The IUPAC definition of a transition element is that it is an element that has an incomplete d-subshell in either the neutral atom or its ion. Thus the group 12 elements are member of the d-block but are not transition elements.
Chemically solft members of the d-block occurs as sulphide minerals and are partially oxidised to obtain the metal, the more electropositive 'hard' metals occurs as oxides and are extracted by reduction.
Opposite to p-block elements, the higher oxidation states are favoured by the heavier elements of d-block Metals on the right of the d-block tend to exist in low oxidation states and form complexes with the ligands. Square-planar complexes are common for the platinum metals and gold in oxidation states that yield `d^8` electronic configuration, which include RH(I),Ir(I),Pd(II),Pt(II) and Au(III).
The most distinctive features/properties of transition metal complex is their wide range of colours.The crystal field theory attributes the colour of the coordination compounds to d-d transition of the electron.It is important to note that (a) in absence of ligand, crystal field spilling does not occur and hence the substances is colourless, (b) the type of ligand also influences the colour of the complexes.
Select the correct statement.

Options

A. `MnO_4^(-) lt Cr_2O_7^(2-) lt VO_2^(+)` respresents the correct increasing order of their oxidising power
B. Complex `[Ti(H_2O)_6]Cl_3` is violet in colour but removal of water from it on heating renders it colourless
C. In the extraction of copper from copper pyrites during smelting , a slag having composition `CaSiO_3` is formed
D. `[PtCl_4]^(2-)` has two unpaired electrons while `[Pt(CN)_4]^(2-)` has no unpaired electron

Answer

B

Explanation

(A)The correct order is `VO_2^(+) lt Cr_2O_7^(2-)ltMnO_4^(-)` and this is attributed to the increasing stability of the lower species to which they are reduced.
(B)According to crystal field theory, the colour of the complex is due to d-d transition of the electrons . In the absence of ligand crystal field splitting does not occur and hence `TiCl_3` becomes colourless.
(C )Copper pyrites is `CuFeS_2` It has iron sulphide as impurity which is removed as slag. `FeSiO_3` is blast furnace during smelting .
`2CuFeS_2 + 2O_2 to Cu_2S+2FeS+SO_2`
`2FeS+3O_2 to 3FeO+2SO_2`
`FeO+SiO_2 to FeSiO_3` (slag)
(D)In both complexes, for the square planar shape (because of `5d^8` configuration ) the hybridisation is `dsp^2`.Hence the unpaired electron in 5d-orbital pair up to make one d-orbital empty for `dsp^2` hybridisation.Thus there is no unpaired electron in `[PtCl_4]^(2-)` and `[Pt(CN)_4]^(2-)` complexes.