The hydridochromium pentacarbonyl anion, HCr(CO)5-, has been isolated and spectroscopically defined as its Et4N+ and Ph4As+ salts. Its great tendency to bind to transition-metal Lewis acids such as [M(CO)5] (M = Mo, W) and [Mo- (PPh3)(CO)4] has allowed for an efficient clean synthesis of heterobinuclear bridging hydride complexes. 1981, American Chemical Society. All rights reserved.