A certain crime is punishable, if attempted but not if committed....What?

I think you've based your question on urban myth rather than your own personal knowledge.

The Criminal Attempts Act of 1981 makes it an offence to attempt to commit many offences.

In the UK, Suicide or attempted suicide has not been a crime since the Suicide Act of 1961.

The crime that is punishable if attempted but not if committed is the taking of ones own life or suicide. The person who attempts suicide and doesn't succeed in killing them self can be charged with attempted murder. The possibility of a district attorney charging someone with attempted murder is very small as the case will be thrown out by any judge because of the mental state of the person who would be charged with attempted murder of him or her self. The accused has the Fifth Amendment right to remain silent and even if the accused does speak anything said will be thrown out in court as inadmissible because of the mental state of the accused at the time the words were spoken. The chances of the accused going to trial are nil because therapy for the accused is what is needed and not incarceration and any judge will see this to be the case and not take up the courts time hearing a case where one attempts to kill oneself and fails unless there are others who were placed in danger or physically hurt by the actions of the accused. The right of the accused to not self incriminate will prevail and this right will also protect the accused from being convicted of attempted murder. By the way, attempted murder is premeditated and for that reason an attempted manslaughter charge won't be brought against the accused.

Suicide