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Yes, it is sigma donor (basicaly all ligands are sigma donors but some of them can be pi donors/acceptors in addition). It has only 1 electron pair so its only sigma donor.How can you tell? Well from examples we were told at lectures (It most likely will not fit all possible ligands):a) has only 1 electron pair at coordinating atom? sigma donorb) has more than 1 electron pair at coordinating atom? sigma and pi donorc) has double or tripple bond at coordinating atom? sigma donor and pi acceptord) is the ligand coordinating by phosphorous? sigma donor and d acceptor
Because the back donation goes into pi-antibonding orbitals. And for them to exist, you need pi bond.