The longest covalent bond reported in scientific literature is the carbon-carbon (C-C) bond in a compound called hexaprismane. Hexaprismane is a hypothetical molecule consisting of six fused cyclobutane rings. The theoretical length of the C-C bond in hexaprismane is approximately 1.54 angstroms (0.154 nanometers).
However, it's important to note that hexaprismane is a highly strained molecule and has not been synthesized or observed experimentally. It exists as a theoretical construct and serves as an example of the upper limit for covalent bond length.
In practical terms, the longest covalent bonds observed or created in laboratories are typically in the range of 1.2 to 1.6 angstroms (0.12 to 0.16 nanometers). These bonds often involve elements such as carbon, silicon, sulfur, or phosphorus, which can form relatively long covalent bonds under certain conditions.