Read the 18-Electron Rule Before Counting EAN

Fe(CO)₅ works because Fe(0) contributes 8 electrons and five CO ligands each donate 2, giving 8+10=18. When a carbonyl can't reach 18 through simple mononuclear counting, it forms M–M bonds instead: two Co(CO)₄ fragments (each short by one electron for 18) dimerise via a Co–Co bond into (CO)₄Co–Co(CO)₄, each metal now reaching 18 by counting the bond partner's contribution. Mn(CO)₅ is similarly a 17-electron radical that dimerises to Mn2(CO)₁₀.

CO, CN⁻ and NO⁺ are π-ACCEPTOR (π-acid) ligands — they have an empty π* orbital that accepts electron density back from filled metal d-orbitals, not the reverse. This back-donation weakens the C≡O bond (increasing C–O bond length in a metal carbonyl versus free CO) while strengthening the M–C bond. More electron density on the metal (a more negative charge/lower oxidation state) means MORE back-donation and correspondingly LOWER C≡O bond order — this is why [Cr(CO)₆]- has a weaker C-O bond than neutral Cr(CO)₆, which in turn is weaker than [Cr(CO)₆]+.

Zeise's salt, K[Pt(η2-C₂H₄)Cl₃], is the classic example of an alkene sigma-and-π-bonded to a metal (the C=C π electrons donate to Pt, while Pt d-electrons back-donate into the alkene's π*). Ferrocene, Fe(η5-C₅H₅)₂, is a sandwich compound. Both are organometallic because they contain at least one direct metal-carbon bond — this is the defining test, which rules out complexes like sodium nitroprusside or cisplatin (no M–C bond) even though they're famous coordination compounds.

(η6-benzene)tricarbonylchromium(0) follows the same alphabetical-ligand-then-metal rule as any coordination compound, just with the hapticity prefix ηⁿ showing how many carbons of the ring bond to the metal. Structurally, Fe(CO)₅ is trigonal bipyramidal and Ni(CO)₄ is tetrahedral, while Fe2(CO)₉ (two Fe bridged by three CO groups plus three terminal CO each) totals 22 σ-bonds and 15 π-bonds once every M–CO and bridging interaction is counted.