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Inner Transition Elements

f-Block elements are also referred to as “inner transition elements.” These are two series of elements formed by the filling of 4f and 5f-subshells. The elements in which 4f-subshell is filled are called lanthanides, and the elements in which 5f-subshell is filled are called actinides.

Electronic configuration

Ce (Z = 58) to Lu (Z = 71): (6th period)
Electronic configuration: [Xe] 4f1–14 5d0–1 6s2
Th (Z = 90) to Lr (Z = 103): (7th period)
Electronic configuration: [Rn] 5d1–14 6d0–1 7s2

Variable oxidation states of lanthanides and actinides

Lanthanides exhibit (III) oxidation state [some elements show (II) and (IV) also]. Many of the compounds are colored. In the lanthanide elements, there is regular decrease in the radius as the period is traversed. This is known as “lanthanide contraction.” In case of actinides, it is called “actinide contraction.” In these elements, the electrons are added to the anti-penultimate shell. The addition of each electron to the 4f-orbitals results in a concomitant increase in the atomic number. Since the addition of electrons is to the anti-penultimate shell, there is no significant change in the ultimate and penultimate shells. As a result of the increasing nuclear charge, there is a regular decrease in the radius along the period.

IUPAC nomenclature of complexes

The following rules are used for naming all types of complexes:
  1. In case of ionic complexes, cation is named first followed by the anion, irrespective of the fact, whether cation or anion or both are complex. Simple cation and anion are named just like naming a simple salt.
  2. Number of cations and anions are not mentioned while writing its name.
  3. There has to be a gap between the cation’s name and anion’s name. The gap should not exist anywhere else and the name of cation and anion should be written in one continuous text.
  4. Within a complex ion, the ligands are named first in the alphabetical order followed by the name of the metal ion, which is followed by the oxidation state of metal ion in Roman numeral in parentheses, except for zero.
  5. Name of all negative ligands ends with “o,” while the name of all positively charged ligands ends with “ium.” Neutral ligands have no special ending.
  6. If the number of a particular ligand is more than one in the complex ion, the number is indicated by using Greek numbers such as di, tri, tetra, penta, hexa, etc., for the number of ligands being 2, 3, 4, 5, and 6, respectively.
    However, when the name of the ligand includes a number, for example, dipyridyl, ethylenediamine, then bis, tris, tetrakis, etc., are used in place of di, tri, tetra, etc. The ligands for which such prefixes are used, their names are placed in parenthesis.
  7. For deciding the alphabetical order of ligands, the first letter of the ligand’s name is to be considered and prefixes di, tri, tetra, bis, tris, tetrakis, etc., are not considered.
  8. Neutral and positive ion complexes have no special ending, but complex negative ion ending with suffix “ate” attached to English names of the metal, but in some cases “ate” is attached to the Latin names of the metal.
  9. For those complexes containing solvent of crystallization: first write thecation’s name followed by anion’s name (obviously after a gap) followed by a gap and then write the number of solvent molecules in Arabic numeral followed by a hyphen which is followed by solvent’s name.
  10. Naming of the bridging ligands of the bridged polynuclear complexesComplexes having two or more metal atoms are called polynuclear complexes. In these complexes, the bridging group is indicated by separating it from the rest of the complex by hyphen and adding the prefix μ before the name of each different bridging group. Two or more bridging groups of the same type are indicated by di-μ-, tri-μ-, etc. When a bridging ligand is attached to more than two metal atoms or ions, this is indicated by a subscript to μ.
    The complex Description: 52480.png (SO4)2
    is named as bis(ethylenediamine) cobalt(III)-μ-amido-μ-hydroxo-bis (ethylenediamine) cobalt(III) sulphate or μ-amido-tetrakis(ethylenediamine)-μ-hydroxo-dicobalt(III) sulphate
    The complex Description: 52491.png(SO4)2
    is named as tetraaquoiron(III)-di-μ-hydroxo-tetraaquoiron(III) sulphate.

