Which of the following statement(s) is(are) true?
a. The coordination number of a metal ion in an octahedral complex ion is 8.
b. All tetrahedral complex ions are low-spin.
c. The formula for triaquatriamminechromium(III) sulfate is $\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{3}\left(\mathrm{NH}_{3}\right)_{3}\right]_{2}\left(\mathrm{SO}_{4}\right)_{3}$
d. The electron configuration of $\mathrm{Hf}^{2+}$ is $[\mathrm{Xe}] 4 f^{12} 6 s^{2}$
e. Hemoglobin contains $\mathrm{Fe}^{3+}$