In BF3, the B—F bond length is 1.30 A. when BF3 is allowed to be treated with Me3N it forms an adduct, Me3N → BF3, the bond length of B—F in the adduct is

Due to back -π bonding in BF3 the B-F bond gets double bond character, the B-F bond length in BF3 is 1.30 A0 .But when it form adduct with NMe3, it can't forms back π-bond with F-filled 2p-orbitals due to lack of empty p-orbital on  B-atom therefore in BF3.NMe3 adduct the B-F bond always single bond. That is in aduct the B-F bond length is>1.30 A0B can't form back π-bond with "F" filled orbitals.