Bonding in complexes

VBT-octahedral complexes
  1. Inner orbital octahedral complexes
    1. Co atom (3d74s24p0)
      Description: 52503.png
    2. Free Co3+ ion (3d64s04p0) in ground state
      Description: 52517.png
    3. Cr3+ ion in [Cr(NH3)6]3+
      Description: 52525.png
    4. [Co(NH3)6]3+
      Description: 52533.png
Here n represents the number of unpaired electrons and “××” represents an electron pair donated by each of free six NH3 ligands. The two electrons of the electron pair have opposite spin. The above complex ion is diamagnetic as all the electrons are paired.
In order to make 3d electrons paired, the two unpaired electrons residing in Description: 50467.png and Description: 50474.png orbitals are forced by the six NH3 ligands to occupy 3dyz and 3dzx orbitals. By doing so, all the 3d electrons become paired and also at the same time, two 3d orbitals, namely Description: 50483.png hybridized together with 4s, 4px, 4py, and 4pz orbitals to give sixd2sp3 hybrid orbitals which, being empty, accepts the six electron pairs donated by six NH3 ligand molecules.
  1. Outer orbital octahedral complexesgd
    Octahedral complexes resulted from sp3d2 hybridization, using outer d- and outer s and p-orbitals are called outer orbital octahedral complexes.
    1. Co atom (3d74s2)
      Description: 52541.png
    2. Co3+ ion in ground state
      Description: 52552.png
    3. Co3+ ion in [CoF6]3– ion in excited state
      Description: 52560.png
    4. [CoF6]3– ion
      Description: 52569.png
In this complex ion, it is Description: 50493.png and Description: 50500.png orbitals that mix with one 4s and three 4p orbitals to give six sp3d2 hybrid orbitals, which being empty, accept the six electron pairs denoted by each of the six F ligands. It is paramagnetic as there are four unpaired electrons.
Description: 50508.png
VBT-tetrahedral complexes (coordination number = 4, sp3 hybridization)
Tetrahedral complexes result from sp3 hybridization. In sp3 hybridization, the s- and three p-orbitals should belong to the same shell. The formation of tetrahedral
complexes by VBT can be explained by considering the complex ion such as Description: 50516.png. This complex ion is paramagnetic corresponding to the presence of five unpaired electrons, and hence the configuration of Mn2+ ion in the free state and in the complex ion remains the same.
  1. Mn (3d54s24p0)
    Description: 52606.png
  2. Free Mn2+ ion (3d54s04p0)
    Description: 52615.png
  3. Mn2+ ion in [MnCl4]2– ion
    Description: 52645.png
  4. [MnCl4]2– ion
    Description: 52629.png
    Description: 50526.png
VBT-square planar complexes (coordination number = 4, dsp2 hybridization)
Square planar complexes result from dsp2 hybridization. In dsp2 hybridization, d-orbital should be Description: 50536.png orbital (belonging to the lower shell) while s and p-orbitals should be from the higher shell. The two p-orbitals should be px and py orbitals. The selection of Description: 50543.pngpx, and py orbitals is based on the fact that all these orbitals lie in the same plane. The formation of square planar complexes by VBT can be explained by considering the complex ion such as [Ni(CN)4]2–. The measurement of magnetic moment value for [Ni(CN)4]2–ion has shown that μ = 0, i.e., the complex ion has no unpaired electron, and hence it is diamagnetic.
  1. Ni atom (3d84s24p0)
    Description: 52671.png
  2. Free Ni2+ ion in ground state (3d84s04p0)
    Description: 52679.png
  3. Ni+2 ion in [Ni(CN)4]2– ion in excited state
    Description: 52687.png
  4. [Ni(CN)4]2– ion
    Description: 52696.png
In order to make all the 3d-electrons paired, one unpaired electron residing in Description: 50551.png orbital is forced by the four CN ligands to occupy Description: 50558.png orbital. Now Description: 50567.png 4s, 4px, 4py, orbitals mix together to form four dsp2 hybrid orbitals which, being empty, accept the four electron pairs donated by the four CN ligand ions.

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