on the insertions of alkynes into the Nb-P bond in the phosphido niobocenes.

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