page_title: Borderline hydrides
section_title: Alternative definition
hierarchical_section_title: Borderline hydrides / Alternative definition
caption_attribution_description: M(H)2 vs. M(H2)
context_page_description: Borderline hydrides typically refer to hydrides formed of hydrogen and elements of the periodic table in group 11 and group 12 and indium and thallium. These compounds have properties intermediate between covalent hydrides and saline hydrides. Hydrides are chemical compounds that contain a metal and hydrogen acting as a negative ion.
context_section_description: A more recent definition of borderline hydrides refers to hydrides that exist between classic and non-classic dihydrides. The classic form is the dihydride M(H)₂ configuration, where the metal is bound to two free hydrogen atoms. The non-classic form contains two hydrogen atoms bound to a central metal atom with an η²-H₂ hapticity, indicating that a single coordination point on the metal atom bonds to two contiguous atoms from another molecule, in this case H₂.  A well-known example of this is from the first such molecule to be synthesized with a coordinated hydrogen ligand (dihydrogen complex): W(CO)₃(PPri₃)₂(η²-H₂).
  Classic dihydrides containing the dihydride M-(H)₂ ligands are typically found as a tautomer with the non-classical dihydrogen complexes containing a M-(η²-H₂) group.
  Borderline hydrides exist with a bond character somewhere between the classical and non-classical hydrides. Those that are thermally unstable exhibit stretching frequencies νHH greater than 2150 cm¹ as a result of poor electron donation from the metal center. An electron dense metal center will yield hydride with a νHH less than 2060 cm¹, while anything between is considered to be in the borderline region. Kubas, et al. state that a stretching frequency of 2090 cm¹ is within the bounds of stable H₂ complexes while 2060 cm¹ is right on the borderline between dihydrogen and dihydrides.
caption: An example of the classical dihydride M(H)2 (left) compared to the non-classical M(H2) dihydrogen bonding (right).
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