  In molecules like hydrogen, most chemical bonds are formed by sharing two
electrons from each atom in the bonding molecular orbital
(two-center-two-electron (2c2e) bonding). There are, however, different kinds
of chemical bonding. The I3- molecule, for example, is noteworthy because three
iodine atoms are linearly united by sharing four electrons
(three-center-four-electron (3c4e) bonding). Some inorganic solids undergo
phase transitions that result in the formation of "molecules" in their
crystalline frameworks, which are often accompanied by dramatic changes in
physical properties; the metal-to-insulator transition (MIT) in vanadium
dioxide, for example, occurs with the formation of dimer molecules with 2c2e
bonding. We repot the creation of a linear ruthenium trimer with 3c4e bonding
in ruthenium monopnictide at its MIT. Charge transfer from polymerized
phosphorous to ruthenium produces this unusual molecule, with all conduction
electrons trapped by the bonding molecular orbital. Our results demonstrate
that molecules are crucial even in solid crystals as they impact their
electronic properties.
