# How to add extra space between the left-side legend and the left parenthesis of a matrix in the presence of a vertical line?

Insert a space within the column specification of your blockarray:

\documentclass{article}

\usepackage{booktabs}
\usepackage{blkarray}

\begin{document}

$\mathbf{R} = \begin{blockarray}{ cccccc } & \lambda_1 & \lambda_2 & \lambda_3 & \lambda_4 \\ \cmidrule{2-6} \begin{block}{@{} c @{\hspace{1ex}} | @{\quad} (ccccc) } \tau_1 & -1 & \phantom{-}1 & \phantom{-}0 & \phantom{-}0 \\ \tau_2 & \phantom{-}0 & -1 & \phantom{-}1 & \phantom{-}0 \\ \tau_3 & \phantom{-}0 & \phantom{-}0 & \phantom{-}1 & -1 \\ \tau_4 & -1 & \phantom{-}0 & \phantom{-}0 & \phantom{-}1 \\ \end{block} \end{blockarray}$

\end{document}


Also note the removal of the inter-column space (using @{}) at the beginning of the nested block.

With {pNiceArray} of nicematrix.

\documentclass{article}
\usepackage{nicematrix,tikz}

\begin{document}

\mathbf{R}= \begin{pNiceArray}{cccc}[first-col,first-row] & \lambda_1 & \lambda_2 & \lambda_3 & \lambda_4 \\ \hline \noalign{\vskip2pt} \tau_1 & -1 & \phantom{-}1 & \phantom{-}0 & \phantom{-}0 \\ \tau_2 & \phantom{-}0 & -1 & \phantom{-}1 & \phantom{-}0 \\ \tau_3 & \phantom{-}0 & \phantom{-}0 & \phantom{-}1 & -1 \\ \tau_4 & -1 & \phantom{-}0 & \phantom{-}0 & \phantom{-}1 \\ \CodeAfter \tikz \draw ([xshift=-2pt]row-1-|col-1) -- ([xshift=-2pt]row-5-|col-1) ; \end{pNiceArray}

\end{document}