Application of Coordination Compounds & Organometallic Compound Importance of co-ordination compounds (in qualitative analysis, extraction of metals and in biological systems).: NEET MCQs with Answers

Explanation: The hapticity symbol η followed by a number tells you how many carbon atoms of the ligand are bound simultaneously to the metal, and its presence is the signature of π-bonding. (CH₃)₄Sn has no η notation. Each methyl group is joined to tin by an ordinary two-electron σ-bond through a single carbon, so tetramethyltin is a σ-bonded organometallic compound and the odd one out. The others are all π-bonded: K[PtCl₃(η²—C₂H₄)] is Zeise's salt, with two carbons of ethene bound side-on; Fe(η⁵—C₅H₅)₂ is ferrocene, with all five carbons of each cyclopentadienyl ring bound; and Cr(η⁶—C₆H₆)₂ is dibenzenechromium, with all six carbons of each benzene ring bound.

Explanation: The structure required is the one with two ammine ligands on adjacent corners of the square plane and two chlorido ligands on the other two, that is cis-[Pt(NH₃)₂Cl₂], cisplatin. Inside the cell the two chloride ligands are displaced by water and the platinum then binds to the N7 atoms of two neighbouring guanine bases in DNA. That intrastrand cross-link distorts the double helix and prevents replication, killing the rapidly dividing tumour cell. The trans isomer has its two chlorides 180° apart, cannot bridge two adjacent bases and is inactive. The ethene complex and tetrachloridoplatinate(II) contain no ammine ligands at all and are not anticancer agents.

Explanation: The formula contains only one iron atom, so the carbonyl is mononuclear. It has a trigonal bipyramidal shape with all five CO groups bonded terminally to that single metal centre.

Explanation: In RMgX the alkyl group is joined to magnesium by a straightforward two-electron sigma bond. Ferrocene, cobaltocene and ruthenocene are pi-bonded sandwich compounds in which the cyclopentadienyl rings donate pi electron density.

Explanation: Excess iodide converts mercury(II) chloride into the soluble tetraiodomercurate(II) complex, [HgI₄]²⁻, while iodine itself combines with iodide to give the triiodide ion, I₃⁻.

Explanation: cis-Diamminedichloridoplatinum(II), cisplatin, is a standard anticancer drug. It cross-links guanine bases of DNA and thereby prevents cell division; the trans isomer is ineffective.

Explanation: In every metal carbonyl the ligand attaches through its carbon atom, never through oxygen. The reason is that the highest occupied molecular orbital of CO is concentrated on the carbon, so it is the carbon lone pair that is donated to the metal to form the σ-bond. The π-component of the bonding also runs through carbon: the metal donates electron density back into the π* orbital of CO, which again has its larger coefficient on the carbon atom. So the metal is bonded to carbon alone, and the linkage is written M—C≡O.

Explanation: Metal carbonyls are the standard example of simultaneous σ and π bonding. Each CO ligand donates its carbon lone pair into a vacant metal hybrid orbital to form a σ-bond, and the metal then returns electron density from a filled d-orbital into the vacant π* orbital of CO to form a π-bond. The two interactions reinforce each other and are described as synergic. In [Co(CO)₅NH₃]²⁺ the cobalt is bound to five CO ligands in this σ-plus-π fashion, together with an ammonia ligand that is a pure σ-donor. Ferrocene is purely π-bonded, Zeise's salt is π-bonded through the ethene, and Fe(CH₃)₃ is purely σ-bonded.

Explanation: A positive charge on the complex lowers the electron density available on the metal for back-donation into the π* orbitals of CO. Less back-bonding leaves the C—O bond closest to free CO, so the cationic [Mn(CO)₆]⁺ has the strongest C—O bond.

Explanation: The trans-effect is the ability of a coordinated ligand to labilise the group directly trans to itself, speeding up its substitution in a square-planar complex. Ligands that are strong π-acceptors or strong σ-donors exert the greatest trans-effect. The established series places CN⁻ and CO at the top, followed by strongly σ-donating carbon ligands such as phenyl, then the heavier halides, with N-donors like NH₃ near the bottom: CN⁻ > C₆H₅⁻ > Br⁻ > NH₃ The trans-effect is of real synthetic value — it is what allows cisplatin and transplatin to be prepared selectively from the same starting materials.

Explanation: The C—O bond length in a carbonyl is governed by synergic π-back-bonding. The metal donates electron density from its filled d-orbitals into the vacant π* antibonding orbitals of CO. Filling an antibonding orbital weakens and therefore lengthens the C—O bond. The more negative the charge on the complex, the greater the electron density available on the metal, and so the more extensive the back-bonding. Ranking by charge: [Fe(CO)₄]²⁻ (−2) > [Co(CO)₄]⁻ (−1) > [Ni(CO)₄] (0) > [Mn(CO)₆]⁺ (+1) Hence [Fe(CO)₄]²⁻ has the weakest and longest C—O bond. Note that stronger back-bonding lengthens C—O while simultaneously shortening and strengthening the M—C bond.

Explanation: A Grignard reagent, RMgX (for example CH₃MgBr), contains a direct two-electron covalent σ-bond between the carbon of the alkyl group and the magnesium atom. That single localised M—C bond is exactly what defines a σ-bonded organometallic compound. Ferrocene, cobaltocene and ruthenocene are all metallocenes of the type M(η⁵-C₅H₅)₂. In these the metal sits between two cyclopentadienyl rings and is bound to the delocalised π-electron cloud of each ring rather than to any one carbon atom, so they are π-bonded organometallic compounds.

Explanation: Excess copper and iron are removed by the chelating ligands D-penicillamine and desferrioxime B through the formation of coordination compounds. EDTA is used in a similar way in lead poisoning. cis-platin is a platinum complex that inhibits the growth of tumours, and cyanocobalamine is vitamin B₁₂, so neither is a chelating drug for metal removal.

Explanation: The ammoniacal medium removes one proton from each dimethylglyoxime molecule to give the dimethylglyoximate anion. Two such anions chelate Ni²⁺ through their nitrogen atoms. The charges cancel, so the chelate [Ni(dmg)₂] is neutral and separates as a rosy red precipitate, which makes it useful for detecting nickel.

Explanation: Mn₂(CO)₁₀ is made of two square pyramidal Mn(CO)₅ units joined by a Mn–Mn bond, so all ten CO groups are terminal. Co₂(CO)₈ has a Co–Co bond that is also bridged by two CO groups, leaving six terminal CO groups.