MPSC 2026 Prelims Paper-I · Question 70 of 100

This question was CANCELLED by the Maharashtra Public Service Commission. It carries no correct answer in the final key and no marks are credited for it, whatever a candidate marked in the examination hall — so nothing turns on how you would have answered it. It is one of six questions struck out of this paper; the others are Q14, Q22, Q25, Q50 and Q51. Because there is no keyed answer, this card deliberately does not argue for one, and no option below is described as right or wrong. What can be said from the printed page is where the question is loose. The stem asks which single element forms coordination compounds with simple ligands such as halides, and offers three named elements plus an escape option. Coordination behaviour with halide ligands, however, is not confined to one of those three: it is the defining habit of the transition metals, and it is also shown by beryllium, whose tetrafluoridoberyllate ion, [BeF4]2−, is part of the standard school treatment of beryllium's anomalous behaviour in the s-block. A stem written in the singular — 'which of the following element forms' — over an option list where more than one named element can be shown to satisfy the description does not isolate a single answer, and the escape option 'None of the above' compounds the difficulty rather than resolving it. That is the visible defect; the Commission's own reasons for cancelling are not published, and are not guessed at here. Two smaller points of printing are worth noting for accuracy: the English stem reads 'which of the following element forms', a number disagreement, and 'Co-ordination' is printed with a hyphen. Neither affects the sense. The topic itself is entirely real and repeatedly examined, so the background layer below is worth reading even though the question is void.

A coordination compound is one in which a central metal atom or ion is surrounded by, and bonded to, a definite number of ions or neutral molecules called ligands. The bond is a coordinate or dative bond: the ligand supplies both electrons of the shared pair from a lone pair it already possesses, and the metal supplies an empty orbital to receive them. The number of donor atoms attached to the central metal is its coordination number. Ligands are classified by how many donor atoms each carries — monodentate ligands such as the halide ions, water, ammonia and cyanide bind through one atom; bidentate ligands such as ethylenediamine and the oxalate ion bind through two; polydentate ligands such as EDTA wrap around the metal through several, forming a ring called a chelate. The whole coordination entity is written inside square brackets, as in [Co(NH3)6]Cl3, and Werner's classical theory explained such formulae by giving a metal two kinds of valence: a primary valence, ionisable and satisfied by negative ions, and a secondary valence, non-ionisable, satisfied by ligands and fixed in number for a given metal — what is now called the coordination number.

Coordination chemistry is dominated by the transition metals, and for reasons that are structural rather than accidental: their ions are small and highly charged, so they pull ligands in strongly, and they possess vacant d orbitals of suitable energy to accept the donated lone pairs. That combination gives the familiar examples — the hexamminecobalt(III) ion, potassium hexacyanoferrate(II), tetracarbonylnickel, and the deep blue tetraamminecopper(II) ion that appears when ammonia is added to a copper sulphate solution. But coordination is not the exclusive property of the d block. Among the s-block elements beryllium is the standout case, because its ion is exceptionally small and its charge density exceptionally high; the standard account of its anomalous behaviour includes complexes such as [BeF4]2−, a halide complex by any definition. Magnesium sits at the heart of chlorophyll, held in a porphyrin ring, and cobalt at the heart of vitamin B12 — two coordination compounds on which life itself depends, though neither involves a halide ligand.

• A coordination compound has a central metal atom or ion bonded to a fixed number of ligands through coordinate bonds, in which the ligand donates a lone pair and the metal provides a vacant orbital. The coordination entity is written in square brackets.
• Coordination number is the number of ligand donor atoms attached to the central metal. Ligands are monodentate (halide ions, water, ammonia, cyanide), bidentate (ethylenediamine, oxalate) or polydentate/chelating (EDTA).
• Transition metals form coordination compounds readily because their ions are small and highly charged and they have vacant d orbitals able to accept donated electron pairs. Standard examples include [Co(NH3)6]Cl3, K4[Fe(CN)6] and [Ni(CO)4].
• Beryllium, though an s-block element, also forms complexes — [BeF4]2− is cited in the standard treatment of beryllium's anomalous behaviour, and it is a complex with a simple halide ligand.
• Coordination compounds of biological importance: haemoglobin, which carries iron; chlorophyll, which carries magnesium; and vitamin B12, which carries cobalt. EDTA is used to estimate the hardness of water and in the treatment of lead poisoning, and cisplatin is used in cancer chemotherapy.

• Assuming only transition metals form complexes. Beryllium's [BeF4]2− and the aluminium fluoride and hydroxide complexes are standard s- and p-block counter-examples.
• Confusing coordination number with oxidation state. They are independent quantities: in [Co(NH3)6]Cl3 the cobalt has coordination number six and oxidation state +3, and either can be asked for on its own.
• Marking an escape option such as 'None of the above' because the named options seem to overlap. An escape option is correct only when every named option can be positively shown to fail; overlap among the named options is a fault in the question, not evidence for the escape.

Coordination chemistry reaches a general-studies paper mostly through its applications rather than its theory. The commonest shape is the metal-in-a-biomolecule question — which metal is present in haemoglobin, chlorophyll or vitamin B12 — which has appeared across many state and central papers. Next commonest is the use question: what EDTA is used for, why cyanide is used in extracting gold and silver, what cisplatin treats. The properly technical shapes, asking for a coordination number, an oxidation state inside a complex ion, or the classification of a ligand, appear in papers with a heavier chemistry weighting. Preparation that covers the three biological metals, four or five named applications, and the meaning of ligand and coordination number will handle nearly everything this topic is likely to send.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which metal is present at the centre of the chlorophyll molecule in green plants, held within a porphyrin ring?

  • (a)Iron
  • (b)Magnesium
  • (c)Cobalt
  • (d)Copper

  Answer(b) Magnesium — chlorophyll is a coordination compound in which a magnesium ion sits at the centre of a porphyrin ring. Iron occupies the corresponding position in haemoglobin, and cobalt in vitamin B12. All three are coordination compounds essential to life, which is why this trio is asked so often.

• practice — not a real PYQ

  In the coordination compound [Co(NH3)6]Cl3, the coordination number and the oxidation state of cobalt are respectively :

  • (a)3 and +6
  • (b)6 and +3
  • (c)6 and +6
  • (d)3 and +3

  Answer(b) 6 and +3 — six ammonia molecules are bonded to the cobalt, so the coordination number is six; the three chloride ions lie outside the square brackets and are ionisable, so the complex ion carries a charge of +3 and, ammonia being neutral, the cobalt itself is in the +3 oxidation state. Coordination number and oxidation state are independent quantities and are frequently asked together to see whether they have been confused.