Partially Formed Bonds in HCN-SO₃ and CH₃CN-SO₃: A Comparison between Donor-Acceptor Complexes of SO₃ and BF₃

The gas-phase structures of HCN-SO₃ and CH₃CN-SO₃ have been determined by microwave spectroscopy in a supersonic jet. Both adducts are symmetric tops with the nitrogen bonded to the SO₃. In HCN-SO₃, the N-S distance is 2.577(6) Å and the NSO angle is 91.8(4)°. In CH₃CN-SO₃, the bond length and bond angle are 2.466(16) Å and 92.0(7)°, respectively. The N-S distances are significantly shorter than the sum of van der Waals radii, and the structures are indicative of N-S dative bonds which are in their early stages of development.¹⁴N nuclear hyperfine structure is consistent with this assessment. The bond length-bond angle relationship for a series of SO₃ complexes with amine and nitrile donors is examined and compared with that for a similar series of complexes of BF₃ and BH₃. The variation of bond angle with bond length is strikingly similar for both sets of systems despite the differing atomic sizes of boron and sulfur. With a given base, however, the degree of bond formation to SO₃ appears to lag that to BF₃.