IR and NMR properties of N-base:PH2F:BeX2 ternary and corresponding binary complexes stabilised by pnicogen and beryllium bonds

Ab initio MP2/aug’-cc-pVTZ calculations have been performed to determine selected stretching frequencies and chemical shieldings for ternary complexes N-base:PH2F:BeX2 and the corresponding binary complexes, with NH3, H2C=NH, and HCN as the nitrogen bases and H, F, and Cl as the substituents X. Be-F and P-F stretching frequencies depend on the Be-F and P-F distances, respectively, while P and F chemical shieldings depend on the N-P and P-F distances, respectively. The graph of the P chemical shieldings versus the P-F distance bears a remarkable resemblance to the graph of the P-F stretching frequencies versus that same distance. EOM-CCSD spin–spin coupling constants have also been evaluated for binary and ternary complexes. 1pJ(N-P) is negative at the longer N-P distances found in ternary complexes with HCN in which the N  …  P bond is a traditional pnicogen bond with some phosphorous-shared character, gains phosphorus-shared character as the N-P distance continues to decrease, and then becomes a phosphorous-transferred bond with ion-pair character at the shorter distances in the complexes with NH3 and H2C=NH. 1J(P-F) values are large and negative in complexes with HCN, but increase and become positive in complexes with H2C=NH and NH3 as the P-F distance increases. GRAPHICAL ABSTRACT


Introduction
In a recent study [1], we reported the structures, binding energies, and bonding properties of two series of binary complexes represented as N-base:PH 2 F and PH 2 F:BeX 2 , and the corresponding ternary complexes N-base:PH 2 F:BeX 2 , with NH 3 , HN= CH 2 , and NCH as the nitrogen bases, the phosphorous molecule fluorophosphane PH 2  BeF 2 , BeCl 2 , BeCO 3 , and BeSO 4 . Two equilibrium isomers labelled A and B were found on the potential surfaces having BeH 2 , BeF 2 , and BeCl 2 as the acids, and these are illustrated in Figure 1 for NH 3 :PH 2 F:BeH 2 . In isomer A, the BeH 2 molecule lies in the symmetry plane of the complex, with the F ... Be bond cis to the bisector of the H-P-H angle with respect to the P-F bond. Isomer B has no symmetry in point group C 1 , since BeH 2 assumes what might be described as a sideways approach to F relative to the plane defined by the bisector of the H-P-H angle and the P-F bond. The presence of the N ... P pnicogen bond and the Be ... F beryllium bond in the ternary complexes leads to large, synergistic cooperative effects on the binding energies and the geometries of these complexes. In ref. 1 it was concluded that as the strength of the nitrogen base as an electron-pair donor and the beryllium acid as an electron pair acceptor increases, the nature of the pnicogen bond changes from traditional, to phosphorus-shared, to phosphorus-transferred. Significant changes are also found for the Be ... F and P-F bonds in the ternary complexes.
In previous studies we have demonstrated that spin-spin coupling constants across hydrogen [31], halogen [32], chalcogen [33], pnicogen [34], and tetrel bonds [35] provide a fingerprint of the nature of the intermolecular bond in a related series of complexes. These coupling constants increase in absolute value as the intermolecular distance decreases in complexes with traditional intermolecular bonds, may reach a maximum as the bond length decreases and the bonds become atomshared bonds, and then further decreases as atom transfer occurs. As a follow-up to ref. 1, we have now carried out EOM-CCSD calculations to evaluate spin-spin coupling constants 1p J(N-P) across the pnicogen bond, 1be J(Be-F) across the beryllium bond, and 1 J(P-F) in the equilibrium ternary complexes N-base:PH 2 F:BeX 2 isomers A and B and the corresponding binary complexes N-base:PH 2 F and PH 2 F:BeX 2 . We have also examined computed IR vibrational frequencies and chemical shieldings in both binary and ternary complexes. It is the purpose of this paper to report these NMR and IR properties, and to interpret them in light of the changing nature of the intermolecular N ... P and F ... Be bonds and the covalent P-F bond of fluorophosphane.

