The interaction of a Lewis acid with a Lewis base results in the formation of a Lewis acid–base adduct. Understanding Lewis acids and bases is central to conceptualizing chemical interactions and constitutes a major portion of metal–ligand chemistry. Sterically encumbered/constrained Lewis pairs cannot form acid–base adducts, but such ‘Frustrated Lewis Pairs’ (FLPs), with their unquenched electronic demands can be elegantly used to simultaneously react with a third species, resulting in unusual reactivity of small molecules. Such unusual reactions, explored only in the last few years, have found several applications, e.g., heterolytic splitting of H₂, activation of small molecules (CO₂, N₂O, etc.). FLPs have opened new opportunities in synthetic chemistry, covering organic, main group as well as transition metal chemistry. The design strategies adopted for FLP systems and their unique reactivity are discussed here.

Ternary copper(II) complexes of salicylaldehyde-histamine Schiff base (HL) and pyridyl ligands, viz. [Cu(bpy)(L)](ClO₄) (1) and [Cu(dppz)(L)](ClO₄) (2), where bpy is 2,2′-bipyridine (in 1) and dppz is dipyrido[3,2-a:2′,3′-c]phenazine (in 2), were synthesized, characterized and their DNA binding, photo-activated DNA cleavage activity and photocytotoxicity studied. The 1:1 electrolytic one-electron paramagnetic complexes showed a d-d band near 670 nm in aqueous DMF (1:1 v/v). The crystal structure of complex 1 showed the metal in CuN₄O distorted square-pyramidal geometry. Complex 2 intercalatively binds to calf-thymus (ct) DNA with a binding constant (𝐾_b) of ∼10⁵ M⁻¹. It exhibited moderate chemical nuclease activity but excellent DNA photocleavage activity in red light of 647 nm forming $^{\bullet}\text{OH}$ radicals. It showed remarkable photocytotoxicity in human cervical cancer cells (HeLa) giving IC₅₀ of 1.6 𝜇M in visible light (400-700 nm) with low dark toxicity. The photo-induced cell death is via generation of oxidative stress by reactive oxygen species.