Syria had previously purchased the S-300 system in 2010 but the contract was eventually torn up & money returned at Israel's request. Russia already has the more advanced S-400 deployed in Syria protecting their Tartus naval facility. The S-400 hasn't been fired & isn't integrated with Syria's existing SAM sites. 

“In 2013 on a request from the Israeli side we suspended the delivery to Syria of the S-300 system, which was ready to be sent with its Syrian crews trained to use it,”

“The situation has changed, and not due to our fault.”

“The most important thing is that it will ensure identification of Russian aircraft by the Syrian air defense forces.”

The third measure announced by the Russian defense ministry is a blanket of electronic countermeasures over Syrian coastline, which would “suppress satellite navigation, onboard radar systems and communications of warplanes attacking targets on Syrian territory.”

Implications of Syria taking delivery of the S-300 system...

Russian MoD presented additional data from their S-400 system to counter the ongoing Israeli denials & propaganda....