Methods
The structures of the isolated monomers; the binary complexes N-base:PH 2 F and PH 2 F:BeX 2 ; and the ternary complexes N-base:PH 2 F:BeX 2 , for N-base equal to NH 3 , H 2 C = NH, and HCN, and BeX 2 equal to BeH 2 , BeF 2 , and BeCl 2 for structures A and B, had been optimised previously at second-order Møller-Plesset perturbation theory (MP2) [36][37][38][39] with the aug'-cc-pVTZ basis set [40]. These structures are reported in ref. 1. Vibrational frequencies were computed to ensure that these structures are equilibrium structures on their potential energy surfaces, and to examine the variation in selected stretching frequencies in the binary and ternary complexes. MP2/aug'-cc-pVTZ absolute chemical shieldings have also been calculated using the GIAO approximation [41]. These calculations were carried out with the Gaussian 16 programme [42].
Spin-spin coupling constants for these complexes have been evaluated using the equation of motion coupled cluster singles and doubles (EOM-CCSD) method in the CI (configuration interaction)-like approximation [43,44] with all electrons correlated. For these calculations, the Ahlrichs [45] qzp basis set was placed on 13 C, 15 N, and 19 F, the qz2p basis set on 31 P and 35 Cl, and the hybrid basis set introduced previously on 9 Be [46]. The Dunning cc-pVDZ basis was placed on 1 H atoms. All terms that contribute to the total coupling constant, namely, the paramagnetic spin orbit (PSO), diamagnetic spin orbit (DSO), Fermi contact (FC) and spin dipole (SD) have been evaluated for the binary complexes and ternary complexes with C 2v symmetry. Computing the SO and SD terms was not feasible for some of the larger ternary complexes with C s symmetry. For three of these we have estimated the values of 1p J(N-P) and 1be J(Be-F) from the computed FC terms and values of the SO and SD terms taken from similar complexes. Coupling constant calculations were performed using ACES II [47] on the HPC cluster Owens at the Ohio Supercomputer Center. Table 1. P-F stretching frequencies and distances, and intermolecular Be-F and P-N stretching frequencies [ν(X-Y), cm -1 ] and X-Y distances [R(X-Y), Å] in binary and ternary complexes.  Table 1 provides the P-F, Be-F, and P-N stretching frequencies and corresponding bond distances in binary and ternary complexes. The P-N stretching frequencies range from 107 cm -1 at a P-N distance of 2.77 Å in the binary complex HCN:PH 2 F, to 310 cm -1 at a P-N distance of 2.10 Å in the ternary complex H 3 N:PH 2 F:BeCl 2 A. These frequencies are intermolecular stretching frequencies across the P ... N pnicogen bond. As expected, the P-N distances are longest in the binary complexes and the ternary complexes HCN:PH 2 F:BeX 2 , and these complexes have the smallest P-N stretching frequencies.

Stretching frequencies in binary and ternary complexes
The P-N distances are shorter in the ternary complexes with NH 3 and H 2 C = NH as the nitrogen bases, and these complexes have greater stretching frequencies. The P-N stretching frequencies exhibit a second order dependence on the charge on the nitrogen base in the ternary complexes, with a correlation coefficient of 0.986. However, these frequencies are relatively small and should be significantly anharmonic. They will not be discussed further in this paper.
There are two bonds of interest which have greater stretching frequencies, namely, the intermolecular Be ... F bond and the intramolecular P-F bond. The data of Table 1 show that the Be-F stretching frequencies across the Be ... F bond vary from 335 cm -1 in the binary complex H 2 PF:BeH 2 A to 777 cm -1 in the ternary complex H 2 C = (H)N:PH 2 F:BeCl 2 B. In the two series of ternary complexes with the strongest nitrogen bases, Be-F stretching frequencies increase with respect to the substituent in the order F < H < Cl. In the binary complexes, this order is H < F < Cl, with the frequencies in the A isomer lower than those in the corresponding B isomer.
