I still remember the first time I saw Zeise's salt written on a blackboard and wondered how on earth a platinum atom was holding onto an ethylene molecule with nothing but a dashed line for support. That confusion is exactly where most students sit when they open the topic of organometallic compounds for the first time — plenty of new symbols, a Greek letter that nobody explained properly in school, and a rule about eighteen electrons that seems to come out of nowhere. I've taught this chapter to enough IIT-JAM, CSIR-NET, GATE and BITSAT aspirants to know where the confusion usually starts, and I've built this piece to clear it up, step by step, while keeping every fact tied to what actually gets asked in these exams.
This note walks through the foundations of organometallic chemistry — what counts as one, how they're classified, why hapticity matters, and how the 18-electron rule (along with its 16-electron cousin) predicts stability. I've kept the original numbers, structures and reaction details intact throughout, because in this subject, a changed subscript can flip the entire answer.
Table of Contents
- What Exactly Is an Organometallic Compound?
- A Bit of History — Cacodyl, Dicacodyl and Zeise's Salt
- Classifying Organometallic Compounds by Periodic Position
- Where These Compounds Actually Get Used
- Naming Rules and Ligand Arrangement
- Hapticity — the η Notation Explained
- The Eighteen Electron Rule
- Electron Counting — Two Methods, One Answer
- The Sixteen Electron Rule
- Exceptions to the 18/16-Electron Rule
- Worked Electron-Counting Examples
- Objective Questions from the Chapter (with Answers)
- Subjective Questions from the Chapter
- 40+ Extra Practice MCQs for Exam Prep
- Common Mistakes I See Students Make
1. What Exactly Is an Organometallic Compound?
I define it the way the chapter does, because this definition gets tested word for word: a compound that contains at least one direct metal-carbon bond. Simple enough on paper, but the exceptions are where exams like to trip people up. Carbides such as CaC₂ and cyanides such as NaCN technically have carbon attached to a metal-adjacent species, yet they're treated as inorganic, not organometallic — so don't fall for a question that lists CaC₂ as an example.
The carbon-containing groups attached to the metal can be carbonyl, alkyl, alkene, aromatic, cyclic, or heterocyclic — Table 1.1 in the source material summarizes these ligand types. What surprised me when I was learning this was the Wilkinson catalyst case: Rh(PPh₃)₃Cl has zero metal-carbon bonds in its resting state, yet it's still classified as organometallic. Why? Because during the catalytic cycle of alkene hydrogenation, a stage arises where the alkene coordinates directly to rhodium, forming a genuine M–C bond. That transient bond is enough to earn the compound its organometallic label — the classification isn't purely about the ground-state structure.
According to the leading journals in this field, the bonding interaction can be ionic or covalent, localized or delocalized, between at least one carbon atom of an organic group and a main group, transition, lanthanide, or actinide metal. There's a second exception worth memorizing: binary metal carbonyls like Ni(CO)₄ are counted as organometallic even though CO itself is an inorganic ligand. Along similar lines, organic derivatives of certain metalloids — boron, silicon, germanium, arsenic, and tellurium — also fall under this umbrella, even though metalloids aren't true metals in the strictest sense.
Exam tip: questions frequently ask "which of the following is NOT organometallic" using CaC₂, NaCN, or a simple metal alkoxide as the trap answer. Keep the M–C bond criterion as your first filter, then check for the Wilkinson-type and carbonyl-type exceptions before finalizing.
2. A Bit of History — Cacodyl, Dicacodyl and Zeise's Salt
Every established field has an origin story, and this one begins with a genuinely unpleasant compound. The first organometallic compound of the main group elements was cacodyl oxide, or dimethylarsinous anhydride, [(CH₃)₂As]₂O. I mention the smell only because the source material does — it's described as repulsive, and the compound itself is toxic. It was originally made by heating arsenic trioxide with potassium acetate:
As₂O₃ + 4CH₃COOK → [(CH₃)₂As]₂O + 2K₂CO₃ + 2CO₂
A closely related compound, dicacodyl or tetramethyldiarsane [(CH₃)₄As₂], is considered among the earliest organometallic compounds ever isolated. Edward Frankland and Robert Bunsen investigated it, and it too was originally produced from arsenic distilled with potassium acetate. Both cacodyl oxide and dicacodyl share that As–CH₃ backbone, so don't confuse the two structures on a quick glance during an exam — one has a bridging oxygen, the other has a direct As–As bond.
Transition metal organometallic chemistry has its own starting point: Zeise's salt, K[Pt(C₂H₄)Cl₃]. This is the compound I opened this article with — an ethylene molecule bound side-on to platinum, with three chloride ligands completing the square-planar geometry around the metal. It's usually the first example an instructor uses to introduce η²-coordination, and for good reason, since it's the oldest known case of that bonding mode.
One structural feature runs through almost all organometallic compounds and is worth internalizing early: the central metal atom is typically found in an unusually low oxidation state — often –1, 0, +1, and occasionally +2. Conventional coordination complexes you might have studied elsewhere rarely sit this low. Keep that pattern in your head; it becomes relevant again when we get to electron counting.
3. Classifying Organometallic Compounds by Periodic Position
The chapter splits these compounds into three broad categories based on where the metal sits on the periodic table.
Main group organometallic compounds involve s- and p-block metals. Examples given include Na⁺C₅H₅⁻, n-BuLi, B(CH₂CH₃)₃, Li₄(CH₃)₄, R₄Pb (where R = CH₃ or C₂H₅), RMgX (Grignard reagents, R = alkyl or aryl, X = Cl, Br, I), and Al₂(CH₃)₆.
Transition metal organometallic compounds form the bulk of what you'll study in this area — think Cp₂TiCl₂, ferrocene (Cp₂Fe), Ni(CO)₄, and Rh(PPh₃)₃Cl.
