# Organometallic Chemistry > Organometallics for JEE Chemistry: metal-carbon bond types, Grignard and organolithium reagents, metal carbonyls, the EAN rule and Ziegler-Natta catalysis. - Canonical URL: https://prepelephant.com/topics/jee/chemistry/organometallic-chemistry - Exam / course: JEE · Subject: Chemistry - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Organometallic Chemistry", PrepElephant, https://prepelephant.com/topics/jee/chemistry/organometallic-chemistry ## Direct answer Grignard reagents owe everything to one polarised bond: carbon carries the negative end of C-Mg, so RMgX behaves as a masked carbanion that must never meet water, an alcohol, an amine or any other active-hydrogen source. That bond defines the field — an organometallic compound contains at least one direct metal-to-carbon bond — and its polarity spans the spectrum from ionic (sodium cyclopentadienide) through sigma-covalent (RLi, RMgX, R2Zn) to pi-complexes where electron density flows the other way, as in Zeise's salt and the sandwich molecule ferrocene. Metal carbonyls close the picture with their synergic sigma donation plus pi back-bonding, obeying the eighteen-electron effective atomic number rule that makes Ni(CO)4, Fe(CO)5 and Cr(CO)6 such tidy examples. ## What you must remember - **Entry ticket:** a direct M-C bond; metal cyanides, carbonates and carbides are excluded by convention, a boundary question in itself. - **Classification trio:** ionic organometallics (Na+ C5H5-, made by deprotonating cyclopentadiene), sigma-bonded covalent (RMgX, RLi, R2Zn), and pi-complexes (Zeise's salt K[PtCl3(C2H4)], the 1827 first organometallic; ferrocene [Fe(C5H5)2], the 1951 sandwich). - **Grignard discipline:** prepare with magnesium turnings in perfectly dry ether; it dies on contact with active hydrogens (RMgX + H2O giving RH + Mg(OH)X), which doubles as a deuterium-labelling trick (RMgX + D2O giving RD). - **Ferrocene's personality:** two aromatic cyclopentadienyl rings sandwiching iron(II), itself undergoing electrophilic substitution like benzene — acetylation with acetic anhydride works smoothly. - **Carbonyl geometry set:** Ni(CO)4 tetrahedral with Ni(0), Fe(CO)5 trigonal bipyramidal with Fe(0), Cr(CO)6 octahedral with Cr(0) — all 18-electron species. - **EAN arithmetic:** effective atomic number = metal atomic number minus oxidation-state electrons plus 2 per carbonyl; each lands on the next noble gas (Ni: 28 - 0 + 8 = 36, krypton). - **Synergic bond:** CO donates its carbon lone pair to the metal, and the metal back-donates d-electrons into CO's empty pi-star orbital — strong bonding with a weakened, red-shifted C-O stretch. - **Industrial cameo:** Ziegler-Natta catalysts (TiCl4 with Al(C2H5)3) polymerise ethene into high-density polythene at mild pressure, earning the 1963 Nobel Prize. ## The EAN count for carbonyls Run the arithmetic on all three flagship carbonyls. Nickel carbonyl: Ni(0) contributes all 28 electrons, and four CO ligands donate 2 each, giving 28 + 8 = 36 — krypton's configuration. Iron pentacarbonyl: 26 + 5 × 2 = 36 again. Chromium hexacarbonyl: 24 + 12 = 36, completing a run of three different metals converging on the same noble-gas count, which is exactly why the rule was worth codifying. When a metal falls short, it binds more ligands or bridges carbonyls; when the oxidation state is not zero, subtract it first — a mistake that ruins the count in mixed complexes. Then pivot to the practical corollary: because CO back-bonding occupies the metal's empty orbitals, these carbonyls are low-spin, diamagnetic, and volatile enough to purify by distillation, nickel's route to ultra-purity Mond process style. ## How the exam frames it No standalone organometallic unit exists in the current JEE Main syllabus — the material is examined obliquely, and that is the honest framing. Grignard chemistry is examinable through the haloalkane-alcohol-ketone conversion questions where a wrong reagent order destroys the reagent; metal carbonyls and EAN surface inside coordination-compounds questions at the JEE Advanced level; ferrocene appears as an aromaticity oddity. The dependable traps: exposing RMgX to a substrate bearing OH or NH2 (write the substrate first, then the Grignard step), forgetting that metal cyanides are not organometallics, and counting ligand electrons as 1 instead of 2 in an EAN tally. ## Frequently asked questions ### Why must Grignard reagents be prepared in scrupulously dry ether? Any active hydrogen — from water, alcohols, amines or acids — protonates the carbanionic carbon instantly, converting the reagent to a plain hydrocarbon and ending its usefulness. ### What is the EAN of iron in Fe(CO)5? Iron(0) contributes 26 electrons and five carbonyls add 10, giving an effective atomic number of 36, the krypton configuration. ### Why is ferrocene described as aromatic? Each cyclopentadienyl ring donates six pi electrons in a planar delocalised loop around the iron, and the molecule undergoes electrophilic aromatic substitution rather than addition. ### What is pi back-bonding in metal carbonyls? The metal donates d-electron density into the empty antibonding pi orbital of carbon monoxide while CO donates its carbon lone pair to the metal — a synergic pair that strengthens the M-C bond and weakens C-O. ### Which was the first organometallic compound recognised? Zeise's salt, potassium trichloro(ethylene)platinate(II), reported in 1827, with its platinum-ethylene pi bond understood only a century later.