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 [Cr(H2O)3(NH3)3]2(SO4)3.
d. The electron configuration of Hf2+ is [Xe]4f126s2.
e. Hemoglobin contains Fe3+.