When beryllium donates a pair of electrons to oxygen, that's oxidation, and the metal is Be(II). When carbon and oxygen share 6 electrons, 2 from carbon and 4 from oxygen, this is also oxidation. Of carbon. Even though it got more electrons, and even, I heard, has a negative charge. When carbonyl donates a pair of electrons to nickel, that is called adduction (or complexation?), and nickel stays at 0. I heard that oxidation states are a bit of voodoo, and can accept the CO thing. But what is so special about nickel carbonyl? Why is this reaction not considered a redox? As I understand, the only difference between this bond (dative covalent) and the covalent bonds in, say, water is that nickel doesn't share electrons of its own. Is that the critical piece?