Love-Electronics: MAGNETIC & SPECTRAL PROPERTIES OF COORDINATE COMPLEXES

ABSTRACT:

Coordination compounds as the term is usually used in inorganic chemistry, include compounds composed of a metal atom or ion and one or more ligands (atoms ,ions or molecules)that formally donate electrons. The presence of unpaired electrons in molecules give rise to magnetic properties which are useful in determining the electronic structure .The experimental distinction between high spin and low spin complexes is based on their magnetic properties.

Therefore , the study of magnetic and spectral properties of complexes will provide a tool to understand the distribution of electrons in various

constituent atoms or ions..

INTRODUCTION

What Is A Coordination Compound?

A coordination complex is the product of a Lewis acid-base reaction in which neutral molecules or anions (called ligands) bond to a central metal atom (or ion) by coordinate covalent bonds.

Ligands are Lewis bases - they contain at least one pair of electrons to donate to a metal atom/ion. Ligands are also called complexing agents.
Metal atoms/ions are Lewis acids - they can accept pairs of electrons from Lewis bases.
Within a ligand, the atom that is directly bonded to the metal atom/ion is called the donor atom.
A coordinate covalent bond is a covalent bond in which one atom (i.e., the donor atom) supplies both electrons. This type of bonding is different from a normal covalent bond in which each atom supplies one electron.
If the coordination complex carries a net charge, the complex is called a complex ion.
Compounds that contain a coordination complex are called coordination compounds.

Coordination compounds and complexes are distinct chemical species - their properties and behavior are different from the metal atom/ion and ligands from which they are composed.

The coordination sphere of a coordination compound or complex consists of the central metal atom/ion plus its attached ligands. The coordination sphere is usually enclosed in brackets when written in a formula. The coordination number is the number of donor atoms bonded to the central metal atom/ion. Example; [Ag(NH₃)₂]^{+ = 2}

Spectral & magnetic properties of coordination complexes

Most of the transition metals are paramagnetic. The paramagnetic character is due to the presence of unpaired electrons .The unpaired electrons behave as tiny magnet as a result of their spin about their axis. The magnetic fields from paired electrons , which have opposite spins cancel out. the molecules with no unpaired electrons cannot be magnetized by external magnetic field and are called diamagnetic. The molecules with unpaired electrons can be magnetized by the external field and are called paramagnetic .In the paramagnetic substances, an external magnetic field lines up the electron spin parallel to the applied magnetic field and therefore, the paramagnetic substances behaves like a magnet in an externally applied magnetic field. On the other hand, a diamagnetic substances is not drawn in to an applied magnetic field and thus, it can be easily distinguished from a paramagnetic substance.

Thus there are mainly two types of magnetic behaviour shown by substances:

Diamagnetic substances:

the substances having paired electron are repelled by magnetic field.since all elements except hydrogen have filled electron shell

Paramagnetic substances:

The substances which are attracted by the magnetic field are called paramagnetic substances. It is caused by the presence of unpaired electrons. Larger the no. of unpaired electrons in a substance, the grater is the paramagnetic character.

Origin of paramagnetic moments:

Magnetic properties as a substance arises from electrons ns nucleons. The contribution of nucleons to magnetic behaviour can b neglected and mainly electrons determine by the magnetic behaviour.

Electron determine the magnetic properties of susbstance in two ways:

(1).each electron can b treated as a small sphere of negative charge spinning on its axis. The spinning of charge produces magnetic moment.

(2). An electron traveling in a closed path around a nucleus will also produce magnetic moment like an electric current flowing in a circular loop of wire.

The magnetic moment due to spin of electron on its axis is called spin magnetic moment of the electron and the magnetic moment due to the motion of the electron around the nucleus is called orbital magnetic moment. The observed magnetic moment of an atom or ion will b the sum of spin and orbital magnetic moment . The magnetic moment of a substance are generally express in terms of units called bohr magneton

m^e =

2 · h · e · s

4p · m*_e

= ± m_Bohr

Where h is planck constant , and c is velocity of light.

The spin magnetic moment can b calculated from the relation :

     μ     =   √[n(n+2)]        Bohr Magneton

where n is the number of unpaired electrons.

Eg:

	*K₃[Fe(oxalate)₃] 3H₂O*
*metal ion*	*Fe³⁺*
*number of d electrons*	5
*stereochemistry*	*octahedral*
*High Spin/Low Spin*	*High Spin*
*magnetic moment*	*√(35) B.M*

Comparison of calculated spin-only magnetic moments with experimental data for some octahedral complexes

Ion

Config

μ_so / B.M.

μ_obs / B.M.

