Organometallic Chemistry
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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.