Lanthanide/actinide organometallic compounds are comparatively rare, and uranocene — a uranium atom sandwiched between two cyclooctatetraenyl rings — is the textbook example. If you see uranocene in a question, it's almost certainly testing whether you can identify f-block organometallic chemistry, since these compounds don't come up nearly as often as the transition metal examples.
4. Where These Compounds Actually Get Used
This is the section I find genuinely interesting to teach, because it connects abstract bonding theory to things people encounter daily — fuel additives, cancer drugs, malaria treatment.
As reagents: alkali and alkaline earth metal organometallics are workhorses in organic synthesis. n-BuLi, s-BuLi, t-BuLi, Grignard reagents (RMgX), and Gilman's reagent (R₂CuLi) all fall in this bucket.
As catalysts: this is where organometallic chemistry earns its industrial relevance. The chapter's Table 1.1 lists several homogeneous catalysts worth memorizing along with their associated processes:
| Catalyst | Process |
|---|---|
| Rh(PPh₃)₃Cl (Wilkinson catalyst) | Hydrogenation |
| Co₂(CO)₈ | Hydroformylation |
| [Rh(CO)₂I₂]⁻ | Methanol carbonylation |
| [Ir(CO)₂I₂]⁻ | Methanol carbonylation |
| Grubb's 1st generation catalyst (Ru, PCy₃ ligands) | Olefin metathesis |
| Grubb's 2nd generation catalyst (N-heterocyclic carbene) | Olefin metathesis |
Note the distinction between the two Grubbs catalysts — the first generation uses two tricyclohexylphosphine (PCy₃) ligands on ruthenium, while the second generation swaps one PCy₃ for an N-heterocyclic carbene (the Mes-substituted imidazolylidene ligand). This swap is a favorite GATE/CSIR-NET distinction question.
Nature has its own organometallic catalyst too: Vitamin B₁₂, also called coenzyme B₁₂, is the only known organometallic compound occurring naturally. It holds a cobalt atom inside a corrin ring and shows genuine catalytic behavior in biological systems.
As drugs: Cp₂TiCl₂ was the first organometallic compound identified with anticancer activity. Ferroquine (FQ) is another one worth remembering — it's an organometallic antimalarial drug built around a ferrocene unit attached to a quinoline scaffold.
As additives: this is where the practical chemistry gets almost historical. Ferrocene is soluble in liquid fuels, air-stable, non-toxic, and thermally stable, so it gets added to diesel to cut down on carbonaceous soot particulates — a genuine health hazard when left unchecked. Tetraethyllead (TEL) served for decades as an antiknocking agent in gasoline, helping fuel burn slowly and smoothly. The catch is that TEL combustion produces Pb and PbO particles, both seriously harmful to health, which is exactly why methylcyclopentadienylmanganese tricarbonyl (MMT) has largely replaced it — MMT was found to be harmless for automobile engines. Bis(tributyltin) oxide (TBT) rounds out this list as an anti-foulant, used to coat ship hulls and prevent marine organism buildup.
5. Naming Rules and Ligand Arrangement
IUPAC nomenclature for organometallic formulas follows a specific order that examiners love testing indirectly through formula-writing questions. The metal is written first. Formally anionic ligands come next, arranged in alphabetical order. Neutral ligands follow, also alphabetized by their chemical symbol. A number of unsaturated organic ligands — allyl, cyclopentadienyl, ethylene, benzene, and similar species — can bind to a metal through more than one coordination mode, which brings us directly to hapticity.
Three worked IUPAC names from the chapter are worth memorizing verbatim, since they show how the naming convention actually plays out:
- Cp₂Fe → bis(cyclopentadienyl)iron(II)
- (η⁵-Cp)Mn(CO)₃ → tricarbonyl(η⁵-cyclopentadienyl)manganese
- (η⁶-benzene)Cr(CO)₃ → (η⁶-benzene)tricarbonylchromium
6. Hapticity — the η Notation Explained
Hapticity confused me longer than it should have, mostly because nobody explained the Greek root behind it. Once I learned that "hapto" comes from the Greek word haptein, meaning "to fasten," the whole notation clicked — pentahapto simply means "fastened in five places."
Formally: a single organic ligand may interact with a central metal atom using one or more of its atoms simultaneously. The number of ligand atoms directly bonded to the metal is denoted by the prefix η (eta), with a superscript showing that count. Most ligands attach through a single atom and are called monohapto (η¹). But some ligands are flexible about how many atoms they use — cyclopentadienyl (C₅H₅⁻ or Cp) is the classic case, since it can bind through one, three, or five carbon atoms, acting as η¹-, η³-, or η⁵-Cp depending on the complex.
Here's the full hapticity ladder, matched to the ligand types given in the source:
η¹ (monohapto) ligands: simple alkyl/aryl groups (M–R, where R = –CH₃, –CH₂CH₃, –C₆H₅), acyl groups (M–C(=O)–R), carbonyl (M–C≡O), thiocarbonyl (M–C≡S), carbene (M=CR₂), carbyne (M≡C–R), and η¹-allyl (M bound through one terminal carbon of the allyl group).
η² (dihapto) ligands: ethylene bound side-on, propene (η²-C₃H₆), and alkynes coordinated through their π system (R–C≡C–R' bound sideways to M).
η³ (trihapto) ligands: the allyl group in its bridging resonance form (η³-allyl), and the cyclopropenyl cation (η³-C₃H₃⁺).
η⁴ (tetrahapto) ligands: 1,3-butadiene (η⁴-C₄H₆), cyclobutadiene (η⁴-C₄H₄), cyclooctadiene (η⁴-COD), and norbornadiene (η⁴-C₇H₈).
