The structure of the title compound was determined previously by Jones & Templeton [Acta Cryst. (1958), 11, 484] at T = 278 K using the Weissenberg technique, and twice by Nahringbauer [Acta Chem. Scand. (1970), 24, 453-462] at T = 278 and 93 K. The present redetermination at T = 40 K provides data of higher precision, reflected in s.u.'s which are about one third those of Nahringbauer.