Chemistry of d-block elements

Definition of d-block elements

• The elements of periodic table belonging to group 3 to 12 are known as d-Block elements. because in these elements last electron enters in d sub shell or d orbital 
• The d -block elements lies in between s- and p-block elements in the long form of periodic table.

Electronic Configuration

•         Across the 1^st row of the d block (Sc to Zn) each element

–        has 1 more electron and 1 more proton

–        Each “additional” electron enters the 3d sub-shell

–        The core configuration for all the period 4 transition elements is that of Ar

•         1s²2s²2p⁶3s²3p⁶

Energy level of Argon

Energy level of Transition metals

Chromium and Copper

•          Cr and Cu don’t fit the pattern of building up the 3d sub-shell, why?

–        In the ground state electrons are always arranged to give lowest total energy

–        Electrons are negatively charged and repel each other

–        Lower total energy is obtained with e^- singly in orbitals rather than if they are paired in an orbital

–        Energies of 3d and 4s orbitals very close together in Period 4

•         At Cr

–        Orbital energies such that putting one e^- into each 3d and 4s orbital gives lower energy than having 2 e^- in the 4s orbital

•         At Cu

–        Putting 2 e^- into the 4s orbital would give a higher energy than filling the  3d orbitals

What is a transition metal?

›  Transition metals [TM’s] have characteristic properties

›  e.g. coloured compounds, variable oxidation states

›  These are due to presence of an inner incomplete d sub-shell

›  Electrons from both inner d sub-shell and outer s sub-shell can be involved in compound formation

›  Not all d block elements have incomplete d sub-shells

›  e.g. Zn has e.c. of  [Ar]3d¹⁰4s², the Zn²⁺ ion ([Ar] 3d¹⁰) is not a typical TM ion

›  Similarly Sc forms Sc³⁺ which has the stable e.c of Ar. Sc³⁺ has no 3d electrons

›  For this reason, a transition metal is defined as being an element which forms at least one ion with a partially filled sub-shell of d electrons.

›  In period 4 only Ti-Cu are TM’s!

›  Note that when d block elements form ions the s electrons are lost first

How are d - Block Elements &

Transition elements different?

• All d block elements are not transition

elements but all transition elements are dblock

elements

• All d block elements are not transition

elements because d block elements like

Zinc have full d10 configuration in their

ground state as well as in their common

oxidation state.which is not according to

definition of transition elements.

 General Properties of the d-Block Elements and Their Trends
Metallic character: All transition elements are metallic in nature, i.e. they have strong metallic bonds. This is because of presence of unpaired electrons. This gives rise to properties like high density, high enthalpies of atomization, and high melting and boiling points

.Lanthanoid Contraction: The steady decrease in the atomic and ionic radii of the transition metals as the atomic number increases. This is because of filling of 4f orbitals before the 5d orbitals. This contraction is size is quite regular. This is called lanthanoid contraction.

Ionisation enthalpy: There is slight and irregular variation in ionization energies of transition metals due to irregular variation of atomic size. The I.E. of 5d transition series is higher than 3d and 4d transition series because of Lanthanoid Contraction

Oxidation state: Transition metals show variable oxidation states due to tendency of (n-1)d as well as ns electrons to take part in bond formation.

Magnetic properties: Most of transition metals are paramagnetic in nature due to presence of unpaired electrons. It increase s from Sc to Cr and then decreases because number of unpaired and then decrease because number of unpaired electrons increases from Sc to Cr and then decreases.

Catalytic properties: Most of transition metals are used as catalyst because of (i) presence of incomplete or empty d – orbitals, (ii) large surface area, (iii) varuable oxidation state, (iv) ability to form complexes, e.g., Fe, Ni, V2O3, Pt, Mo, Co and used as catalyst

Formation of coloured compounds: They form coloured ions due to presence of incompletely filled d – orbitals and unpaired electrons, they can undergo d – d transition by absorbing colour from visible region and radiating complementary colour.

Color: The complexes of the d-block metal ions are usually colored, except, very often, those of d0 and d10 metal ions. The colors are due to:a) electronic transitions of d-electrons within the d sub-shell. These are known as d→d transitions. d0 and d10 metal ions do not show these transitions.b) electronic transitions from the metal ion to the ligand (M→L transitions) or ligand to the metal ion (L→M transitions), which are known as charge-transfer transitions, and these can occur for d0 to d10 metal ions.c) The ligands themselves may be colored, and this color may contribute to the color of the complex.

Paramagnetism: When there are unpaired electrons in the d sub-shell, these will lead to paramagnetism. Thus, in [Cr(H2O)6]3+ the three d electrons (it is d3) are unpaired. Thus, like the O2 molecule which is paramagnetic, Cr(III) is paramagnetic. A d10 metal ion (e.g. Zn(II)) has a filled d sub-shell, and a d0 metal ion (e.g. Ti(IV)) has no d-electrons, so neither of these can be paramagnetic

.Variable oxidation states: Most d-block metal ions display variable oxidation states. Thus, for example, Mn displays oxidation states from Mn(-III) (in [Mn(CO)(NO)3]) through Mn(0) (in [Mn2(CO)10]) to Mn(VII) (in [MnO4]-).