If you have arranged for the Retirement administrator to send the money directly to the ex, then he/she gets her own 1099-R and you do not pay tax on that half. So, you get no deduction.
If you collect all the retirement and get a 1099-R for the full amount, then what you sent her is usually considered alimony and is deductible as such.
If the divorce papers specifically say it is not to be treated as alimony, then there is no other way to "get credit"
