Carlton confirmed on Wednesday afternoon that they are not willing to match St Kilda's offer for the restricted free agent.
De Koning chalked up his 100th game for the Blues against Essendon last Thursday night and etched his name onto the number 12 locker along with famous names such as Matthew Lappin, Adrian Gleeson and Barry Armstrong.
It was no secret that St Kilda were willing to throw big money De Koning's way to attract him to Moorabbin, and whilst the move has not yet been iced, it's all but set to happen after De Koning had his exit meeting on Wednesday.
"Tom De Koning has informed the Carlton Football Club of his decision to take up a free agency option at St Kilda," the Blues said in a statement.
"Although De Koning is a restricted free agent, the Club understands that the significant size of the deal presented by St Kilda is not one the Club is prepared to match.
"The Club will now wait to formalise the process and subsequently receive the appropriate compensation pick from the AFL during the upcoming Trade and Free Agency Period in October."
After several at times tumultuous and difficult years at Ikon Park, De Koning described his departure as "the toughest decision I have ever had to make".
“In the end I have made a decision that is best for me," De Koning said.
"I really want to thank the entire Carlton community who has embraced me since I was drafted – in particular the cheer squad, who have sponsored me and been an amazing support.”
Carlton's compensation is expected to be in the form of a first round draft pick.
For the Saints, it continues a bumper off-season in which they have secured Nasiah Wanganeen-Milera and have been linked to both Liam Ryan and Jack Silvagni among others.