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−SO 3 and CH 3 CN−SO 3 have been determined by microwave spectroscopy in a supersonic jet. Both adducts are symmetric tops with the nitrogen bonded to the SO 3 . In HCN−SO 3, the N−S distance is 2.577(6) Å and the NSO angle is 91.8(4)°. In CH 3 CN−SO 3, 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. 14 N nuclear hyperfine structure is consistent with this assessment. The bond length−bond angle relationship for a series of SO 3 complexes with amine and nitrile donors is examined and compared with that for a similar series of complexes of BF 3 and BH 3 . 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 SO 3 appears to lag that to BF 3 .