Arrange the following compounds in increasing order of length of their C - N bond(III) CH3−CH=NH

The bond order of C - N bond is 2 in III, shortest, I in II, longest bond. There is greater extent of delocalization of nitrogen lone pair in IV because it brings negative charge on oxygen than in I where negative charge moves on carbon. Hence,       C - N bond acquire greater partial double bond character in IV than in I. This results in greater shortening of C - N bond length in IV than in I, although I has more resonating structures.