Phosphorus force constants

Force constant calculations for Ni(PF3)4 (190, 326) indicate that K(P-F) is similar to that in free trifluorophosphine and the metal-phosphorus force constant (about 2.4 mdyne/A), is only in the range expected for a single bond. Very recently, structural data for Ni(PF3)4 have become available from electron diffraction studies (la, 279a) (Section IX,C) and the phosphorus-fluorine bond length (1.561 ... 

A comprehensive discussion about vibrational spectra and force constants of phosphorus compounds is supplied in Ref. 124. 

Since force constants are approximately proportional to the bond order, the following comparison of molecules containing multiply bonded phosphorus is informative ... 

Table 3 Nickel-phosphorus stretching force constants for [Ni(PX3)4] complexes ...

Nitrile derivatives of the metal carbonyls have been discussed together with other nitrogen donor molecules in a number of contexts. Much of the early work has been reviewed by Manuel 337) in his article on Lewis base-metal carbonyl complexes in Volume 3 of this series, and by Stroh-meier 436) in his review of photochemical substitution reactions. In general, nitriles are weaker Lewis donors than phosphorus and nitrogen bases 436), but compared to carbon monoxide, better electron donors but poorer acceptors 427). Force constants and assignments for a series of complexes [(MeCN)jjM(CO)g J (M = Cr, Mo, W) were studied 165, 228, 296) and... 

The positive value found for PCI3 (and for other haldgenophosphines) can be understood as an inductive effect or as a very strong t bond which deshields the phosphorus atom. Many authors prefer the first explanation. In fact, in the case of PF3 the Ni-P force constant was 2.7 (206) or 2.37 mdyne/A. (123)—a rather low value which does not indicate the presence of a double bond. On the other hand, the examination of the force constants in the series Ni(CO)4 n(PF3)n (n = 0, 1, 2, 3, and 4) indicates that PF3 has about the same properties as CO (59, 123). This is supported also by the statistical distribution of the products in the exchange reaction between Ni(CO)4 and PF3 and by the little variation of NMR chemical shift on these products (56). Loutellier and Bigorgne (123) prefer to consider trifluorophosphine as a weak a donor and weak 7T acceptor for the following reasons. 

The P-F force constant is about the same in both the free and com-plexed ligand, and this could not happen if the electronic density of phosphorus were perturbed by t back donation. Besides, the correlation of Taft s a constants, with the CO stretching in the compounds Ni(CO)4 n- (PX3)n (X = Et, Ph, OR, F, C=CPh, CF3), shows that the decrease of CO with a different X is fundamentally of an inductive nature (20). 

Force constants for Zrl were estimated from those of the triiodides of phosphorus and arsenic. These were used in a valence force field calculation giving = 195, = 88, = 181, and = 65 cra". For the fluorides and chlorides of P, As, and Sb,... 

Vibrational spectra, force constants, and conformations of molecules of trivalent tricoordinated phosphorus compounds including P-heterocycles 00UK817. 

Studies of Bonding.—Further work is reported on the force constants of the PCI, PO, and PS bonds. Calculations on some compounds containing the phosphoryl bond indicate that the polarity of the PO bond is reduced by inductive donation by the remaining substituents. A number of reports of calculations on the bonding in various phosphorus halides have also been published. " The intensities of the NH fundamental band in spirophosphoranes such as (135) indicate sp character for the nitrogen atoms owing to P—N conjugation. "... 

Bonds to Carbon.—Phosphorus(jn) Compounds. Force constants, f p = 9.100 mdyn A and/ch = 6.250 mdyn A, have been calculated from an anharmonic force field for HCP. A simple preparative method for trimethyl-phosphine involves the reaction between the trichloride and methyl-lithium in ether at —78 °C and gives yields of ca. 60%. 

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