Halogen‐Metal Secondary Bonding). An NQR Study in Mercury and Cadmium Halide Adducts of Polyethers

Halogen atoms bridge metal atoms by forming two M — X bonds of about equal length („symmetric bridging”︁, as in cadmium halide adducts of oxygen‐donor ligands), or by forming two bonds of significantly different lengths („secondary bridging”︁, as mercuric halide adducts of the same ligands). Halogen (35Cl, 81Br, 127I) NQR spectra of the title and related complexes show characteristic differences for the two types of bridging. Bridging halogen atoms (in nontransition metal halides) have lower frequencies than do analogous terminal halogen atoms; if the bridging is symmetric, the lowering depends on the bridging angle and is quite variable; if the bridging is secondary, the lowering is less but relatively constant. Asymmetry parameters for symmetrically‐bridging halogens are often large and depend on the bridging angle; those of secondary‐bridging halogens are small. The temperature dependence of the NQR frequencies of symmetric bridging halogens is normal negative; that of secondary bridging halogens is less negative than for analogous terminal halogens, and may even be positive. Two Townes‐Dailey treatments of secondary bridging have been developed: one assumes the use of sp hybrid orbitals by the halogen, and the other uses pure p orbitals for the secondary bond. Analysis of the data leads to a picture of the formation of a “typical” secondary bond: 0.03 electrons are polarized or transferred towards the metal atom from the halogen p, orbital, and the halogen acquires an additional 0.01 electron in the original M — X bond.