Coordination chemistry of perhalogenated cyclopentadienes and alkynes, XXVII. Synthesis of bimetallic perchlorcymantrene derivatives [C5Cl4−x(R)x (MLn)]Mn(CO)3 with Ni, Pd or Pt – Complex fragments as ring substituents. Structures of {C5Br4[Ni(PMe3)2(Cl,Br)]}Mn(CO)3 and {C5Cl2[SMe]2[Ni(PMe3)2Cl]}Mn(CO)3