The most interesting relationship is found for the Be-X stretching frequencies in the binary and ternary complexes when these are analysed as a function of the substituent X, as illustrated in Figure 2. For each BeX 2 acid, the Be-F stretching frequency across the beryllium bond increases linearly as the Be-F distance decreases. The correlation coefficients of the trendlines are greater than 0.990. The trendlines for the complexes with BeH 2 and BeCl 2 are essentially parallel, while that for the complexes with BeF 2 has a reduced slope. There are eight points for each set of complexes, but values of the stretching frequencies for ternary complexes with NH 3 and H 2 C = NH as the nitrogen bases and BeF 2 and BeCl 2 as the beryllium acids may have values for the A and B isomers which are indistinguishable in the graph. Table 1 also presents the P-F stretching frequencies and the P-F distances in the monomer PH 2 F, the binary complexes N-base:PH 2 F and PH 2 F:BeX 2 , and the ternary complexes N-base:PH 2 F:BeX 2 . The P-F stretching frequency in the monomer PH 2 F is 808 cm -1 . Complex formation reduces the P-F stretching frequency, which has its smallest value of 406 cm -1 in the ternary complex with H 2 C = NH as the base and Cl as the substituent, and its largest value of 756 cm -1 in the binary complex with HCN as the base. From Table 1, it is apparent that the P-F stretching frequencies depend on the nature of the complex, that is, whether it is a binary complex with PH 2 F as the electron-pair donor or acceptor, or a ternary complex with HCN, H 2 C = (H)N, or NH 3 as the nitrogen base. Thus, the order of decreasing P-F stretching frequencies is PH 2 F > N-base:PH 2 F > PH 2 F:BeX 2 > HCN:PH 2 F:BeX 2 > H 3 N:PH 2 F:BeX 2 ≈ H 2 C = (H)N:PH 2 F:BeX 2 .
There is also some dependence on the nature of X in these complexes. These relationships can be readily seen in Figure 3 which is a plot of the P-F stretching frequencies versus the P-F distance. The third-order trendline has a correlation coefficient of 0.956.   Table 2 presents the N, P, F, and Be chemical shieldings in the binary and ternary complexes. The Be chemical shieldings in the binary complexes PH 2 F:BeX 2 are 87, 110, 100 ppm for X = H, F, and Cl, respectively. In the ternary complexes N-base:PH 2 F:BeX 2 for X = H, these shieldings increase to about 95 ppm. Smaller increases of no more than 3 ppm are found in the ternary complexes with X = F and Cl. Thus, the formation of ternary complexes has little effect on these Be chemical shieldings. However, the 11 Be chemical shieldings in the binary and ternary complexes do exhibit a second order correlation with the charge on 11 Be, with a correlation coefficient of 0.98.

Chemical shieldings in binary and ternary complexes
The N chemical shieldings in H 3 N:PH 2 F, H 2 C(H)N: PH 2 F, and HCN:PH 2 F have values of 267, -44, and +16 ppm. These shieldings decrease upon the formation of ternary complexes with NH 3 , but vary over a narrow range from 247 to 251 ppm. The chemical shieldings increase to between -9 and -22 ppm when H 2 C = NH is the base, and to between 26 and 34 ppm when HCN is the base. Once again, these are relatively small changes associated with the formation of ternary complexes. Much greater changes upon binary and ternary complex formation are found for the chemical shieldings of the P and F atoms of PH 2 F. The chemical shielding of the P atom in the isolated monomer is 273 ppm. The shieldings increase to between 305 and 350 ppm in the binary complexes with the nitrogen bases, but decrease to between 206 and 230 ppm in the binary complexes with the beryllium acids. The values of the P chemical shieldings in the ternary complexes HCN:PH 2 F:BeX 2 are small relative to the values in the complexes with NH 3 and H 2 C = NH, as can be seen in Table 2. In the latter complexes, the P chemical shieldings increase dramatically with values between 394 ppm in the H 2 C = (H)N:PH 2 F:BeH 2 A complex and 441 ppm in the H 3 N:PH 2 F:BeCl 2 B complex. It is interesting to note that the P shieldings in ternary complexes with NH 3 as the base are greater than the shieldings in corresponding complexes with H 2 C = NH as the base, even though the P-N distance is longer in the complexes with NH 3 . Figure 4 presents a plot of the P chemical shieldings versus the N-P distance which has a second-order trendline with a correlation coefficient of 0.890. This trendline demonstrates the relationships among the P chemical shieldings in ternary and binary complexes.
