The electronic spectra of transition metal complexes can arise from two types of transitions: d-d transitions and charge transfer transitions. D-d transitions involve electron movement within the d-orbitals of the metal, while charge transfer transitions move electrons from the metal to the ligand or vice versa. Charge transfer complexes form when partial charge is transferred between molecules or parts of a large molecule, resulting in electrostatic attraction. They have characteristic intense colors due to electronic transitions in the visible light range. Examples of charge transfer complexes include iodine-starch complex and potassium nitrate.

1. The electronic spectra of 1 st
row transition metal complexes can arise from several different
types of transition. Describe these transitions with relevant examples.
Solution
Electronic absorption spectrum is of two types. d-d spectrum and charge transfer spectrum. d-d
spectrum deals with the electronic transitions within the d-orbitals. In the charge –transfer
spectrum, electronictransitions occur from metal to ligand or vice-versa.
A charge-transfer complex ( CT complex ) or electron-donor-acceptor complex is an
association of two or more molecules, or of different parts of one large molecule, in which a
fraction of electronic charge is transferred between the molecular entities. The resulting
electrostatic attraction provides a stabilizing force for the molecular complex. The source
molecule from which the charge is transferred is called the electron donor and the receiving
species is called the electron acceptor.
The nature of the attraction in a charge-transfer complex is not a stable chemical bond, and is
thus much weaker than covalent forces. Many such complexes can undergo an electronic
transition into an excited electronic state. The excitation energy of this transition occurs very
frequently in the visible region of the electromagnetic spectrum, which produces the
characteristic intense color for these complexes. These optical absorption bands are often
referred to as charge-transfer bands (CT bands). Optical spectroscopy is a powerful technique to
characterize charge-transfer bands.
Charge-transfer complexes exist in many types of molecules, inorganic as well as organic, and in
solids, liquids, and solutions. A well-known example is the complex formed by iodine when
combined with starch, which exhibits an intense blue charge-transfer band.eg kno4
d-d transition occurs in d orbitals of the transition metals, but it not occurs in all d orbitals, it
occurs only in asymmetric d orbital that means it occurs only in thode orbitals which have
diffrent energy levels. Initially all d orbitals have same energy but due to vibrations, the
degenracy of d orbitals split out, and some orbital move towards higher energy level, and some
are towards lower energy level.

2. When we provide energy to these orbital then the electrons of lower energy level, get excited and
move towards higher level or vice versa, such type of transitions are called d-d transition. And
it is responsible for color in some transition elements.eg- [Fe(CN)6]3-