The transfer of phosphate groups is an essential function of many intracellular biological enzymes. The transfer is in many cases facilitated by a protein scaffold involving two closely spaced magnesium "ions". It has long been a mystery how these "ions" can retain their closely spaced positions throughout enzymatic phosphate transfer: Coulomb's law would dictate large repulsive forces between these ions at the observed distances. Here we show, however, that the electron density can be borrowed from nearby electron-rich oxygens to populate a bonding molecular orbital that is largely localized between the magnesium "ions". The result is that the Mg-Mg core of these phosphate transfer enzymes is surprisingly similar to a metastable [Mg-2](2+) ion in the gas phase, an ion that has been identified experimentally and studied with high-level quantum-mechanical calculations. This similarity is confirmed by comparative computations of the electron densities of [Mg-2](2+) in the gas phase and the Mg-Mg core in the structures derived from QM/MM studies of high-resolution X-ray crystal structures. That there is a level of covalent bonding between the two Mg "ions" at the core of these enzymes is a novel concept that enables an improved vision of how these enzymes function at the molecular level. The concept is broader than magnesium-other biologically relevant metals (e.g., Mn and Zn) can also form similar stabilizing covalent Me-Me bonds in both organometallic and inorganic crystals.