Transition metal hydrides include compounds that can be classified as covalent hydrides. Some are even classified as interstitial hydrides and other bridging hydrides. Classical transition metal hydride feature a single bond between the hydrogen centre and the transition metal.  Some transition metal hydrides are acidic, e.g.,  and . The anions potassium nonahydridorhenate  and  are examples from the growing collection of known molecular homoleptic metal hydrides.  As pseudohalides, hydride ligands are capable of bonding with positively polarized hydrogen centres.  This interaction, called dihydrogen bonding, is similar to hydrogen bonding, which exists between positively polarized protons and electronegative atoms with open lone pairs.