In contrast to the P chemical shieldings, the F chemical shieldings decrease upon complex formation. Among the binary complexes the smallest chemical shielding of 426 ppm is found for H 2 C = (H)N:PH 2 F and the largest with a value of 490 ppm is found for the PH 2 F:BeF 2 A and B isomers. The F chemical shieldings decrease further in the ternary complexes, ranging from 320 ppm in H 2 C = (H)N:PH 2 F:BeCl 2 A to 416 ppm in HCN:PH 2 F:BeF 2 B. Corresponding ternary complexes with NH 3 and H 2 C = NH have smaller, similar values. These changes are illustrated in Figure 5 in a plot of the F chemical shieldings versus the P-F distance. The trendline has a correlation coefficient of 0.861. Figure 5 presents an excellent representation of the variation of the F chemical shieldings from the PH 2 F monomer, to the binary complexes with the nitrogen bases and the beryllium acids, and then to the ternary complexes. It is also interesting to compare this graph of the F chemical shieldings versus the P-F distance to Figure 3 which illustrates the P-F stretching frequencies versus the P-F distance. Except for the points for the binary complexes N-base:PH 2 F, these two graphs look remarkably alike. Thus, they emphasise what is an expected result, namely, that as the P-F distance decreases, the P-F stretching frequencies as well as the F chemical shieldings increase.

Spin-spin coupling constants for binary and ternary complexes
Table S1 of the Supplemental Data provides the components of 1p J(N-P) and 1 J(P-F) for the binary complexes H 3 N:PH 2 F, H 2 C = (H)N:PH 2 F, and HCN:PH 2 F; and 1 J(P-F) and 1be J(Be-F) for the binary complexes PH 2 F:BeX 2 structures A and B, with X = H, F, and Cl. Table S2 provides corresponding data for the ternary complexes H 3 N:PH 2 F:BeX 2 , H 2 C = (H)N:PH 2 F:BeX 2 , and HCN:PH 2 F:BeX 2 . When analysing these data, it should be kept in mind that the magnetogyric ratios of 9 Be and 15 N are negative, while those of 19 F and 31 P are positive. The data of Table S1 show that the FC term is a good approximation to 1p J(N-P) for the binary complexes with the three nitrogen bases, but is not a good approximation to 1 J(P-F) since both the PSO and SD terms are non-negligible, as is usually the case for coupling constants involving F [48]. The FC term is a fairly good approximation to 1be J(Be-F) since there is a partial cancellation between the negative DSO and SD terms and the positive PSO term, but cancellation is not something that can be relied on. Table S2 provides the components of coupling constants for the ternary complexes. For these, the FC term is generally not a good approximation to total J, except for 1p J(N-P) in complexes with HCN as the nitrogen base. Only total J values will be discussed in this paper.   Table 3 presents P-F and P-N distances and the corresponding coupling constants 1 J(P-F) and 1p J(P-N) for the binary complexes H 3 N:PH 2 F, H 2 C = (H)N:PH 2 F, and HCN:PH 2 F. Table 3 also presents P-F and Be-F distances and 1 J(P-F) and 1be J(Be-F) coupling constants for the complexes PH 2 F:BeH 2 , PH 2 F:BeF 2 , and PH 2 F:BeCl 2 structures A and B. For the complexes H 3 N:PH 2 F, H 2 C = (H)N:PH 2 F, and HCN:PH 2 F, 1 J(P-F) for the P-F covalent bond is negative as it is in the PH 2 F monomer, even though both P and F have positive magnetogyric ratios. Thus, the reduced coupling constant 1 K(P-F) would also be negative. This is another example of a violation of the Dirac Vector Model [49] which states that all reduced one-bond coupling constants are positive.

Coupling constants for binary complexes.
