Complex matrix representation of quaternions ($\left(\begin{array}{cc}a+ib& c+id\\ -c+id& a-ib\end{array}\right)$)

The displayed complex matrices, with real coefficients, form a four-dimensional real associative algebra. Their three imaginary basis matrices square to minus the identity and satisfy $JK=L$, $KL=J$, $LJ=K$, with the reversed products negative. For $\alpha =aI+bJ+cK+dL$, quaternionic conjugation gives $\alpha \overline{\alpha }=(a^2+b^2+c^2+d^2)I$. Every nonzero matrix therefore has inverse $\overline{\alpha }/(a^2+b^2+c^2+d^2)$ inside this algebra. This realizes the real division algebra of quaternions within $M_2(\mathbb{C})$; it is not a complex vector subspace.