η⁵ (pentahapto) ligands: cyclopentadienyl in its fully bound form (η⁵-Cp) is the headline example here. What I find genuinely useful for exams is that Cp doesn't have to stay pentahapto within the same molecule — a titanocene-type complex can have one Cp bound η⁵ and another bound η¹ simultaneously, and tungsten carbonyl-Cp complexes show η⁵ and η³ coexisting. Don't assume every Cp in a formula shares the same hapticity; check the structure.
η⁶ (hexahapto) ligands: benzene is the standard example, giving rise to compounds like (η⁶-benzene)Cr(CO)₃. Cycloheptatriene (C₇H₈) can also bind η⁶.
η⁷ (heptahapto) ligands: the cycloheptatrienyl cation (η⁷-C₇H₇⁺), often called tropylium when free.
η⁸ (octahapto) ligands: cyclooctatetraene (COT) is flexible enough to bind as η², η⁴, η⁶, or η⁸ depending on the complex — uranocene, mentioned earlier, uses the η⁸ mode on both COT rings.
A ligand capable of switching its hapticity within a molecule, or over the course of a reaction, is called a fluxional ligand. This term shows up regularly in CSIR-NET and GATE questions, so don't skip past it. One more rule that trips people up in formula-based MCQs: if a question doesn't explicitly state the hapticity of a ligand, you're expected to assume its highest possible hapticity by default — Cp defaults to η⁵, allyl defaults to η³.
7. The Eighteen Electron Rule
Main group chemistry runs on the octet rule — eight valence electrons, a filled s and p shell, and you're done. PbEt₄ is the clean example: lead uses all four valence electrons to form four σ bonds, hits eight total valence electrons, and sits stable. PbEt₃, by contrast, only manages six valence electrons, fails to satisfy the octet, and is correspondingly unstable. Transition metals play a different game entirely. Because they have d-orbitals in the mix, the relevant "magic number" isn't eight — it's eighteen.
Historically, this idea started as Sidgwick's effective atomic number (EAN) rule: the effective atomic number of a metal equals the sum of the metal's own electrons plus the electrons donated by all attached ligands, and stability is predicted when this sum matches the atomic number of the nearest noble gas — 36 (Kr), 54 (Xe), or 86 (Rn).
The more general and more commonly used version today is simply the 18-electron rule: in an organometallic compound (or any coordination complex), the sum of the valence shell electrons on the transition metal or metal ion plus the electrons donated by the ligands should equal 18 for a stable, closed-shell configuration — (n–1)d¹⁰ns²np⁶. Here, both the (n–1)d electrons and the ns electrons of the transition metal count as valence electrons.
Why do people prefer the 18-electron rule over EAN? Mostly convenience — you don't need to memorize noble gas atomic numbers, and it maps more intuitively onto the nine bonding molecular orbitals available in an octahedral transition metal complex (one s, three p, five d — nine total, each holding two electrons, giving eighteen). In most transition metal complexes, electrons preferentially fill these nine bonding MOs while the antibonding ones stay empty, which is precisely why eighteen electrons corresponds to a filled bonding-orbital manifold and enhanced stability.
First-row transition metal carbonyls mostly obey this rule cleanly. Each metal contributes electrons equal to its group number, and each CO ligand donates two electrons regardless of how much π-back bonding is happening — back bonding doesn't change the formal electron count assigned to the metal.
The interesting cases show up with odd-electron metals, where you can never reach an even number like 18 just by adding two-electron CO ligands. Different metals solve this differently, and I think walking through all four resolution strategies side by side makes the pattern click faster than reading them in isolation:
- V(CO)₆ has only 17 valence electrons. Vanadium is too small to accommodate a seventh CO ligand without severe steric clash, and it's also too small to form a stable V–V bonded dimer. Instead, the system resolves the odd-electron problem through simple one-electron reduction: V(CO)₆ is readily reduced to the 18-electron anion [V(CO)₆]⁻.
- Mn(CO)₅, also a 17-electron fragment, takes the opposite route — it dimerizes to Mn₂(CO)₁₀, forming a genuine Mn–M bond. The 5-coordinate manganese fragment has enough open space to accommodate that metal-metal bond, unlike vanadium. This is analogous to how the 7-electron methyl radical dimerizes into 8-electron ethane, C₂H₆ — pairing up the unpaired electron from each fragment.
- Co(CO)₄, another 17-electron species, dimerizes too, but with a twist — a pair of CO ligands bridge the two metal centers rather than staying purely terminal. This doesn't change the overall electron count, because a bridging CO still contributes one electron to each metal, so the M–M bond is still required to reach 18.
- Even-electron metals such as Cr, Fe, and Ni don't need any M–M bonding at all — they simply bind the right number of terminal CO ligands (6, 5, and 4 respectively) to land directly on 18 electrons.
The coordination numbers that emerge from this — 6, 5, and 4 — correspond to octahedral, trigonal bipyramidal/square pyramidal, and tetrahedral/square planar geometries respectively. It's worth pausing on this, because a question that gives you a coordination number and asks for the likely geometry is really just asking you to reverse-engineer this exact logic.
Exam tip: mononuclear complexes with an odd total valence electron count never satisfy either EAN or the 18-electron rule on their own — they always need reduction, oxidation, or dimerization to get there. If a question shows you an odd-electron mononuclear species and asks whether it obeys the 18e rule "as written," the answer is no, full stop.
8. Electron Counting — Two Methods, One Answer
There are two accepted ways to count valence electrons around a metal center, and both are fair game in written-answer questions. They differ only in how they assign electrons — from the metal or from the ligands — but they converge on the exact same total every time, provided you don't mix the two conventions mid-calculation.
