Synthesis, characterisation and some reactions of alkynyl derivatives [Ni(η5-C5H5)(PPh3)C=CX]

Oxalic acid catalyses the hydrolysis of the Ni(II) acetylide [Ni(eta(5)-C5H5)(PPh3)Cequivalent toCCH(OEt)(2)] 1, to the alkynylaldehyde [Ni(eta(5)-C5H5)(PPh3)Cequivalent toCCHO] 2, in high yield. Condensation reactions of 2 with phenylhydrazine and dinitrophenylhydrazine in the presence of acetic acid, and with malononitrile and 3-phenyl-5-isoxazolone (C9H7NO2) in the presence of triethylamine yield [Ni(eta(5)-C5H5)(PPh3)Cequivalent toCX] derivatives where X=CH=NNHC6H5 3, CH=NNHC6H3(NO2)(2)-2,4 4, CH=C(CN)(2) 5, and CH=C9H5NO2 6. The reactivity of [Ni(eta(5)-C5H5)(PPh3)Cequivalent toCX] Complexes towards [Co-2(CO)(8)] is a function of X. Thus 1 and 2, where X=CH(OEt)(2) or CHO, react readily to give the bridging alkyne derivatives [{mu-eta(1):eta(1)-Ni(eta(5)-C5H5)(PPh3)Cequivalent toCCH(OEt)(2)}{Co-2(CO)(6)}] 7, and [{mu-eta(1):eta(1)-Ni(eta(5)-C5H5)(PPh3)-Cequivalent toCCHO} {Co-2(CO)(6)}] 8. but 5, where X is the strongly electron-withdrawing CH=C(CN)(2) group, does not react even after 24 h at room temperature. Furthermore, coordination of the alkyne to a Co-2(CO)(6) fragment appears to inhibit the normal reactions of the group X in 7 and 8. Thus the acetal grouping in 7 does not undergo oxalic acid-catalysed hydrolysis to an aldehyde in 8, and the aldehyde function in 8 does not undergo a Knoevenagel condensation with CH2(CN)(2). The IR spectra of 1, and 3-6 show a single v(Cequivalent toC) band the frequency of which decreases along the series X = CH(OEt)(2)>CH=NNHC6H5>CH=NNHC6H3(NO2)(2)-2,4>CH=C(CN)(2) approximate to CH=C9H5NO2: that of 2 is anomalous in that it can show two v(Cequivalent toC) bands. The UV-visible spectra of 1-6 show a strong charge transfer absorption band which increases in wavelength 1<3<2<4<5<6. These spectroscopic data and the C-13 chemical shifts suggest that the (η(5)-C5H5)(Ph3P)Ni moiety is a donor and, when X is an acceptor, charge separated cumulenic mesomers such as Ni+=C=C=X- contribute to the description of the bonding in 1-6. This is not reflected in the molecular dimensions of 1, 2 and 5 as determined by X-ray diffraction. However, the crystal structure of [{μ-η(1):η(1)-Ni(η(5)-C5H5)(PPh3)C≡CCHO}{Co-2(CO)(6)}], 8, shows that the C2Co2 cluster core is severely distorted because of the strong donor (Ni) and acceptor (CHO) substituents on the acetylenic carbon atoms.