This article discusses the second-order nonlinear optical properties of a series of dipolar bis(dibutylaminostyryl) bipyridine M(II) dichloride complexes (M = Co, Ni, Cu, Zn) and octupolar tris(bipyridine) M(II) complexes of D 3 symmetry (M = Fe, Ru, Os, Ni, Cu, Zn). Their quadratic NLO-susceptibilities have been determined by the Harmonic Light Scattering technique at 1.91 μm. This study shows that the optical and NLO properties are strongly influenced by the nature of the metallic centers, and the first hyperpolarizability constants β can be fine tuned by varying the nature of the metal ion. Finally, we describe an efficient strategy for the design of switchable nonlinear optical (NLO) materials. For that purpose, we have used a photochromic metal complexes which gives rise to an efficient ON/OFF reversible switching of the μβ values determined by EFISH at 1.91μm. © 2012 Old City Publishing, Inc.