Neutral atom (covalent) method: every metal atom and every ligand is treated as electrically neutral. If the overall complex carries a charge, you add electrons for a negative charge or subtract them for a positive charge. Ligands are considered to donate a number of electrons equal to their negative charge as free ions — practically, this means most X-type ligands (H, halogens, alkyl, aryl) donate 1 electron, while neutral L-type ligands (CO, phosphines, amines) donate 2.
Oxidation state (ionic) method: here you first assign a formal oxidation state to the metal by figuring out the charge on every ligand, then count electrons accordingly. Under this method, halides, alkyls, and aryls are treated as anionic (donating 2 electrons each as X⁻), while CO and phosphines remain neutral 2-electron donors either way.
The chapter is explicit that CO and phosphine ligand counts don't change between the two methods — both give 2 electrons regardless of approach. Where the two methods diverge sharply is on ligands like amide (NR₂), alkoxide (OR), and halogen — the neutral atom method assigns these just 1 electron, while the oxidation state method assigns 2 (treating them as NR₂⁻, OR⁻, X⁻).
Practically speaking, I'd recommend the neutral atom method for low-valent transition metal complexes, especially ones with unsaturated ligands, simply because you're not forced to guess an oxidation state that might genuinely be ambiguous. The oxidation state method earns its keep in high-valent complexes involving N, O, or halide ligands, and it's the natural choice whenever a reaction changes the metal's oxidation state, since you're already tracking that variable.
Table 1.2 in the source gives the full electron contribution list for common ligands under both methods — I've reproduced the key entries below because they come up constantly in electron-counting problems:
| Ligand | Neutral Atom Method | Oxidation State Method |
|---|---|---|
| Carbonyl (M–CO) | 2 | 2 |
| Phosphine (M–PR₃) | 2 | 2 |
| Halogen (M–X) | 1 | 2 (X⁻) |
| Hydrogen (M–H) | 1 | 2 (H⁻) |
| Alkyl (M–R) | 1 | 2 (R⁻) |
| Nitrosyl, bent (M–N=O) | 1 | 2 |
| Nitrosyl, linear (M–N≡O) | 3 | 2 |
| Carbene/alkylidene (M=CR₂) | 2 | 4 (CR₂²⁻) |
| Carbyne/alkylidyne (M≡CR) | 3 | 6 (CR³⁻) |
| η¹-allyl | 1 | 2 |
| η³-allyl | 3 | 4 (or 2×C₃H₅⁺) |
| η¹-cyclopentadienyl | 1 | 2 (C₅H₅⁻) |
| η³-cyclopentadienyl | 3 | 4 (C₅H₅⁻) |
| η⁵-cyclopentadienyl | 5 | 6 (C₅H₅⁻) |
| η⁴-1,3-butadiene | 4 | 4 |
| η⁴-cyclobutadiene | 4 | 6 (C₄H₄²⁻) |
| η⁶-benzene | 6 | 6 |
| η⁸-cyclooctatetraenyl | 8 | 10 (C₈H₈²⁻) |
Bridging ligands get their own set of rules, since a bridge donates electrons to two metals at once. A bridging carbonyl donates 2 electrons under both methods, but bridging halogen donates 3 (neutral atom) or 4 (oxidation state), bridging hydrogen donates 1 or 2, and bridging amide or phosphide donate 3 or 4. Finally, M–M bonds themselves contribute electrons in multiples of two: a single M–M bond contributes 2 electrons, a double bond 4, a triple bond 6, and a quadruple bond 8.
9. The Sixteen Electron Rule
Not every stable organometallic complex hits eighteen electrons, and this isn't a failure of the theory — it's a separate, equally valid stability pattern. An important class of complexes follows a 16-electron rule instead, because one of the nine available bonding/nonbonding orbitals sits very high in energy and typically stays vacant.
These 16-electron complexes are especially common for d⁸ transition metals, particularly those in groups 9 and 10, and they're usually square planar in geometry. The chapter's examples include RhCl(CO)(PPh₃)₂, IrCl(CO)(PPh₃)₂, and the anion of Zeise's salt, [PtCl₃(C₂H₄)]⁻. If you total it up — eight metal d-electrons plus two electrons from each of four ligands — you land squarely on sixteen. These square planar 16-electron complexes are typically found among the heavier d⁸ metals: Rh(I), Ir(I), Pd(II), and Pt(II) especially.
Here's a subtlety worth remembering for CSIR-NET level questions: 16-electron complexes aren't inherently less stable than 18-electron ones. They can be just as stable, or occasionally more stable, than an 18-electron complex of the very same metal. Don't treat "16-electron" as automatically meaning "unstable" — that's a common misreading of this rule.
10. Exceptions to the 18/16-Electron Rule
Plenty of stable complexes simply ignore both the 18- and 16-electron rules, and these exceptions cluster around transition metals on the left side of the periodic table. The underlying reason is steric and electronic — early transition metals often can't accommodate the extra ligands or the dimerization needed to climb up to 18 electrons, so the rule gets violated and the complex remains perfectly stable anyway.
Some named examples from the chapter: Ta(CH₂ᵗBu)₃(=CHᵗBu)-type complexes, MeTiCl₃ (an 8-electron species), Me₂NbCl₃ (10 electrons), WMe₆ (12 electrons), Pt(PCy₃)₂ (14 electrons), and (η⁵-Cp)Cr(CO)₂(PPh₃), which sits at 17 electrons. That last one is a genuinely instructive case — it shows the role steric bulk plays directly. When the bulky PPh₃ ligand in this complex gets swapped for a smaller, more compact CO ligand, the resulting species dimerizes both in the solid state and in solution, since the extra space now available allows an M–M bond to form.
