Answer
a) 3+
b) 3+
c) 2+
Work Step by Step
a) The coordination compound has a total charge of 3-. The cyanide ions contribute a total charge of 6- and thus iron must be 3+ to even the charge
b) The coordination compound has a total charge of 3-. The oxalate has a total charge of 6- and thus chromium must be 3+.
c) The coordination compound has a total charge of 2-. The cyanide ions contribute a total charge of 4- and thus nickel must be 2+.
