In Search of Singlet Phosphinidenes

Abstract

We have examined singlet-triplet energy separations in different phosphinidenes (RP) substituted by first- and second-row elements, making use of ab initio molecular orbital theory. Our main purpose is to find out the substituents that particularly favor the singlet electronic state. The QCISD(T)/6-311++G(3df,2p) + ZPE level has been applied to small molecules and the CISD(Q) and QCISD(T) with the 6-311G(d,p) basis set for all species considered. We have identified few factors that come into play rendering the singlet phosphinidene more stable than the triplet. The parent phosphinidene, PH, has a triplet ground state lying 28 kcal/mol below the closed-shell singlet excited state. The triplet ground state is mainly favored when negative hyperconjugation is involved. In the boryl-, alkyl-, and silyl-substituted phosphinidenes, the triplet state remains by far the ground state. When the substituents have pi-type lone pair electrons (i.e., -NX(2), -PX(2), -OX, -SX), the singlet state becomes stabilized by such an amount that both states have similar energies or even a change in ground state occurs. The most stabilized singlet ground states are attributed to PSF and PSCl. P and S have similar p-orbital sizes, making pi-delocalization easier. Implantation of alkyl and/or amino groups in the beta-position of amino- and phosphinophosphinidenes also contributes to a singlet stabilization. Bulky beta-groups also destabilize the triplet state by a steric effect. From a practical viewpoint, amino (P-NR(2)) and phosphino (P-PR(2)) derivatives bearing large alkyl groups (R) are the most plausible and feasible targets for preparing phosphinidenes possessing a closed-shell singlet ground state.

Phosphinidene Transition Metal Complexes: A Combined Ab Initio MO-DFT Study of Cr(CO)5–PR

“…The B3LYP values are consistently slightly seem to correlate with the observed π bond strengths dislarger than UMP2 values obtained in previous work. [3] cussed in the previous section and shown in Figures 4 and However, on the average, the difference in the calculated 5. As expected, the PϪCr bond becomes shorter as the π PϪR distances is only about 0.01 Å , which is too small to bond strength increases.…”

“…The charges at the Cr center are 6Ϫ31G(d,p) geometries given in ref. [3] . A negative value corresponds to a triplet ground state.…”

“…(see for instance ref. [3] ). As a consequence, the second phosThe obtained relative energies are shown in Table 3.…”

“…[16] singlet ground state. [3] The theoretical study presented here, includes full geMathey [9] [10] and Lammertsma [11] suggested from numer-ometry optimizations and high-level energy calculations on ous experimental observations of the complexed species a series of simple carbonylchromium complexes, that the phosphinidene ligand within the coordination Cr(CO) 5 ϪPR, in which R ϭ H, CH 3 , SiH 3 , NH 2 , PH 2 , OH, sphere of a metal fragment behaves as a typical singlet and SH. monovalent intermediate giving a higher and cleaner reactivity.…”

Terminal, Anionic Carbide, Nitride, and Phosphide Transition‐Metal Complexes as Synthetic Entries to Low‐Coordinate Phosphorus Derivatives

“…[78] Singlet phosphinidenes are being sought, and the phosphino substituent has been proposed as one of the most viable for their stabilization. [79] Recently the terminally bonded phosphinophosphinidene complex [(h 1 -PPtBu 2 ){Cp 2 Zr(PPhMe 2 )}] was reported as the product of reaction between zirconocene dichloride and the reagent tBu 2 P-P(SiMe 3 )Li in the presence of excess [80] The reagent tBu 2 P-P(SiMe 3 )Li joins the tBu 2 P-P= P(X)tBu 2 (X = Br or Me) family as agents effective for the delivery of the PPtBu 2 unit to metal centers, in what has been termed "phosphinophosphinidene transfer". [81][82][83] While earlier reported syntheses proceed by the delivery of an intact {P-PR 2 } fragment to a metal center, our method provides an alternative in that the PÀP bond of the complexed diorganophosphanylphosphinidene ligand is formed by the reaction of the terminal phosphide ion [PNb(NNpAr) 3 ] À with the desired chlorodiorganophosphane, ClPR 2 .…”