The neutral bis(cyclopentadienyl) complexes of most transition metals — CoCp₂ and NiCp₂ among them — don't obey the 16/18-electron rule either. Group 8 metals are the exception within this exception: FeCp₂ (ferrocene) does satisfy the 18-electron count cleanly, and complexes that hit 18 electrons tend to be the most kinetically and thermodynamically robust of the bunch. The others are comparatively less stable, and you can actually verify this experimentally — through bond length measurements and through redox behavior.
A complex sitting at 17 electrons behaves as a strong oxidizing agent and shows paramagnetism, which makes physical sense — it's one electron short of the stable 18-electron configuration and "wants" to grab one more. The reduction of V(CO)₆ illustrates this directly:
V(CO)₆ + Na → Na⁺[V(CO)₆]⁻
Conversely, a complex with 19 or 20 electrons behaves as a strong reducing agent, since it's carrying one or two electrons beyond the stable count and readily gives them up. CoCp₂ (cobaltocene), a 19-electron complex, oxidizes readily to the 18-electron cation CoCp₂⁺:
CoCp₂ → (Cp₂Co)⁺ + e⁻
NiCp₂ (nickelocene) takes this even further as a 20-electron complex, and it oxidizes to the 18-electron dication:
NiCp₂ → [NiCp₂]²⁺ + 2e⁻
This redox pattern — 17e oxidizes readily, 19e/20e reduces or gets oxidized down toward 18e — is a frequently tested conceptual question, often disguised as "which of these metallocenes is the strongest reducing agent."
11. Worked Electron-Counting Examples
The chapter's Table 1.3 works through several full electron counts using both methods side by side, and I think seeing several of them consecutively builds the pattern-recognition you actually need for exam speed.
HMn(CO)₅ — Oxidation state method: Mn⁺ contributes 6e⁻, five CO ligands contribute 10e⁻ (2 each), and the hydride H⁻ contributes 2e⁻, totaling 18e⁻. Neutral atom method: neutral Mn contributes 7e⁻, five CO contribute 10e⁻, and neutral H contributes 1e⁻, again totaling 18e⁻.
(η⁵-C₅H₅)₂Fe (ferrocene) — Oxidation state method: Fe²⁺ gives 6e⁻, and two C₅H₅⁻ rings give 12e⁻ (6 each), totaling 18e⁻. Neutral atom method: neutral Fe gives 8e⁻, and two neutral Cp rings give 10e⁻ (5 each), totaling 18e⁻ again.
(η⁵-C₅H₅)Fe(CO)₂Cl — Oxidation state method: Fe²⁺ (6e⁻) + η⁵-C₅H₅⁻ (6e⁻) + 2CO (4e⁻) + Cl⁻ (2e⁻) = 18e⁻. Neutral atom method: Fe (8e⁻) + η⁵-C₅H₅ (5e⁻) + 2CO (4e⁻) + Cl (1e⁻) = 18e⁻.
These three should convince you that whichever method you use, as long as you apply it consistently within a single calculation, the final electron count will match. Where students lose marks isn't the arithmetic — it's mixing an oxidation-state-style ligand charge with a neutral-atom-style metal count in the same calculation. Pick one method and stay inside it from start to finish.
Two more complex cases from the table, involving mixed hapticities on the same metal, are worth flagging. In the CH₃Mn(CO)₅ case, both methods again converge cleanly to 18e⁻ (Mn⁺ 6e⁻ + CH₃⁻ 2e⁻ + 5CO 10e⁻ under oxidation state; Mn 7e⁻ + CH₃ 1e⁻ + 5CO 10e⁻ under neutral atom). And in mixed molybdenum-nitrosyl-allyl-Cp complexes from the table, you'll see 16-electron totals emerging instead of 18 — a good reminder that not every complex in this chapter is built to hit the "magic" eighteen, and you shouldn't force the arithmetic to reach 18 if the ligand set genuinely gives 16.
12. Objective Questions from the Chapter (with Answers)
These are the exact MCQs from the source material. I've kept them in original order, and the answer key follows immediately after.
- The complex that does not obey the 18-electron rule is:
(a) [(η⁵-C₅H₅)RuCl(CO)(PPh₃)] (b) [W(CO)₃(SiMe₃)(Cl)(NCMe)₂] (c) [IrCl₃(PPh₃)₂(AsPh₂)]⁻ (d) [Os(N)Br₂(PMe₃)(NMe₂)]⁻ - Among the following, the unstable carbonyl species is:
(a) Mn(CO)₅Cl (b) [Mn(CO)₅]⁻ (c) [Mn(CO)₅]⁺ (d) Mn(CO)₅ - In which complex are the formal oxidation state and coordination number of Co equal to –1 and 4 respectively?
(a) Co₂(CO)₈ (b) RCo(CO)₄ (c) Na[Co₄(CO)₁₂] (d) Na[Co(CO)₄] - Identify the 18e species below:
(a) Ni(H₂O)₆²⁺ (b) Fe(CO)₅ (c) (η⁶C₆H₆)Ru (d) V(CO)₆ - The oxidation state of molybdenum in [(η⁷-tropylium)Mo(CO)₃]⁺ is:
(a) +2 (b) +1 (c) 0 (d) –1 - The correct formulation for tetra(cyclopentadienyl)titanium(IV) complex is:
(a) [Ti(η⁵-C₅H₅)₄] (b) [Ti(η¹-C₅H₅)₄] (c) [Ti(η⁵-C₅H₅)(η¹-C₅H₅)₃] (d) [Ti(η⁵-C₅H₅)₂(η¹-C₅H₅)₂] - Among (i) (C₆H₆)₂Cr, (ii) [HMn(CO)₅], (iii) [(CH₃CO)Rh(CO)I₃]⁻, and (iv) CpFe(CO)₂(CH₃), the 18-electron rule is not followed in:
(a) (iii) only (b) (ii) and (iii) (c) (i) and (iv) (d) (ii) only - Oxidation number of Fe in [Fe(NO)(CN)₅]²⁻ is:
(a) 1 (b) 2 (c) 3 (d) 0 - Oxidation number of Co in Co(NO)(CO)₃ is:
(a) 1 (b) 2 (c) 3 (d) –1 - The organometallic compound W(C₅H₅)₂(CO)₂ follows the 18-electron rule. The hapticities of the two cyclopentadienyl groups are:
(a) 5 and 5 (b) 3 and 5 (c) 3 and 3 (d) 1 and 5 - Which molecule does not obey the 18-electron rule?
