Synthesis, crystal structure, and carbon-13 NMR spectra of .alpha.-pyrrolidonato-bridged head-to-head [PtIII2(NH3)4(C4H6NO)2(NO2)(NO3)](NO3)2.cntdot.H2O and electrochemical study on conversion of the binuclear platinum(III) complex into a tetranuclear platinum(III) complex