Structure-property relationships in subnanometric transition metal tetramers

We present a systematic and exhaustive exploration of transition metal tetramer low-energy minima (M, M' = Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, x = 0-4), combining stochastic structural searches with density functional theory. Our results reveal a pronounced structural diversity, multiple low-lying spin states, and widespread fluxionality at the subnanometric scale. While binding energies broadly follow periodic trends, selected heterometallic combinations exhibit enhanced cohesion, identifying "magic" compositions with increased stability. Time-dependent DFT calculations predict intense UV-visible absorptions in several tetramers and distinctive and potentially useful optical signatures in a number of Ni-containing heterotetramers. Conceptual DFT descriptors uncover a wide range of electrophilic and nucleophilic behaviors across the series, offering initial, qualitative insights into potential reactivity patterns across the series. These results establish clear links between atomic-scale composition, geometry, and functionality in transition metal subnanoclusters, providing transferable insights relevant to nanocatalysis, optical nanomaterials, and the rational design of functional nanoalloys.