Werner was the first to describe the bonding features in coordination compounds. But his theory could not answer basic questions like :
(i) Why only certain elements possess the remarkable property of forming coordination compounds ?
(ii) Why the bonds in coordination compounds have directional properties ?
(iii) Why coordination compounds have characteristic magnetic and optical properties ?
Many approaches have been put forth to explain the nature of bonding in coordination compounds viz. Valence Bond Theory (VBT), Crystal Field Theory (CFT), Ligand Field Theory (LFT) and Molecular Orbital Theory (MOT). We shall focus our attention on elementary treatment of the application of VBT and CFT to coordination compounds.
According to this theory, the metal atom or ion under the influence of ligands can use its (n−1)d, ns np or ns, np, nd orbitals for hybridisation to yield a set of equivalent orbitals of definite geometry such as octahedral, tetrahedral, square planar and so on. These hybridised orbitals are allowed to overlap with ligand orbitals that can donate electron pairs for bonding.
The formula which correctly represents Potassium tetra hydroxo zincate (II) is :
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → K2[Zn(OH)4]
The formula is built from the name. In potassium tetrahydroxozincate(II) (current IUPAC: tetrahydroxidozincate(II)):
“-ate” shows that the coordination entity is an anion, so potassium is the counter-cation.
tetrahydroxo = four \( \ce{OH^{-}} \) ligands; zincate(II) = zinc in the +2 state.
Charge on the entity: +2 + 4(−1) = −2, so \( \ce{[Zn(OH)4]^{2-}} \).
Two \( \ce{K^{+}} \) ions balance this charge: K2[Zn(OH)4].
Why the other options are incorrect
Option (1) K[Zn(OH)4] — Incorrect. One \( \ce{K^{+}} \) would require \( \ce{[Zn(OH)4]^{-}} \), i.e. zinc in the +3 state, which zinc does not show.
Option (2) K3[Zn(OH)4]3 — Incorrect. Three entities of charge −2 (total −6) are not balanced by three \( \ce{K^{+}} \) (+3).
Option (3) K4[Zn(OH)4] — Incorrect. Four \( \ce{K^{+}} \) would need an entity of charge −4, i.e. zinc in the 0 state, not zincate(II).
NCERT (2026-27 edition):
Naming of Mononuclear Coordination Compounds – “… (v) Oxidation state of the metal in cation, anion or neutral coordination entity is indicated by Roman numeral in parenthesis.
(vi) If the complex ion is a cation, the metal is named same as the element. For example, Co in a complex cation is called cobalt and Pt is called platinum. If the complex ion is an anion, the name of the metal ends with the suffix – ate. For example, Co in a complex anion, [Co(SCN)4]2− is called cobaltate.”