How do you summon mobs to dropped items?

/summon doesn't take a a target to summon the mob at, so you'll need to use /execute, and selectors can't select items by type, so you'll need to use /scoreboard players set (which can set a score for items with specific NBT data) to set the entity's score first, and then select it based on that scoreboard objective.

First, set up a dummy scoreboard objective that we will use to keep track of items that are iron ingots:

/scoreboard objectives add isIronIngot dummy

Then run this on a clock, to set all dropped iron ingot's isIronIngot score to 1:

/scoreboard players set @e[type=Item] isIronIngot 1 {Item:{id:"minecraft:iron_ingot"}}

Now, whenever you want to spawn a zombie at the iron ingots, use this command:

/execute @e[score_isIronIngot_min=1] ~ ~ ~ /summon Zombie

Which will make anything with an isIronIngot score of (at least) 1 summon a zombie.

To have this only work with ingots named "Zombie", replace the second command with:

/scoreboard players set @e[type=Item] isIronIngot 1 {Item:{id:"minecraft:iron_ingot",tag:{display:{Name:"Zombie"}}}}

I feel that there is definitely an easier way:

/execute @e[type=Item] ~ ~ ~ summon Zombie ~ ~ ~

It should work, just the only downside is that /execute cannot support NBT's like {Item:{id:"minecraft:iron_ingot"}}

You could use the following command:

/entitydata @e[type=Item,c=1] {CustomName:"ironIngot"}

to give the item a name. (ironIngot is only an example)

Then you use this command:

/execute @e[name=ironIngot] ~ ~ ~ summon Zombie ~ ~ ~ {}

to summon the zombie.

I left the brackets empty to indicate that you could give tags to the zombie to customise it