Ti(III)

d1 (t_2g1)

√3 = 1.73

1.6-1.7

V(III)

d2 (t_2g2)

√8 = 2.83

2.7-2.9

Cr(III)

d3 (t_2g3)

√15 = 3.88

3.7-3.9

Cr(II)

d4 high spin (t_2g3 e_g1)

√24 = 4.90

4.7-4.9

Cr(II)

d4 low spin (t_2g4)

√8 = 2.83

3.2-3.3

Mn(II)/ Fe(III)

d5 high spin (t_2g3 e_g2)

√35 = 5.92

5.6-6.1

Mn(II)/ Fe(III)

d5 low spin (t_2g5)

√3 = 1.73

1.8-2.1

Fe(II)

d6 high spin (t_2g4 e_g2)

√24 = 4.90

5.1-5.7

Co(III)

d6 low spin (t_2g6)

Co(II)

d7 high spin (t_2g5 e_g2)

√15 = 3.88

4.3-5.2

Co(II)

d7 low spin (t_2g6 e_g1)

√3 = 1.73

1.8

Ni(II)

d8 (t_2g6 e_g2)

√8 = 2.83

2.9-3.3

Cu(II)

d9 (t_2g6 e_g3)

√3 = 1.73

1.7-2.2

Experimental values of magnetic moment if some transition metals ions agree very well with spin only values. in many cases experiment value differ from

Spin only values this is because in addition to spin of the electron orbital motion of electron also make contribution to the magnetic moment. In these cases the total magnetic moment which is dependent on both spin and orbital motion.

The magnetic moment observed in first transition series are calculated by spin contribution only that means the contribution to magnetic moments due to orbital motion of electron is very small or the orbital moment is quenched because the electric field of ligand surrounding the metal ion restrict the orbital motion.

In the second and third row transition elements and particularly in the lanthanide series occupy 4f orbital motion is not prevented or quenched. In these elements, the 4f orbital are well shielded from the surrounding by the overlying 5s orbital and 5p subshell. The electric field of ligands surrounding the metal ions does not restrict the orbital motion of electrons. The orbital motion to magnetic moment is not quenched and thus lanthanide ions are due to both electron spin and orbital motion of electron. Three types of interactions are possible spin-spin ,spin-orbital ,orbital-orbital interactions.

Lande splitting factor calculated for unpaired electrons;

the g-factor or Landé-factor; it is given by

1 +

J(J +1) + S(S + 1) - L(L + 1)

2J(J + 1)

Measurement of magnetic moment

In a large class of materials there exists an approximately linear relationship between ${\bfm M}$ and ${\bfm H}$ . If the material is isotropic then

$\begin{displaymath} {\bfm M} = \chi_m {\bfm H}, \end{displaymath}$

where $\chi_m$ is called the magnetic susceptibility. If $\chi_m$ is positive the material is called paramagnetic, and the magnetic field is strengthened by the presence of the material. If $\chi_m$ is negative then the material is diamagnetic and the magnetic field is weakened in the presence of the material. The magnetic susceptibilities of paramagnetic and diamagnetic materials are generally extremely small.

A linear relationship between ${\bfm M}$ and ${\bfm H}$ also implies a linear relationship between ${\bfm B}$ and ${\bfm H}$ . In fact, we can write

$\begin{displaymath} {\bfm B} = \mu {\bfm H}, \end{displaymath}$

where

$\begin{displaymath} \mu = \mu_0(1+ \chi_m) \end{displaymath}$

is termed the magnetic permeability of the material in question. (Likewise, $\mu_0$ is termed the permeability of free space.) It is clear from Table 1 that the permeabilities of common diamagnetic and paramagnetic materials do not differ substantially from that of free space. In fact, to all intents and purposes the magnetic properties of such materials can be safely neglected (i.e., $\mu =\mu_0$ ).

Experimental measurement of magnetic moment

1. Gouy’s method:

Gouy’s balance use 2 measure paramagnetism in this method the finely powdered substance or solution is taken in a pyrex glass tube called gouy tube.the substance is weigh first without megnatic field then with the presence of magnetic field. If the substance is paramagnetic it will weigh more in the presence of magnetic field then in the absence. The increase in weight is a measure of paramagnetism.

Calculation of Magnetic Moment


1	2

In the Laboratory Experiment involving the Gouy method, you are required to make 7 weighings from which the magnetic moment can be determined.

Magnetic properties observed in crystal field theory

• the electron configuration of the metal ion with split d orbitals depends on the strength of the crystal field

• the 4th and 5th electrons will go into the higher energy dx2-y2 and dz2 if the field is weak and the energy gap is small – leading to unpaired electrons and a paramagnetic complex

• the 4th thru 6th electrons will pair the electrons in the dxy, dyz and dxz if the field is strong and the energy gap is large – leading to paired electrons and a diamagnetic complex