(a) [Mn(CO)₆]⁺ (b) Fe(CO)₅ (c) [Cr(CO)₅]²⁻ (d) [Mn(CO)₄Cl₂]²⁻ - The hapticity of cyclohepta-1,3,5-triene in (C₇H₈)Fe(CO)₃ is:
(a) 2 (b) 4 (c) 6 (d) 7 - The complex which obeys the 18-electron rule is:
(a) Fe(CO)₄ (b) Ni(CO)₃(PPh₃) (c) Cr(CO)₅ (d) Cr(C₅H₅)₂ - To satisfy the 18-electron rule in [cycloheptatriene·Mo(CO)₃], the hapticity of the coordinated cycloheptatriene ligand must be:
(a) 6 (b) 5 (c) 4 (d) 2 - The oxidation state of Fe in [Cp(Fe(CO)₂)]₂ is:
(a) +2 (b) +1 (c) 0 (d) –1 - The complex [(η³-allyl)Ru(CO)(PPh₃)₂]⁺ follows:
(a) 18e rule and stable (b) 16e⁻ rule and unstable (c) 18e⁻ rule and thermodynamically unstable (d) 16e⁻ rule and stable - The complex that obeys the 18-electron rule is:
(a) [Mn(CO)₅] (b) [(η⁵-C₅H₅)₂Co] (c) [Mo(CO)₃(CH₃CN)₃] (d) [(η⁵-C₅H₅)₂Ti] - Identify the complex which does not obey the 18-electron rule:
(a) [Fe(H₂O)₆]²⁺ (b) [Ru(η⁶-C₆H₆)(η⁶-C₆H₆)] (c) Na[Co(CO)₃(PPh₃)] (d) [Mn(CO)₅]⁻ - The hapticity of nitrosyl in [Mo(η¹-allyl)₃(η³-allyl)₂NO] is:
(a) 1 (b) 2 (c) 3 (d) 0
Answer key: 1-(d), 2-(d), 3-(d), 4-(b), 5-(c), 6-(d), 7-(a), 8-(b), 9-(d), 10-(b), 11-(d), 12-(b), 13-(b), 14-(a), 15-(b), 16-(d), 17-(c), 18-(b), 19-(a)
13. Subjective Questions from the Chapter
These are longer-format questions straight from the source, useful for TGT/PGT written exams and for building the kind of derivation fluency that MCQs alone won't give you.
- Show that the metal centres in the following complexes obey the 18-electron rule:
(i) RhCl(H)₂(η²-C₂H₄)(PPh₃)₂ (ii) (η⁵-C₅H₅)Ir(η²-C₂H₄)(PPh₃) (iii) (η³-C₃H₅)₂Rh(μ-Cl)₂Rh(η³-C₃H₅)₂ (iv) (OC)₅Fe(η²-alkene) (v) [(η⁵-C₅H₅)Rh(CH₃)(PPh₃)₂]⁺ - Compute the coordination number, oxidation number of metal, VEC (valence electron count), and name the following species. Also identify which obey the 18-electron rule:
(a) Cp(CO)Ir(μ-CO)₂Re(CO)Cp (b) [Fe(C₆H₆)(C₅H₅)]⁺ (c) Ti(η⁵-Cp)₂Cl₂ (d) [CpFe(CO)(PPh₃)(CF₂)][BF₄] (e) Cp*₂ZrCl₂ (f) [Cr(NO)(CN)₅]⁴⁻ (g) [Fe(NO)(CN)₅]²⁻ (h) Cr(NO)₄ (i) Mn(NO)₃(CO) (j) [Ir(CO)(PPh₃)Cl(NO)]⁺ (k) Cp₂Co(μ-NO)₂ (l) [Ir(PPh₃)₃(N₂)₂]⁺ (m) Np(Cp)₃Me (n) (η⁴-cot)Fe(CO)₃ (o) [(η³-C₃H₅)₂(μ-Cl)Rh]₂ (Rh–Rh single bond) - Following the 18-electron rule, determine the unknown quantity in the following complexes:
(a) [CpFe(CO)₃]ˣ (b) [CpMn(CO)ₓ]₂ (an Mn=Mn double bond) (c) Naₐ[Fe(CO)ₓ] (d) [CpW(CO)ₓ]₂ (W–W single bond) (e) [CpM(CO)₃]₂ (a single M–M bond, M is a 4d transition metal) - A metal complex has a methyl-substituted Cp and a CO ligand, with formula FeC₉H₇O₃⁺. The complex is highly soluble in polar solvent and obeys the 18-electron rule. Determine the structure of the complex.
- Define organometallic compounds. Which of the following are organometallic compounds?
(i) CH₃MgBr (ii) (C₂H₅)₂Zn (iii) Ti(OEt)₄ (iv) Zeise's salt (v) (η⁶-C₆H₆)₂Cr - (a) What is meant by the term "hapticity"? Explain with suitable examples.
