Final answer:
The metal ion for iron triphosphide is iron(II) or Fe2+. Balancing the charges of the ions leads to the conclusion that three iron(II) ions are required to neutralize two phosphate ions in the compound.
Step-by-step explanation:
The metal ion for iron triphosphide can be derived from understanding the composition of the compound and the charges involved. Iron, a transition metal, can form more than one ion, typically iron(II) with a 2+ charge or iron(III) with a +3 charge.
As explained by the reference, to obtain a neutral compound, the charges of the ions must balance out. Since iron triphosphide is composed of iron and phosphate ions (P33-), and referring to the information that two phosphate ions (with a charge of 6-) require three iron(II) ions to balance out with a charge of 6+, it is clear that the metal ion for iron triphosphide is iron(II) or Fe2+.