在过渡金属的羰基配合物中,为什么过渡金属的价态经常为0价,有的时候还会出现负价态?

CO ligand is a strong ligand,particularly it has very strong pi electron withdrawing capability from metal centre (back bonding).To satisfy carbonyl's this property,or to stable the complex,the metal centre generally has low oxidation state,and is electron-rich.
BTW,18 electron rule does not count pi bonding electrons.