The new linker molecule 4-phosphono-biphenyl-4′-carboxylic acid (H
2O
3P-(C
6H
4)
2-COOH, H
3BPPA) has been structurally elucidated in hydrogen-bonded networks with the ammonium cation NH
4(H
2BPPA)(H
3BPPA) (
1) and the hexaamminecobalt(III) cation [Co(NH
3)
6](BPPA)·4H
2O (
2). The protic O-H and N-H hydrogen atoms were found and refined in the low-temperature single-crystal X-ray structures. The hydrogen bonds in both structures are so-called charge-assisted; that is, the H-bond donor and/or acceptor carry positive and/or negative ionic charges, respectively. The H-bonded network in
1 consists of one formally mono-deprotonated 4-phosphonato-biphenyl-4′-carboxylic acid group; that is, a H
2BPPA
− anion and a neutral H
3BPPA molecule, which together form a 3D hydrogen-bonded network. However, an almost symmetric resonance-assisted hydrogen bond (RAHB) bond [O···H = 1.17 (3) and 1.26 (3) Å, O···H···O = 180 (3)°] signals charge delocalization between the formal H
2BPPA
− anion and the formally neutral H
3BPPA molecule. Hence, the anion in
1 is better formulated as [H
2BPPA···H···H
2BPPA]
−. In the H-bonded network of
2 the 4-phosphonato-biphenyl-4′-carboxylic acid is triply deprotonated, BPPA
3−. The [Co(NH
3)
6]
3+ cation is embedded between H-bond acceptor groups, –COO
− and –PO
3− and H
2O molecules. The incorporation of sixteen H
2O molecules per unit cell makes
2 an analogue of the well-studied guanidinium sulfonate frameworks.
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