What distinguishes the strength and the effect of a Lewis base: insights with a strong chromogenic silicon Lewis acid

Quantifying Lewis basicity (LB) is essential for understanding chemical reactivity. Yet, the relationship between the intrinsic thermodynamic strength of a Lewis base and the effect it induces at a bound Lewis acid remains poorly defined, especially across structurally diverse bases. Here, we introduce a chromogenic silicon-based Lewis acid to disentangle this relationship <i>via</i> optical spectroscopy and computational analysis. Extending our previously developed concept of global and effective Lewis acidity (gLA and eLA), we propose analogous descriptors for Lewis basicity: global Lewis basicity (gLB) and effective Lewis basicity (eLB). Our findings reveal that gLB and eLB are distinct and independently varying properties. However, unlike for Lewis acidity-where the offset of gLA and eLA is governed by deformation energy-the divergence of gLB and eLB is dominated by solvation of the Lewis base. Specifically, solvation energy significantly affects adduct formation thermodynamics (gLB) but has minimal influence on the induced optical response (eLB). Furthermore, the chromogenic probe enables identification of π-type Lewis basicity contributions. The distinction developed in this work refines the conceptual framework of Lewis pair interactions and highlights the importance of solvation and electronic structure when applying LB descriptors in different chemical contexts.