Mechanism of formation of the peroxocarbonate complex (PCy3)2Ni(CO4) from solid (PCy3)2Ni(CO2) and dioxygen: an example of solid-state metallorganic reaction involving CO2 deco-ordination and reinsertion into the O-O bond of (PCy3)2Ni(O2). Reactivity of the peroxocarbonate complex towards olefins in the solid state and in solution