Electron movement arrow and an extra pair of electrons

Question

How can I show that one, then another pair of electrons is moving from N to C in HCN? This is what I tried, and my "hacky way" of creating the extra pair between :C: and :N: worked for the last diagram, but for the life of me I could not get the 2nd diagram to show the correct arrow - from the pair above N to the pair just left of :N! It is pointing to the bottom of free space between C: and :N instead of the top of : next to N.

I tried playing with \phantom{:} and a few other things I can't even remember now.

\documentclass{scrartcl}
\usepackage[version=3]{mhchem}
\usepackage{chemfig}

\begin{document}

\chemfig{\lewis{,H}\quad \lewis{0:4:,C}\quad \lewis{0:2:6:,N}}

\bigskip
% Moving one pair to in between C and N
\schemestart
\chemfig{\lewis{,H}\quad @{C}\lewis{0:4:,C}@{b}\quad @{N}\lewis{0:4:6:,N}}
\schemestop
\chemmove{
% This is pointing to the bottom of blank space between the moved pair and :C:
\draw(N) ..controls +(90:5mm) and +(90:5mm) .. (b);
}

\bigskip
\schemestart
\chemfig{\lewis{,H}\quad @{C}\lewis{0:4:,C}@{b}\quad @{N}\lewis{0:4:,N}}
\schemestop
\chemmove{
% Here I did not know how to create another pair of electrons and rigged TikZ draw
\draw[fill] (-13.7pt,5.4pt) circle (0.7pt);
\draw[fill] (-13.7pt,2.1pt) circle (0.7pt);
\draw(N) ..controls +(-90:5mm) and +(-90:5mm) .. (b);
}

\end{document}

Answer 1

This is how I understand you want it. My suggestion is to stick to one type of unit and not mess around with pt, mm, and unit-less in the same graphics. Other than that you had pretty much done it correctly.

\documentclass{article}
%\usepackage[version=3]{mhchem}
\usepackage{chemfig}

\begin{document}

\chemfig{\lewis{,H}\quad \lewis{0:4:,C}\quad \lewis{0:2:6:,N}}

\bigskip

\schemestart
\chemfig{\lewis{,H}\quad @{C}\lewis{0:4:,C}@{b}\quad @{N}\lewis{0:4:6:,N}}
\schemestop
\chemmove{
\draw(N) ..controls +(90:5mm) and ++(90:5mm) .. ++(-1.95mm,1mm);
}

\bigskip
\schemestart
\chemfig{\lewis{,H}\quad @{C}\lewis{0:4:,C}@{b}\quad @{N}\lewis{0:4:,N}}
\schemestop
\chemmove{
\draw[fill] (b) ++(0,0.65mm)circle (0.25mm);
\draw[fill] (b) ++(0,1.75mm) circle (0.25mm);
\draw(N) ..controls +(-90:5mm) and +(-90:5mm) .. (b);
}

\end{document}

Sample output: