The molybdenum nitride complex [(Ph(3)SiO)(3)Mo[triple bond]N] (10), which can be conveniently prepared in situ from readily available [(Me(3)SiO)(2)((Me(3)Si)(2)N)Mo[triple bond]N] (9) and Ph(3)SiOH, and the pyridine adduct of 10, [(pyridine)(Ph(3)SiO)(3)Mo[triple bond]N] (11), exhibit excellent catalytic activity in alkyne metathesis reactions of all kinds. Adduct 11 is sufficiently stable to be weighed in air and is therefore much easier to use than any of the established, structurally defined alkyne metathesis precatalysts known to date. This new system is compatible with a host of functional groups, including esters, amides, alkenes, carbamates, ethers, silyl ethers, sulfonates, thioethers, THP acetals, glycosides, ketones, enoates, thiophenes, pyridines, thiazoles, and nitro groups, and even accepts protected propargyl alcohols as substrates. It has been used for the preparation of key intermediates for bioactive natural products, such as gallicynoic acid I, homoepilachnene, epothilone A, cruentaren A, and a fully synthetic latrunculin analogue. However, 9 and 11 react with aldehydes and acid chlorides to give the corresponding nitrile derivatives.

Density functional theory calculations were used to study a given complex for the whole series of lanthanide cations: [Ln(C3H5)Cp(OMe)] (1) [Ln = La (Z = 57)-Lu (Z = 71)], the radioactive lanthanide promethium (Z = 61) excepted. Contrary to the common assumptions, the calculations suggested a significant, albeit indirect, contribution of f electrons to bonding. Relativistic effects were considered in the calculations of the bonding energies, as well as in geometry optimizations in both spin-restricted and unrestricted formalisms. The unrestricted orbitals were finally used for the analysis of the charges and the composition of the frontier orbitals. It was confirmed that the ionic character was more pronounced for complexes of the late lanthanides.