leave a pension to/for

What is the context?

A "pension" mean a sum of money. Is that what you mean?

What do you mean by "leave"?

A man died and left a pension for/to his wife.

You can use either 'for' or 'to'. You might use 'for' if the pension was in the control e.g. of a bank or lawyer and the wife just received regular payments (for example an annuity), and you might use 'to' if the wife had complete control, e.g. could release the capital. I say "might" because many people would use 'for and 'to' interchangeably without this distinction.

Worth remembering that the correct word for a woman whose husband is dead is 'widow'.

Both are right. It's almost the same when you put the receiver's name on a letter or card. You may write "to Mary" or "for Mary". I remember my teacher told that "to" means the direction or intention the action to be done. "For" means who is intended receiver of the action.