Transition metals

TenSaru SG

For transition metals when do I need to include water in the complex ion formula?

In 2008 paper 2 question 4, the 1st part of the question says "What is the formula of the cation present in CuCl2 (aq) and cation present in C(when excess NH3 added to CuCl2)." In this case the answer scheme include water as [Cu(H2O)6]2+ and [Cu(NH3)4(H2O)2]2+

But in the next part CuCl2 reacts with conc. HCl to form D(yellow green solution) and also reaction with Cu with conc HCl to form E (colourless solution). Suggest the formula of complex ion present in D and E.

In this case the answer wrote [CuCl4]2- and [CuCl2]-. Why this time no need to include water??

Thanks :)

UltimaOnline SG

For transition metals when do I need to include water in the complex ion formula?

In this case the answer wrote [CuCl4]2- and [CuCl2]-. Why this time no need to include water??

Thanks :)

In the case of the deep blue tetraaminediaquocopper(II) ions, this is a well-known complex ion in the H2 syllabus which you should be familiar with (ie. you have to memorize as such), and its full formula is known to be [Cu(NH3)4(H2O)2]2+ and its geometry is known to be square planar (in regard to the NH3 ligands), not tetrahedral.

(Note that Cambridge will also accept the simpler [Cu(NH3)4]2+ as the formula of the complex ion, but you must still state its geometry as square planar, not tetrahedral).

But in the case of Cl- ligands, which is a less well-known complex ion (as far as the H2 syllabus is concerned), and you cannot assume that there are water ligands present, as is the case for tetraaminediaquocopper(II) ions.

For [CuCl4]2-, there are two possible complex ion geometries, which is tetrahedral and square planar. Cambridge will accept either geometry, for two reasons :

(i) both geometries, as well as in-between (ie. "flattened tetrahedral") geometries do exist for complex ions with 4 ligands. [CuCl4]2- can exist as either geometry, unlike tetraaminediaquocopper(II) ions.

(ii) it is impossible at 'A' levels to determine whether the geometry of a complex ion with 4 ligands is tetrahedral or square planar. To determine this involves complex calculations beyond 'A' level syllabus. Hence at 'A' levels you have no choice but to simply memorize a common list of complex ions, their geometries and their colours, included in your JC notes.