Trigonal planar phosphorus

Note The geometry around phosphorus is an important factor influencing the chemical shift value. Trigonal planar phosphorus atoms resonate downfield from... 

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 most interesting results have been obtained using cationic Lewis aeids. Indeed, in 1989 we showed that addition of trimethylsilyl triflate to the carbene 17 led to the so-called methylenephosphonium cation 36, which was the first example of a stable trieoordinated phosphorus cation. [53] This compound features a short phosphorus-carbon double bond (1.62 A) and trigonal planar phosphorus and carbon centers, as expeeted for a eompound isovalent and isoeleetronie with olefins however, the double bond is dramatically twisted (60°). 

This extremely air-sensitive compound, which is valence isoelectronic to an olefin, has been structurally characterized by X-ray diffraction. It has a short carbon-phosphorus double bond (1.62 A) the phosphorus and carbon atoms adopt a trigonal planar geometry with a dihedral angle of 60° (Fig. 3). This value is significantly larger than that reported for the most crowded olefin.61 Formally, this compound can be viewed as the product of a car-bene-carbenoid coupling between bis(trimethylsilyl)carbene and bis(diiso-propylamino)phosphenium triflate. Note that another route to methylene-phosphonium salt has been reported by Griitzmacher et al.62... 

The deshielded 31P and 13C NMR chemical shifts observed for 41-43 ( 31P +130, 13C +76, /PC 85 Hz) are consistent with the presence of a P = C double bond and positive charge development at phosphorus these spectroscopic data are in fact very similar to those observed for the methylenephosphonium salt 40.59,6° This similarity is reinforced by the X-ray analysis of the gallium adduct 42 (Fig. 4) (i) The phosphorus and carbon atoms adopt a trigonal planar geometry, (ii) there is a twist angle between the two planes of 34.1°, and (iii) the phosphorus-carbon bond distance is rather short (1.61 A). 

There is a recent review of two-coordinate phosphorus complexes.306 Malisch et a/.307 observed the reversible reaction (86), in which a metal-arsenic(III) double bond is formed, i.e. the M—As a bond is augmented by the arsenic lone pair to form a n bond system (since the cyclopentadienyl coligand is not coplanar with the M=As, the arsenic double bond is isolated). Complex (58) undergoes reactions typical of double bond molecules (Scheme 14). Phosphorus analogues have also been prepared (Scheme 15) the crystal structure of product (c) in Scheme 15 has been solved (59a). The d(W—P) of 2.181 A is shorter than the predicted rf(W=P) of 2.26 A, and the trigonal planar coordination of phosphorus indicates sp2 hybridization.308... 

An even broader structural variety has been evidenced for ethylene-bridged PBs. As for methylene-bridged systems, the presence of n-donat-ing substituents at boron (such as catechol or pinacol) prevents intramolecular P-B interaction and leads to linear monomeric structures (compounds 40g and 40h).56 This is clearly apparent in solution from the 3IP and 11B NMR data, and was further confirmed crystallographi-cally for 40h (Figure 5). The environment around boron remains trigonal planar and the anti arrangement of the PCH2CH2B skeleton maintains the distal phosphorus and boron atoms far away from each other (4.231(2) A). 

The structure of 70 was established by X-ray diffraction analysis (Figure 22). The palladium center is only coordinated by the phosphorus atom of 32 (the sulfur atom remains pendant) and the dvds coligand (r 2 r 2 fashion), resulting in a slightly distorted trigonal planar geometry around the metal. The rigid thioxanthene linker maintains the boron atom remote from the coordination sphere of the metal. 

BC13 has a hexagonal bimolecular cell, Cj , P63, with a = 6.08 and c = 6.55 A. The trigonal planar molecules are in A and B positions as for SbCls and Sb(CH3)3Br2 (Figures 4.26 and 4.27). All other boron halides are also planar. Phosphorus trihalides, like NH3 and nitrogen halides, are pyramidal because of the lone electron pair on P and N. 

There are unusual compounds in which the elements, formally in the I oxidation state, form trigonal planar compounds of type (10-11) dn-pn interaction is doubtless involved in the bonding. In another (10-III) the phosphorus is formally pentavalent. 

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