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

Abstract

Chemische Bindung i Kernquadrupolresonanz I Kristallstruktur I MolekiilstrukturHalogen 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 (%, "Br, '"I) 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 ofteii large and depend on the bridging angle; those of secondary-bridging halogens are small. The temperature dependence of the NQR frequencies of symmetric bridgng 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 pr orbital, and the halogen acquires an additional 0.01 electron in the original M -X bond.Halogenatome bilden mit Metallatomen Briickenbindungen aus, wobei die beiden Bindungslangen ungefahr gleich sein konnen (,,symmetrische Briickenbindung", wie in Cd-halidaddukten mit Sauerstoff-Donorliganden) oder wesentlich ungleich lang sein konnen (,,sekundare Briickenbindung", wie in Hg-halidaddukten). Halogen ("CI, "Br, '271)-NQR-spektren der genannten und ahnlicher Komplexe zeigen fur die beiden Typen von Briickenbindung charakteristische Unterschiede. Halogenbriickenatome zeigen in Verbindungen mit Hauptgruppenmetallen niedrigere Frequenzen als die endstandigen Halogene. 1st die Briicke symmetrisch, so hangt die Frequenzabsenkung 475 empfindlich vom Bruckenwinkel ab. Sekundare Bruckenbindung ist weniger effektiv. Die Asymmetrieparameter symmetrischer Briickenhalogenatome sind meist hoch und vom Winkel abhangig; bei sekundarer Briickenbindung sind sie niedrig. Die Temperaturabhangigkeit der NQR-frequenzen symmetrischer Briickenhalogenatome ist normal, d. h. negativ: in sekundareren Bruckenbindungen ist sie schwacher negativ als die terminaler Halogenatome, ja manchmal positiv. Zwei verschiedene Berechnungen der sekundiren Bruckenbindung nach Townes und Dailey wurden durchgefiihrt: einmal werden sp-Hybrid-Orbitale am Halogen angenommen, im anderen Fall reine p-Orbitale. Daraus ergibt sich fur eine ,,typische" sekundare Bruckenbindung von Halogenen: 0.03 Elektronen aus dem py-Orbital des Halogens sind in Richtung Metallato...

Coordination of Ortho-Chlorines in Copper (I) and Silver (I) 2,6-Di- and 2,4,6-Trichlorophenolates

“…Crystallographic data for each point is from the reference with the same number as that point; NQR data is from refs. [12,13,27,28], or data to be published from this laboratory. chlorine bond distance.…”

“…35,37q NQR spectra were obtained on a Decca superregenerative spectrometer using Zeeman modulation at 77 K, 195 K, and 273 K; for room-temperature searches for 63,65 Cu signals in the frequency range [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] MHz, a Wilks NQR-1A spectrometer was also employed.…”

“…An approximate TownesDailey treatment [27] of the covalent interaction shows that the 35 C1 NQR frequency of a coordinated chlorophenolate orr/io-chlorine (Cl 2 ) should be lowered relative to that of the non-coordinated orthochlorine (Cl 6 ). This has been observed in practice in several crystallographically-characterized chlorophenolates of first-row transition metals [28,29].…”

“…26 _C (27) 136.4 (9) C,41 _C(42, 140.8 (9) p<2) _C (31) 181.6 (5) C(12)_ C (7) 141.7 (7) _C<2) 141.1 (7) C(17)_ C (18) 142.2(11) c 32) _C (33) 137.7 (8) C (2) _C (3) 140.1 (8) 142.2(11) 137.7(8) Table 3b. Crystal structure of (2,4,6-Trichlorophenolato-0,Cl)bis(triphenylphosphine)silver(I), C 42 H 32 AgCl 3 OP 2 ; Intramolecular angles (°).…”

Nuclear Quadrupole Resonance (NQR) Spectroscopy

“…Within a given crystalline modification of a compound, NQR spectra include distinct frequencies for each crystallographically distinct atom: as an extreme example, CCl 4 (s) adopts a complex lattice with 16 crystallographically inequivalent chlorine sites. CCl 4 (s) has 16 35 Cl NQR frequencies averaging 40.64 MHz, with a spectral width between the highest and lowest frequency of 0.351 MHz. Spectral widths in covalent organochlorine compounds normally are limited to about 0.8 MHz.…”