Reactivity of the methylcyclopentadienylmanganese cyanometalate Na[Cp'Mn(CO)2CN] with titanium, zirconium, and hafnium halides. Crystal and molecular structure of the .mu.-cyano .mu.-oxo tetranuclear complex [Cp2Zr{(.mu.-NC)MnCp'(CO)2}]2(.mu.-O) (Cp = .eta.-C5H5; Cp' = .eta.-MeC5H4)