\documentclass[tikz,border=10pt]{standalone}
\begin{document}
\begin{tikzpicture}

\foreach \ii in {0,1,...,5} {
  \foreach \jj in {0,1,...,4} {
    \node at ({\ii+\jj*0.5}, {-\jj*0.8}) {$(\ii,\jj)$};
  }
}

\end{tikzpicture}
\end{document}

用最简单的方式，或者定义两个函数

\documentclass[tikz,border=10pt]{standalone}
\begin{document}
\begin{tikzpicture}[
    declare function={
        px(\i,\j) = \i + \j*0.5;   % x 坐标
        py(\i,\j) = -\j*0.8;       % y 坐标
    }
]
\foreach \i in {0,...,5}
  \foreach \j in {0,...,4}
    \node at (px(\i,\j), py(\i,\j)) {$(\i,\j)$};
\end{tikzpicture}
\end{document}

非要一次返回一对坐标，用command方式，但不推荐

\documentclass[tikz,border=10pt]{standalone}
\newcommand\labelpos[2]{#1+#2*0.5, -#2*0.8} % 👈 一次返回 x,y
\begin{document}
\begin{tikzpicture}
\foreach \i in {0,...,5}
  \foreach \j in {0,...,4}
    \node at (\labelpos{\i}{\j}) {$(\i,\j)$}; % 👈 完美可用
\end{tikzpicture}
\end{document}

\documentclass{article}
\usepackage{chemfig}
\usepackage{mhchem}
\usepackage{xcolor}
\begin{document}

\[
        \setchemfig{atom sep=2em}
        \hreac
            $n$ \chemfig{[:-60]*6(-=-=(-[0]CH_2OH)-(-[2]OH)=)}
            >[{\ce{H+}}][{$\triangle$}]
            \chemfig{H-[@{op,.6}]([:-30]*6(-=-=(-[0]CH_2-[@{cl,0.4}:0]OH)-(-[2]OH)=-=))}\polymerdelim[delimiters ={[]}, height = 5pt, indice = n]{op}{cl}
            + $(n - 1)$\ce{H2O}
            %\polymerdelim[delimiters ={[]}, height = 5pt, indice = \!\!n]{op}{cl}
        \endhreac
\]
\end{document}

大体已解决，如上代码