IUPAC_SMILES_plus
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Add support for zero-order bond
Clark, 2011 argues for introduction of zero-order bonds to ease the representation of non-organic compounds. This suggestion was directed at MDL format, but I believe it is also applicable to SMILES. Currently the representation of metal coordination in SMILES is ambiguous: one may represent it using single bond or not represent interaction at all, losing an important piece of information.
As for the symbol, I think !
could be easy to remember as it resembles both a dashed line (used in Clark, (2011)) and asymmetry due to donor-acceptor interactions.