An investigation of the chemistry of the tert-butyl-substituted molybdenocene system, {(Cp-But)(2)Mo} (Cp-But = C5H4But), has demonstrated that the eta (2)-acetonitrile ligand in (Cp-But)(2)Mo(eta (2)-MeCN) may be alkylated by RI (R = Me, Et) at nitrogen to give iminoacyl derivatives, [(Cp-But)(2)Mo(eta (2)-MeC=NR)](+), which is a new type of reactivity for mononuclear acetonitrile complexes. A series of chalcogenolate-hydride complexes (Cp-But)(2)Mo(EPh)H (E = S, Se, Te) have been obtained by reaction of (Cp-But)(2)MoH2 with Ph2E2, and may be alkylated by MeI to give the chalcogenoether adducts [(Cp-But)(2)Mo(PhEMe)H]I. Dynamic NMR studies on [(Cp-But)(2)Mo(PhEMe)H](+) indicate that the barrier to inversion at the chalcogen increases in the sequence S < Se < Te. The oxo complex (Cp-But)(2)MoO reacts with 2 equivalents of Me3SiX (X = Cl, Br, I, O2CMe, O3SMe) to yield (Cp-But)(2)MoX2; for X = CN and NCS, (Cp-But)(2)Mo(OSiMe3)X, the product of reaction with 1 equivalent may be isolated. (Cp-But)(2)Mo(OSiMe3)X (X = CN, NCS) reacts with Me3SiNCS to give the thiocyanate complex (Cp-But)(2)Mo(SCN)X, rather than the isocyanate isomer, (Cp-But)(2)Mo(NCS)X, thereby indicating that the reactions do not involve a simple metathesis of the Si-NCS bond. Other complexes that are reported include: (Cp-But)(2)MoX2 (X = H, Cl, Br, I, Me, CH2Ph, CH2SiMe3), (Cp-But)(2)MoL (L = CO, C2H4, C2H2, MeCN, PMe3), [(Cp-But)(2)Mo(mu -E)](2), (Cp-But)(2)Mo(eta (2)-E-2) and (Cp-But)(2)Mo(EPh)(2) (E = S, Se, Te).
