Stepwise metal-assisted conversion of .eta.2-carbon diselenide to .eta.1-phosphoniodiselenoformate, .eta.2-diselenocarbonate, and .eta.2-diselenium. Crystal structure of the rhodium complexes [(triphos)Rh(Se2CO)]BPh4.cntdot.0.5CH2Cl2.cntdot.0.5C4H9OH