The new paramagnetic compounds [M (eta-C5H4R)X2(NR')] (M = Mo, X = Cl, R' = Bu(t), R = H, Me or Pr(i); M = Mo, X = Cl, R = H, R'= Pr(i), Pr(n) or Ph; M = Mo, X = Cl, R = Me, R'= Ph; M = Mo, X = Br, R = Me, R' = Bu(t); M = W, X = Cl, R' = Bu(t), R = H or Me) have been prepared and the crystal structure of [Mo(eta-C5H5)Cl2(NBut)] has been determined. The new diamagnetic molybdenum(IV) compounds of general formula [Mo(eta-C2H4R)Cl(L)(NR')] where L=ethene, propene, styrene, but-2-yne or carbon monoxide have been synthesised. The mercury-bridged compounds [{Mo(eta-C2H4R)(C2H4)(NBut)}2(mu-Hg)] (R = H or Me) have also been obtained, together with the compound (Mo(NBut)Cl2(PMe3)3]. The crystal structures of [Mo(eta-C5H4Me)Cl(C2H4)(NBut)], [{Mo(eta-C5H5)-(C2H4)(NBut)}2(mu-Hg)] and [Mo(NBut)Cl2(PMe3)3] have been determined.