Infrared spectra of Si n H 4 n -1 + ions ( n = 2-8): inorganic H-(Si-H) n -1 hydride wires of penta-coordinated Si in 3c-2e and charge-inverted hydrogen bonds.

Si n H m + cations are important constituents in silane plasmas and astrochemical environments. Protonated disilane (Si 2 H 7 + ) was shown to have a symmetric three-centre two-electron (3c-2e) Si-H-Si bond that can also be considered as a strong ionic charge-inverted hydrogen bond with polarity Si δ + -H δ - -Si δ + . Herein, we extend our previous work to larger Si n H 4 n- 1 + cations, formally resulting from adding SiH 4 molecules to a SiH 3 + core. Infrared spectra of size-selected Si n H 4 n- 1 + ions ( n = 2-8) produced in a cold SiH 4 /H 2 /He plasma expansion are analysed in the SiH stretch range by complementary dispersion-corrected density functional theory calculations (B3LYP-D3/aug-cc-pVTZ) to reveal their bonding characteristics and cluster growth. The ions with n = 2-4 form a linear inorganic H-(Si-H) n hydride wire with adjacent Si-H-Si 3c-2e bridges, whose strength decreases with n , as evident from their characteristic and strongly IR active SiH stretch fundamentals in the range 1850-2100 cm -1 . These 3c-2e bonds result from the lowest-energy valence orbitals, and their high stability arises from their delocalization along the whole hydride wire. For Si n H 4 n -1 + with n ≥ 5, the added SiH 4 ligands form weak van der Waals bonds to the Si 4 H 19 + chain. Significantly, because the Si n H 4 n -1 + hydride wires are based on penta-coordinated Si atoms leading to supersaturated hydrosilane ions, analogous wires cannot be formed by isovalent carbon.