Electronic structure control of silicon-hydrogen bond activation by transition metals. 2. Valence photoelectron spectra of (.eta.5-C5H4CH3)Mn(CO)2HSiPh3, (.eta.5-C5H4CH3)Mn(CO)2HSiHPh2 and (.eta.5-C5H4CH3)Mn(CO)2HSiFPh2 (Ph = C6H5)