Binuclear orbit

In a seminar talk given in 1925, Pauling discussed the idea of binuclear orbits (Pauling Papers, Box 242, Popular Scientific Lectures 1925-1935, "Modem Theories of Valence and Chemical Combination"). 

A crystal-structure determination on [Ni(PhCH2CS2)2] showed evidence of a Ni-Ni bond (Ni—Ni distance, 256 pm) in a bridging, acetate-cage, binuclear complex (363). Each nickel atom is 5-coordinate and is in a tetragonally distorted, square-pyramid spectroscopic evidence for a Ni-Ni bond has been obtained (364). The polarized crystal spectra showed more bands than predicted for a mononuclear, diamagnetic, square-planar nickel(Il), and the spectra are indicative of substantial overlap of the d-orbitals between the two nickel atoms. The bis(dithiobenzation)nickeKII) complex was found to exhibit unusual spectrochemical behavior (365). 

The one-step reaction of H2prCl6] with MeC02Li under 02 in a mixed solvent of acetic acid and acetic anhydride yields the Ir11 binuclear complex [Ir2(/u-02CMe)2Cl2(C0)2].483 Crystal-structure determinations of [Ir2(/x-02CMe)2Cl2(C0)2L2], (295), where L = MeCN, DMSO, and py, are reported. The one-electron oxidation product for (295), L = py, is EPR active at 77 K the odd electron occupies the 6Ir Ir orbital. 

Binuclear and polynuclear compounds with direct metal-metal interaction constitute a large class of transition metal complexes which play an important role also in organometallic chemistry. Generally, the frontier orbitals of these compounds are engaged in metal-metal bonding. Consequently, redox processes affect the metal-metal interaction. The same is true for the luminescence of such complexes since it involves also thg frontier orbitals. The binuclear complex Pt2(pop) 36) (pop = diphosphonate) and... 

The ri-ionization patterns are more complex in binuclear carbonyls. For the 14c electrons of Mn2(CO)10 and Re2(CO)10, P.E. spectra clearly indicate (15) an orbital energy sequence of three e levels (the two lowest ones being very close in energy) probably containing the u(xz,yz) and 8(xy) orbitals, followed by a nondegenerate orbital, possibly the M—M bonding o(d), all of these levels being contained in a relatively narrow interval of 1.0—1.5 eV. 

The oxidative degradations of binuclear azaarenes (quinoline, isoquinoline, and benzodrazines) by hydroxyl and sulfate radicals and halogen radicals have been studied under both photochemical and dark-reaction conditions. A shift from oxidation of the benzene moiety to the pyridine moiety was observed in the quinoline and isoquinoline systems upon changing the reaction from the dark to photochemical conditions. The results were interpreted using frontier-orbital calculations. The reaction of OH with the dye 3,3,6,6-tetramethyl-3,4,6,7,9,10-hexahydro-(l,8)(2//,5//)-acridinedione has been studied, and the transient absorption bands assigned in neutral solution.The redox potential (and also the pA a of the transient species) was determined. Hydroxyl radicals have been found to react with thioanisole via both electron transfer to give radical cations (73%) and OH-adduct formation (23%). The bimolec-ular rate constant was determined (3.5 x lO lmoU s ). " ... 

Consider a binuclear complex with metals A and B and assume that each metal contains one unpaired electron allocated to orbitals a and b, respectively (see Figure 6). Let us assume further that the states la> and lb> contain the contributions of the ligands (including the bridging ligands) and that we have orthogonalized the two functions, = 0. If only those two "active electrons are taken into account the electrostatic part of the Hamiltonian is... 

The electronic spectra of niobium(IV) and -(V) and zirconium(IV) complexes 126,127) have been reported but not interpreted. The spectrum of Nb(ethyl-dtp)4 is of particular interest since the compound is probably 8-co-ordinate. Discussion of the spectrum of binuclear molybdenum complexes 130,131) employed the molecular orbital model of Blake, Cotton and Wood for MO2O3LX complexes s). 

Two main aspects of the HDW approach are important for the present discussion. One aspect is that the model is valid only in the absence of orbital angular momentum contributions to the system, so that its use has been broadly restricted to (say) binuclear complexes of iron(LLl), octahedral nickel(II), or distorted octahedral copper(II). The other aspect concerns the fact that the parameter is a global parameter of the system sequestering many contributions from various ligands and orbitals and, in this respect, may be compared with how Dq in ligand-field theory represents the net effect of electrostatic and covalent (o and rr) effects in the tjg — eg orbital splitting. Attempts to localize our conception of the exchange phenomenon have been made by various... 

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