Highly reduced organometallics. Part 25. Reactions of trisodium tetracarbonylmetalates(3-) of manganese and rhenium with Broensted acids and other electrophiles. Synthesis of H2M(CO)4- (M = Mn and Re), (CH3)2Re(CO)4-, the first dialkyl derivative of a carbonylmetalate trianion, and related anionic species