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 m icrowave 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) Angstrom and the NSO angle is 91.8(4)degrees. In CH3CN-SO3, the bond len gth and bond angle are 2.466(16) Angstrom and 92.0(7)degrees, 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 t heir early stages of development. N-14 nuclear hyperfine structure is consi stent with this assessment. The bond length-bond angle relationship for a s eries of SO3 complexes with amine and nitrile donors is examined and compar ed with that for a similar series of complexes of BF3 and BH3. The variatio n of bond angle with bond length is strikingly similar for both sets of sys tems despite the differing atomic sizes of boron and sulfur. With a given b ase, however, the degree of bond formation to SO3 appears to lag that to BF 3.