`CN^(-)` ion is known but `CP^(-)` ion is unknown-why?

Give reasons for the following : (a) CN^(-) ion is known but CP^(-) ion is not known (b) NO_(2) dimerises to form N_(2)O_(4)

CN^(-) is known but CP^(-) is not known.

PCl , is known but 'NCl_5' is unknown. Why?

Explain why Cu(NH_(3))_4]^(2+) ion is coloured but [Cu(CN)_4]^(3-) ion is colourless.

[Fe(CN)_6]^(4-) ion is:

Why H^(+) ion is known as proton?