Does H₃O⁺ Really Act as a Ligand in the Solid State?

Abstract

The evidence for the existence of metal complexes containing H 3 O + as a ligand in the solid state is examined. Each of the 68 examples in the Cambridge Structural Database in which H 3 O + is bound to a transition metal, lanthanoid, actinoid, or main group metal ion is detailed and critically appraised. It is concluded that none of the reported examples of complexes containing coordinated H 3 O + have been unequivocally characterized and that they result from either curation errors or misinterpretations of the crystallographic data. These conclusions are supported by computational techniques, which show that three purported H 3 O + complexes based on the 1,4,7,10,13,16,21,24-octaazabicyclo(8.8.8)hexacosane azacryptand skeleton are better described as aqua complexes, with protonation occurring at the amine ligand.

Chelated O2BOH2– or Chelated O2CO2–: An Isoelectronic Conundrum

“…The complex was prepared under basic conditions (pH ∼ 9), and we believed it extremely unlikely that a hydronium ion of crystallization would be obtained from such an environment. 21 We therefore repeated the reported synthesis exactly, and using a variety of techniques, we herein show unequivocally that the crystalline material isolated is We obtained a pink crystalline material from the above synthesis, which looked similar to that reported by Pan and coworkers. 2 The only difference was that our product took slightly longer (11 days) to crystallize in reasonable amounts.…”

“…A Co­(III) oxidation state was further supported by examination of the reported X-ray structure; the Co–N bond lengths, which range from 1.913 to 1.944 Å, are consistent with those found in Co­(III) complexes containing two bipy ligands , and are significantly shorter than those in analogous Co­(II) complexes ( d Co–N ∼ 2.1 Å). , Similarly, the reported B–O bond lengths are almost exactly the same as the C–O distances found in Co­(III) amine complexes containing chelated carbonate. ,, Finally, we were concerned about the presence of an H 3 O + ion in the structure. The complex was prepared under basic conditions (pH ∼ 9), and we believed it extremely unlikely that a hydronium ion of crystallization would be obtained from such an environment . We therefore repeated the reported synthesis exactly, and using a variety of techniques, we herein show unequivocally that the crystalline material isolated is [Co­(bipy) 2 O 2 CO]·[B 5 O 6 (OH) 4 ]·H 3 BO 3 ·2H 2 O, a diamagnetic Co­(III) complex, with the carbonate ligand being derived from adventitious CO 2 .…”