As noted above, values of 1 J(P-F) are negative in the N-base:PH 2 F complexes, and increase in absolute value as the P-F distance decreases, as illustrated in Figure 6. The linear trendline has a correlation coefficient of 0.995. Different patterns are observed for this coupling constant  Table 3 that 1be J(Be-F) for coupling across the beryllium bond depends on the substituent X of BeX 2 , and to a much lesser extent on the isomer A or B. Complexes with X = F have small negative values of 1be J(Be-F), while those with X = Cl have small positive values. 1be J(Be-F) values are much greater and positive when X = H. In addition, for a fixed X, isomer A has the larger coupling constant even though the Be-F distance is longer in A than in B. Table 4 reports the N-P, Be-F, and P-F distances and the corresponding coupling constants 1p J(N-P), 1be J(Be-F), and 1 J(P-F) for the ternary complexes NH 3 :PH 2 F:BeX 2, H 2 C = (H)N:PH 2 F:BeX 2 , and HCN:PH 2 F:BeX 2 . Total J values 1p J(N-P) and 1be J(Be-F) for the B isomers of NH 3 :PH 2 F:BeCl 2 , H 2 C = (H)N:PH 2 F:BeCl 2 , and HCN: PH 2 F:BeCl 2 were approximated based on the computed value of the FC term as modified to take into account the contributions of the SO and SD terms. How this was done can be seen from the footnotes in Table 4 and the values of these terms in related complexes, which are found in Table S2. The three coupling constants 1p J(N-P), 1be J(Be-F), and 1 J(P-F) will be discussed in the following three sections. Table 4 provides N-P distances and coupling constants 1p J(N-P) across the pnicogen bond for the ternary complexes. 1p J(N-P) values are always negative, varying from -73 Hz at an N-P distance of 2.23 Å in H 3 N:PH 2 F:BeH 2 B, to -27 Hz at a N-P distance of Table 4. Distances (R, Å) and spin-spin coupling constants 1p J(N-P), 1 J(P-F), and 1be J(Be-F) (Hz) for ternary complexes NH 3 :PH 2 F:BeX 2 , H 2 C = (H)N:PH 2 F:BeX 2 , and HCN:PH 2 F:BeX 2 NH3 R(N-P) 1p J(N-P) R(P-F) 1 J(P-F) R(Be-F) 1be J(Be-F)  The variation of 1p J(N-P) as a function of the N-P distance is illustrated in Figure 7. The vertical axis has been reversed to reflect the negative magnetogyric ratio of 15 N. The behaviour of 1p J(N-P) as a function of the N-P distance can be interpreted from Figure 7 in terms of the changing nature of the pnicogen bond in these complexes. At the longest N-P distances are the complexes with HCN as the base. These complexes are stabilised by traditional pnicogen bonds that have significant phosphorus-shared character. As the N-P distance decreases, 1p J(N-P) becomes less negative as the N ... P bond in the complexes with NH 3 as the base becomes a phosphorus-shared pnicogen bond with some phosphorus-transferred character. At even shorter distances, the N ... P bond becomes a phosphoroustransferred bond in the complexes with H 2 C = NH. The complex with the greatest degree of phosphoroustransferred character is H 2 C = (H)N:PH 2 F:BeF 2 A. It should be noted that it is unusual for the weak base HCN to form complexes with a high degree of phosphorus-shared character as it does in the HCN:PH 2 F:BeX 2 complexes. This may be attributed to cooperativity between the beryllium bond and the pnicogen bond.

1be J(Be-F)
The Be-F distances and coupling constants 1be J(Be-F) for the ternary complexes are reported in Table 4. The values of this coupling constant range from -24 Hz at a Be-F distance of 1.616 Å in H 2 C = (H)N:PH 2 F:BeF 2 A and B, to 34 Hz at a Be-F distance of 1.704 Å in the HCN:PH 2 F:BeH 2 A complex. Figure 8 provides graphical insight into the variation of this coupling constant as functions of the Be-F distance, the substituent X, and the nitrogen base.
