Lewis acids and bases ionic bonding. Classically, the term "Lewis acid" is restricted to trigonal planar species with an empty p orbital, such as BR where R can be an organic substituent or a halide. For the purposes of discussion, even complex compounds such as EtAlCl and AlCl are treated as trigonal planar Lewis acids. Metal ions such as Na, Mg, and Ce, which are invariably complexed with additional ligands, are often sources of coordinatively unsaturated derivatives that form Lewis adducts upon reaction with a Lewis base[citation needed]. Other reactions might simply be referred to as "acid-catalyzed" reactions. Some compounds, such as HO, are
Internet-related prefixes as a cultural reference to the rising popularity at that time of the prefix in product names. The word "virtual" is used in a similar way to the prefixes above, but it is an adjective instead of a prefix. For example, it is used in the terms "virtual reality", "virtual world", and "virtual sex". These prefixes are productive. Michael Quinion notes that most of these formations are nonce words that will never be seen again. He writes that new terms such as "e-health" are unneeded; in this case "telemedicine" already exists to describe the application of telecommunications to medicine. He