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

SiH cations are important constituents in silane plasmas and astrochemical environments. Protonated disilane (SiH) 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-H-Si. Herein, we extend our previous work to larger SiH cations, formally resulting from adding SiH molecules to a SiH core. Infrared spectra of size-selected SiH ions ( = 2-8) produced in a cold SiH/H/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 = 2-4 form a linear inorganic H-(Si-H) hydride wire with adjacent Si-H-Si 3c-2e bridges, whose strength decreases with , as evident from their characteristic and strongly IR active SiH stretch fundamentals in the range 1850-2100 cm. 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 SiH with ≥ 5, the added SiH ligands form weak van der Waals bonds to the SiH chain. Significantly, because the SiH hydride wires are based on penta-coordinated Si atoms leading to supersaturated hydrosilane ions, analogous wires cannot be formed by isovalent carbon.