Two-Electron and One-Electron Reduction of the Indenyl Complex [Mn(.eta.5-C9H7)(CO)3] and Reversible Counterion-Controlled Comproportionation of [Mn(.eta.5-C9H7)(CO)3] and [Mn(.eta.3-C9H7)(CO)3]2- To Give [Mn(.eta.5-C9H7)(CO)3]-