Low-Dimensional Perovskites and Metal Halides across Structures and Functions: Connectivity, Dimensionality, and Emerging Principles.

Hybrid metal halides have experienced rapid growth in research activity, achieving remarkable advances over the past decade and a half. This expansion stems from valuable traits including synthetic and optoelectronic tunability, high absorption coefficients, solution processability, and high defect tolerance. Studies on these materials have expanded from 3D perovskites to include their low- dimensional 2D counterparts, whose performance and improved stability have made them model systems for various applications, yet this progress has only begun to scratch the surface of structural possibilities. Other low-dimensional metal halides, including perovskitoids and nonperovskite structures, remain far less explored despite offering immense structural diversity and functional potential. In this Review, we provide an updated, integrative perspective on these overlooked systems. We begin by describing synthetic strategies for low-dimensional metal halides, covering both established and emerging approaches. We then examine structural motifs, illustrating how octahedral connectivity defines dimensionality and lattice architecture. Optoelectronic properties follow, focusing on how composition, distortion, and the organic spacer chemistry modulate optical and electronic behavior in these materials. A dedicated section highlights optoelectronically active organics. Finally, we address stability and degradation mechanisms and feature recent advances and representative case studies, including their translation into thin films and next-generation optoelectronic and radiation-detection devices. The advances achieved to date reveal general principles that connect dimensionality, connectivity, and organic-inorganic interactions across the full landscape of hybrid metal halides, while also exposing the limits of current predictive understanding. The remaining challenge is to translate these qualitative insights into design rules that reliably link structure to function and guide the deliberate creation of next-generation low-dimensional materials.