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.