This lecture covers the principles of Infrared Spectroscopy (IR) in the context of bonding with a focus on carbonyl compounds, explaining concepts such as bond order, bonding and nonbonding electrons, and the impact of different orbitals on the spectroscopic properties. The lecture also delves into the factors influencing the color of transition metal complexes, including the role of ligands and charge transfer processes. Additionally, it explores magnetic properties in coordination complexes, discussing spin states, magnetic susceptibility, and the effects of different field strengths on the magnetic behavior of materials.