On the Stability and the Bonding Model of n-->sigma Type Molecular Complexes, R(2)Z-X-X: Proposal of 3c-4e Description for Z-X-X in the Adducts.

Abstract

The stability of the Z-(1)X-(2)X bonds in R(2)Z-(1)X-(2)X molecular complexes (MC) was examined for O(CH(2)CH(2))(2)Se-I-X, PhMeSe-I-X (X = Cl, Br), and Se(C(6)H(4))(2)Se-Br-Br. The MC adducts were shown to be comparably stable or more stable than the corresponding trigonal bipyramidal adducts (TB) which equilibrate with the MC in some cases. To clarify the reason for the stability of the MC, the model adducts of H(2)Z(1)X(2)X (MC and TB) (Z = O, S, Se and X = Cl, Br), together with the related species (H(2)Z, X(2), H(2)ZX(+), and H(2)ZX*), were optimized with the 6-311++G(3df,2pd) basis sets at the MP2 and/or B3LYP levels. Calculations were also performed with different distances between (1)Cl and (2)Cl (r((1)Cl,(2)Cl)) in H(2)S-(1)Cl-(2)Cl (MC) and r(S,(2)Cl) in H(2)S(1)Cl(2)Cl (TB), where r = r(o) + 0.1m Å (r(o): the optimized distance and m = -1, (0), 1, 2, and 3). A charge transfer (CT) occurs from S to (2)Cl in the S-(1)Cl-(2)Cl bond of the MC as r((1)Cl,(2)Cl) becomes larger, assuming a singlet multiplicity in the calculations. The situation is equal to that of (1)Cl-S-(2)Cl in the TB, for which CT occurs from (1)Cl to (2)Cl. A 3c-4e description of the Z-(1)X-(2)X bond in R(2)Z-(1)X-(2)X (MC) is proposed based on the ab initio MO calculations by exhibiting the 10-(1)X-2 character, the N-X-L cording system for hypervalent bonds proposed by Martin, for R(2)Z-(1)X-(2)X (MC) practically. Bond orders for typical TB and MC were calculated from literature data according to Pauling's equation. The bond orders agree with the proposed 3c-4e model for the MC.