P/ -backbonding

Parvalbumin, 46 443, 445 calcium binding sites, 42 113-114 p backbonding, osmium complexes, 37 316, 320... 

Let us consider the P(I)/Ta(III) couple nonetheless. The phosphorus atom would retain one of the lone pairs it possesses as a P(I) entity and use the other to form a (T-donor bond to the metal. The empty p-orbital on phosphorus would then be available to accept the 7r-donor interaction originating from the lone pair on tantalum (M—P backbonding). The result is a trigonal planar phosphorus atom with a stereoactive lone pair and a Ta=P double bond, as observed. However, since the 7r-donor interaction now involves a flUed metal orbital, the phosphorus resonance would be seen relatively upheld. 

The orange, air-stable, homoleptic tetrakis( 71-phosphabenzene)nickel (1046) is tetrahedral (point symmetry 54) and can be obtained from phosphabenzene and [Ni(cod)2].2 25 It features a short Ni—P bond length of 2.1274(5) A with considerable N i P 7r-backbonding and a i/(Ni—P) stretch at 168 cm-1. In solution, partial dissociation of one phosphabenzene ligand is observed. 2-Diphenylphosphino-3-methylphosphinine forms with [Ni(cod)2] in the presence of the CO the dinuclear complex (1047) with a W-frame structure.2526... 

Complexes 41 and 42 were characterized by their IR and H-NMR spectra, and 41 also by elemental analysis. Table III contains the pertinent spectral data. Noteworthy are the very low energy terminal carbonyl bands for 41 and 42 at 1864 cm-1 (hexane). The weak 7r-accepting abilities of PR3 (R = Et, Ph) allow the lone CO ligand to 77-backbond to the Ti(II) center to a much greater degree. The -NMR spectrum of 41 exhibited a doublet (/H-p = 1.5 Hz) at 8 4.75 due to the coupling of the cyclo-pentadienyl protons with the 31P nucleus, while complex 42 exhibited a broad cyclopentadienyl singlet at 8 4.67. 

It has been speculated that there is a common origin of the reduced chemical etch rate for (111) oriented silicon substrates and for highly p-type doped substrates. But the electrochemical investigations discussed above indicate that the passivation of highly doped p-type Si can be ascribed to an oxide film already present at OCP, while no such oxide film is observed on (111) silicon below PP. This supports models that ascribe the reduced chemical etch rate on (111) planes to a retarded kinetic for Si surface atoms with three backbonds, present at (111) interfaces [Gil, A12], as discussed in Section 4.1. 

The two eflPects above constitute what is called central field covalency since they aflFect both the a and the tt orbitals on the metal to the same extent. There is also, of course, symmetry restricted covalency which acts difiFerently on metal orbitals of diflFerent symmetries. This type of covalency shows up in optical absorption spectra as differences in the values of Ps and p -, as compared with 35. The first two s refer to transitions within a given symmetry subshell while 635 refers to transitions between the two subshells. This evidence of covalency almost of necessity forces one to admit the existence of chemical bonds since it is difficult to explain on a solely electrostatic model. The expansion of the metal orbitals can be caused either by backbonding to vacant ligand orbitals, or it may be a result of more or less extensive overlap of ligand electron density in the bond region. Whether or not this overlap density can properly be assigned metal 3d character is what we questioned above. At any... 

The decisive difference between, e.g., [Cp(CO)2Fe]" and H4Ta- is the smaller amount of orbital overlap of the former with the carbene 2 p orbital, resulting in less efficient transfer of electron density from the metal to C . Although Fischer-type carbene complexes are formally low valent, backbonding to the carbene is less effective than in tantalum alkylidene complexes. 

Molecular orbital theory may provide an explanation for stereochemical differences between carboxylate-metal ion and phosphate-metal ion interactions. Detailed ab initio calculations demonstrate that the semipo-lar 1 0 double bond of RsP=0 is electronically different from the C=0 double bond, for example, as found in H2C=0 (Kutzelnigg, 1977 Wallmeier and Kutzelnigg, 1979). The P=0 double bond is best described as a partial triple bond, that is, as one full a bond and two mutually perpendicular half-7r bonds (formed by backbonding between the electrons of oxygen and the empty d orbitals of phosphorus). Given this situation, a lone electron pair should be oriented on oxygen nearly opposite the P=0 bond, and these molecular orbital considerations for P=0 may extend to the phosphinyl monoanion 0-P=0. If this extension is valid, then the electronic structure of 0-P=0 should not favor bidentate metal complexation by phosphate this is in accord with the results by Alexander et al. (1990). 

Does the phosphine group function simply as a a donor, or is there a significant amount of Re 5d — P 3d 7r backbonding ... 

Re 5cf — P 3cf 7r backbonding is a minor effect compared with P 3p — Re 5d a donation. The net transfer of negative charge to PH3 largely results from Re-H — Pa donor interactions. 

Veith et al.736 have prepared the unique four-membered cyclic phosphenium ion 291. X-ray crystal structure analysis of the tetrachloroaluminate salt clearly indicates the intramolecular backbonding from ring nitrogen atoms (average N—P bond distance = 1.633 A). 

---