(b) What hapticities are possible for the interaction of each of the following ligands with a single d-block metal atom? (i) C₃H₅ (ii) butadiene (iii) cyclobutadiene (iv) cyclopentadienyl (v) C₆H₆ (vi) cyclooctatetraene - (a) Give the valence electron count in the following: (i) [Mn(π-C₃H₃)(CO)₄] (ii) [HCo(CO)₄] (iii) Fe(π-C₅H₅)₂ (iv) [Mn(CO)₅Cl]
(b) Predict whether the following obey the EAN rule and explain: (i) [Mn(CO)₅(C₂H₄)]⁺ (ii) [Co(π-C₃H₅)(CO)₃] (iii) Cr(CO)₆ (iv) [V(CO)₆]⁻ - (a) Using the 18-electron rule, indicate the probable number of n, m, and the 3d metal (M) in the following: (i) [W(η⁶-C₆H₆)(CO)ₙ] (ii) [(η⁶-C₆H₆)ₙCr(CO)ₘ] (iii) [(η⁵-C₅H₅)M(C₂H₄)₂] (iv) [Rh(η⁵-C₅H₅)(CO)ₙ] (v) Ru₃(CO)ₙ
(b) Write the formulae of three organometallic compounds having multicentred bonding.
14. 40+ Extra Practice MCQs for Exam Prep
I've written these additional questions to stress-test the same concepts — hapticity, the 18/16-electron rule, electron counting methods, and classification — from angles the original chapter doesn't explicitly cover. Answers are given right after each question in bold so you can self-check quickly.
- Which of the following is NOT classified as an organometallic compound? (a) Ferrocene (b) NaCN (c) Grignard reagent (d) Zeise's salt
Answer: (b) - The metal-carbon bond in Wilkinson's catalyst forms during: (a) Its synthesis (b) Its storage (c) The catalytic cycle of hydrogenation (d) It never forms
Answer: (c) - Cacodyl oxide has the formula: (a) (CH₃)₄As₂ (b) [(CH₃)₂As]₂O (c) (CH₃)₃As (d) (CH₃)₂AsCl
Answer: (b) - Zeise's salt is best described as an example of: (a) η¹-alkyl coordination (b) η²-alkene coordination (c) η⁵-Cp coordination (d) η⁶-arene coordination
Answer: (b) - Which pair correctly matches metal oxidation states typical of organometallic compounds? (a) +3, +4 (b) –1, 0, +1 (c) +5, +6 (d) +2, +3 only
Answer: (b) - Uranocene is an example of a: (a) Main group organometallic compound (b) Transition metal organometallic compound (c) Lanthanide/actinide organometallic compound (d) Non-organometallic complex
Answer: (c) - Which catalyst is used specifically for hydroformylation? (a) Rh(PPh₃)₃Cl (b) Co₂(CO)₈ (c) Grubb's 1st generation catalyst (d) [Ir(CO)₂I₂]⁻
Answer: (b) - The key structural difference between Grubbs 1st and 2nd generation catalysts is: (a) Different metal centre (b) Replacement of one PCy₃ with an N-heterocyclic carbene (c) Addition of a Cp ring (d) Loss of the Ru=CHPh unit
Answer: (b) - Vitamin B₁₂ contains which metal at its centre? (a) Iron (b) Cobalt (c) Nickel (d) Zinc
Answer: (b) - Ferroquine is primarily used as a/an: (a) Anticancer drug (b) Antimalarial drug (c) Antibiotic (d) Antifoulant
Answer: (b) - Which compound replaced tetraethyllead as an antiknocking agent? (a) TBT (b) MMT (c) Ferrocene (d) Cacodyl oxide
Answer: (b) - Bis(tributyltin) oxide (TBT) is primarily used as a/an: (a) Fuel additive (b) Anti-foulant coating (c) Anticancer drug (d) Hydrogenation catalyst
Answer: (b) - In IUPAC organometallic nomenclature, which class of ligand is cited first after the metal? (a) Neutral ligands (b) Anionic ligands (c) Solvent molecules (d) Counter-ions
Answer: (b) - The Greek root "hapto" means: (a) To donate (b) To fasten (c) To rotate (d) To bridge
Answer: (b) - A ligand that changes its hapticity during a reaction or within different complexes is called: (a) Ambidentate (b) Fluxional (c) Chelating (d) Bridging
Answer: (b) - If hapticity is not explicitly specified for an allyl ligand, it should be assumed to be: (a) η¹ (b) η² (c) η³ (d) η⁴
Answer: (c) - Which hapticity is NOT possible for cyclopentadienyl coordination? (a) η¹ (b) η² (c) η³ (d) η⁵
Answer: (b) - Benzene coordinated in η⁶ fashion contributes how many electrons under the neutral atom method? (a) 2 (b) 4 (c) 6 (d) 8
Answer: (c) - Cyclooctatetraene (COT) shows a maximum hapticity of: (a) η⁴ (b) η⁶ (c) η⁷ (d) η⁸
Answer: (d) - The cycloheptatrienyl cation typically coordinates with hapticity: (a) η³ (b) η⁵ (c) η⁶ (d) η⁷
Answer: (d) - PbEt₄ is stable because it satisfies the: (a) 18-electron rule (b) Octet rule (c) 16-electron rule (d) EAN rule only
Answer: (b) - According to the EAN rule, a stable metal complex has an effective atomic number equal to that of: (a) The metal's own atomic number (b) The nearest alkali metal (c) The nearest noble gas (d) Zero
Answer: (c) - Which noble gas atomic number is relevant for third-row transition metal EAN calculations? (a) 36 (b) 54 (c) 86 (d) 18
Answer: (c) - V(CO)₆ has how many valence electrons? (a) 16 (b) 17 (c) 18 (d) 19
Answer: (b) - V(CO)₆ achieves an 18-electron configuration by: (a) Dimerizing (b) One-electron reduction to [V(CO)₆]⁻ (c) Losing a CO ligand (d) Oxidation to V²⁺
Answer: (b) - Mn(CO)₅ (17e⁻) achieves stability primarily through: (a) Reduction (b) Oxidation (c) Dimerization via an Mn–Mn bond (d) Ligand substitution
