Comparison of the self-reactivity of [CpCo(PP)I]+ complexes, PP = Ph2P(CH2)nPPh2 (n = 1-4), leading to bridged and oxidized dangling PP. Reactions, spectral studies, and structures of [CpCo(dppm)I]I.CHCl3, CpCo[dppm(O)]I2, and Cp2Co2I4(.mu.-dpppent)