Chemistry MCQs for NEET — Practice Questions with Answers

The NCERT text states, 'Werner proposed the term secondary valence for the number of groups bound directly to the metal ion; in each of these examples the secondary valences are six.' It also mentions, 'The secondary valences are non ionisable. These are satisfied by neutral molecules or negative ions. The secondary valence is equal to the coordination number and is fixed for a metal.'

The NCERT text provides this specific example: '1 mol $CoCl_3\cdot 5NH_3$ (Purple) gave 2 mol AgCl'. This means 2 moles of chloride ions are ionisable, indicating they are in the primary valence sphere and not directly bonded to the metal. The remaining one chloride ion must be in the secondary valence sphere, directly bonded and non-ionisable.

The NCERT states, 'Binary compounds such as $CrCl_3$, $CoCl_2$ or $PdCl_2$ have primary valence of 3, 2 and 2 respectively.' For $PdCl_2$, the primary valence is given as 2.

The NCERT states, 'The ions/groups bound by the secondary linkages to the metal have characteristic spatial arrangements corresponding to different coordination numbers. In modern formulations, such spatial arrangements are called coordination polyhedra.'

The NCERT states, 'His theory postulated the use of two types of linkages (primary and secondary) by a metal atom/ion in a coordination compound. In the modern language of chemistry these linkages are recognised as the ionisable (ionic) and non-ionisable (covalent) bonds, respectively.'

The NCERT text notes, 'He further postulated that octahedral, tetrahedral and square planar geometrical shapes are more common in coordination compounds of transition metals.'

The NCERT lists the limitations: '(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?' However, the text also mentions 'Using the property of isomerism, Werner predicted the geometrical shapes of a large number of coordination entities', implying his theory could explain isomerism to some extent.

The NCERT states, 'The species within the square bracket are coordination entities or complexes and the ions outside the square bracket are called counter ions.' It also says, 'The primary valences are normally ionisable and are satisfied by negative ions.' The examples with AgCl precipitation clearly demonstrate that ions outside the square bracket (counter ions) are ionisable.

The NCERT states: '1 mol $CoCl_3\cdot 4NH_3$ (Green) gave 1 mol AgCl'. This means only one chloride ion is ionisable (outside the coordination sphere). The remaining two chloride ions and four ammonia molecules must be inside the coordination sphere. Thus, the formula is $[Co(NH_3)_4Cl_2]Cl$.

Werner's theory established the concepts of primary and secondary valences and the idea of characteristic spatial arrangements (coordination polyhedra). Crystal Field Theory (CFT) is mentioned as a later theory ('The Crystal Field Theory (CFT) to coordination compounds is based on the effect of different crystal fields...'), building upon earlier ideas but not part of Werner's original postulates.