Answer: (c) - Why does V(CO)₆ not dimerize like Mn(CO)₅ does? (a) Vanadium is too small, causing steric hindrance for a 7th site (b) Vanadium has no d-electrons (c) CO cannot bind to vanadium (d) V(CO)₆ is already 18-electron
Answer: (a) - Co(CO)₄, a 17-electron species, dimerizes with: (a) No bridging ligands (b) Bridging CO ligands and an M–M bond (c) Loss of all CO ligands (d) Formation of a Co=Co double bond only
Answer: (b) - Even-electron metals like Cr, Fe, and Ni achieve 18 electrons by: (a) Forming M–M bonds exclusively (b) Binding an appropriate number of terminal CO ligands (c) Dimerizing always (d) Losing electrons
Answer: (b) - Cr(CO)₆, Fe(CO)₅, and Ni(CO)₄ adopt geometries of: (a) Tetrahedral, square planar, octahedral (b) Octahedral, trigonal bipyramidal, tetrahedral (c) Square planar for all three (d) Linear, bent, trigonal
Answer: (b) - The number of bonding molecular orbitals responsible for the 18-electron limit is: (a) 5 (b) 7 (c) 9 (d) 11
Answer: (c) - 16-electron complexes are especially common for metals with which d-electron configuration? (a) d⁴ (b) d⁶ (c) d⁸ (d) d¹⁰
Answer: (c) - Square planar 16-electron complexes are typical for which oxidation states/metals? (a) Rh(I), Ir(I), Pd(II), Pt(II) (b) Fe(0), Ni(0) (c) Sc(III), Ti(IV) (d) Mn(VII)
Answer: (a) - A 16-electron complex compared to an 18-electron complex of the same metal is: (a) Always less stable (b) Always more stable (c) Can be just as stable or more stable (d) Never observed
Answer: (c) - Complexes that violate both the 18- and 16-electron rules are most common for: (a) Late transition metals (b) Early transition metals on the left of the periodic table (c) Lanthanides only (d) Main group metals only
Answer: (b) - WMe₆ has a valence electron count of: (a) 8 (b) 10 (c) 12 (d) 14
Answer: (c) - Which bis(cyclopentadienyl) complex of a group 8 metal DOES satisfy the 18-electron rule? (a) CoCp₂ (b) NiCp₂ (c) FeCp₂ (d) CrCp₂
Answer: (c) - A 17-electron organometallic complex is best described as: (a) A strong reducing agent, diamagnetic (b) A strong oxidizing agent, paramagnetic (c) Inert (d) Always dimeric
Answer: (b) - A 19- or 20-electron complex is best described as: (a) A strong oxidizing agent (b) A strong reducing agent (c) Diamagnetic and inert (d) Always anionic
Answer: (b) - CoCp₂ (cobaltocene) has a valence electron count of: (a) 17 (b) 18 (c) 19 (d) 20
Answer: (c) - Cobaltocene is readily converted to the cobaltocenium cation via: (a) Reduction (b) Oxidation, losing one electron (c) Protonation (d) Ligand substitution
Answer: (b) - NiCp₂ (nickelocene) has a valence electron count of: (a) 18 (b) 19 (c) 20 (d) 16
Answer: (c) - Nickelocene is oxidized to [NiCp₂]²⁺ because this cation is: (a) 16-electron and unstable (b) 18-electron and stable (c) 20-electron still (d) Not isolable
Answer: (b) - In the neutral atom method, a terminal halogen ligand donates: (a) 1 electron (b) 2 electrons (c) 3 electrons (d) 0 electrons
Answer: (a) - In the oxidation state method, a terminal halogen ligand (as X⁻) donates: (a) 1 electron (b) 2 electrons (c) 3 electrons (d) 4 electrons
Answer: (b) - Under the oxidation state method, a bridging halogen donates how many electrons? (a) 1 (b) 2 (c) 3 (d) 4
Answer: (d) - A linear nitrosyl ligand (M–N≡O) donates how many electrons under the neutral atom method? (a) 1 (b) 2 (c) 3 (d) 4
Answer: (c) - A bent nitrosyl ligand donates how many electrons under the neutral atom method? (a) 1 (b) 2 (c) 3 (d) 0
Answer: (a) - Which ligand type shows the SAME electron count under both the neutral atom and oxidation state methods for a neutral donor like CO? (a) Always different (b) Always the same (c) Same only for anionic ligands (d) Same only for bridging ligands
Answer: (b) - A carbyne (alkylidyne, M≡CR) ligand donates how many electrons in the oxidation state method? (a) 2 (b) 3 (c) 4 (d) 6
Answer: (d)
15. Common Mistakes I See Students Make
A few patterns show up again and again when I grade practice tests on this chapter. First, students forget that hapticity isn't fixed for a ligand type — it depends on the specific complex, and a single molecule can have two Cp rings with different hapticities simultaneously. Second, people mix the neutral atom and oxidation state methods mid-problem, usually by assigning an anionic charge to a ligand while also treating the metal as neutral — pick one convention and stick with it end to end. Third, there's a tendency to assume every stable complex must hit exactly 18 electrons; plenty of genuinely stable species sit at 16, and a fair number of early transition metal complexes don't obey either rule at all.
One last thing I'd say to anyone prepping seriously for GATE, CSIR-NET, or IIT-JAM: don't just memorize the answer key. Redo the electron counts for Table 1.3 by hand, using both methods, until the arithmetic becomes automatic. That's genuinely where the marks come from — not the definitions, but the speed and accuracy of counting electrons under exam pressure.
