Ligand substitution labile terminal ligands

The octahedral early transition-metal-halide clusters [Mo Clg]" and [M6Xi2] + (M = Nb, Ta, Re X = F, Cl, 1 n = 2, 3, 4) contain both strongly anchored inner halide ligands that bridge metals and substitution-labile outer chloride ligands that are terminal in the six axial/equatorial positions and give rise to a rich hexasubstitution chemistry. 

In place of the commonly used metal halides as the metal precursors, transition metal complexes with labile leaving groups have also been used for the preparations of metal alkynyls (Scheme 10.8). The presence of good leaving groups, for example, N2, MeCN or H2O ligands, would provide potential sites for ligand substitutions. These kinds of reactions are usually carried out in the presence of sodium hydroxide or trialkylamine which act as a base to deprotonate the terminal alkynes. 

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