To reveal the diverse chemistry of beryllium, we employ multireference methodologies to study the ground and several excited electronic states of the titled beryllium oxides and carbonyls. The two types of complexes serve as model systems to describe the various ways that beryllium can form chemical bonds. Be(O-2), its isomer OBeO, and Be(O-2)(2) are ionic compounds where beryllium is best represented in its Be(II) oxidation state. On the other hand, CO induces the excitation of one or two 2s electrons of beryllium to its 2p shell. In this manner, the beryllium core (Be2+.) is exposed and enables the formation of dative bonds from the lone pair of carbonyls to Be. For all of the considered electronic states, we provide accurate optimal geometries and excitation energies.