From Table 4 and Figure 8 it can be seen that in the ternary complexes which have H as the substituent, 1be J(Be-F) is positive, independent of the nature of the nitrogen base and independent of the structure A or B. Among these complexes, the longest distance is found in HCN:PH 2 F:BeH 2 A, with 1be J(Be-F) equal to 34 Hz. The shortest Be-F distance is 1.616 Å in H 2 C = (H)N:PH 2 F:BeF 2 A and B, with 1be J(Be-F) equal to -24 Hz. 1be J(Be-F) is negative in complexes H 3 N:PH 2 F:BeX 2 , H 2 C = (H)N:PH 2 F:BeX 2 , and HCN:PH 2 F:BeX 2 when X is F or Cl, independent of the conformation A or B. Among these, the longest Be-F distance of 1.68 Å is found in HCN:PH 2 F:BeF 2 A, with a value of -13 Hz for 1be J(Be-F). The shortest Be-F distance of 1.57 Å is found in NH 3 :PH 2 F:BeCl 2 A which has 1be J(Be-F) equal to -18 Hz. The smallest negative value of -6 Hz for 1be J(Be-F) is found for HCN:PH 2 F:BeCl 2 structure A at a Be-F distances of 1.62 Å.
1be J(Be-F) for the ternary complexes may be compared to 1be J(Be-F) for the binary complexes PH 2 F:BeX 2 . When X = H, 1be J(Be-F) for the binary complexes are positive, with values for structures A and B greater than the values of 1be J(Be-F) for the corresponding ternary complexes A and B. When X = F, 1be J(Be-F) for the binary complexes are small and negative, but when X = Cl, they are small and positive. In the ternary complexes, 1be J(Be-F) values are always negative in complexes with F and Cl as the substituents. In these complexes, the presence of the nitrogen bases significantly changes the electron densities at Be and F in the ternary complexes, thereby dramatically changing the values of 1be J(Be-F).

1 J(P-F)
Coupling constants 1 J(P-F) and P-F distances for the ternary complexes are reported in Table 4. When HCN is the nitrogen base, 1 J(P-F) values range from -365 Hz  respectively. When X = Cl, 1 J(P-F) increases to 187 Hz at a P-F distance of 1.90 Å in isomer A. Thus, the value of 1 J(P-F) for H 3 N:PH 2 F:BeCl 2 A is large and positive, in contrast to 1 J(P-F) values which are large and negative for the binary complexes and the other ternary complexes with NH 3 as the base. This difference arises from the small positive value of 15 Hz for the FC term in H 3 N:PH 2 F:BeCl 2 A, compared to the large negative values of this term in the binary complexes and other ternary complexes with NH 3 , as can be seen in Tables S1 and S2.
The changing values of 1 J(P-F) as functions of the P-F distance and the nitrogen base are evident from Figure 9. The complexes with HCN as the nitrogen base are set apart with values of 1 J(P-F) that are large and negative at the shortest P-F distances. As the base changes to H 2 C = NH and NH 3 , the values of 1 J(P-F) increase and become positive as the P-F distance increases. There is considerable overlapping of the points representing complexes with H 2 C = NH and NH 3 . At the longest P-F distance is the large and positive value of 187 Hz for 1 J(P-F) in the complex with NH 3 as the base and Cl as the substituent. The trend of increasing 1 J(P-F) with increasing P-F distance is clearly indicated by the second-order trendline in Figure 9 which has a correlation coefficient of 0.990. A comparison between 1p J(P-F) values for complexes with NH 3 and H 2 C = NH as the nitrogen bases suggests that if it were possible to compute coupling constants for complexes H 2 C = (H)N:PH 2 F:BeCl 2 , these would have the largest positive values at the longest P-F distances of 1.92 Å for isomers A and B.

Conclusions
Ab initio calculations have been performed to evaluate IR stretching frequencies, NMR chemical shieldings, and NMR spin-spin coupling constants involving the atoms N, P, F, and Be in ternary complexes N-base:PH 2 F:BeX 2 and the corresponding binary complexes N-base:PH 2 F and PH 2 F:BeX 2 . The nitrogen bases are NH 3 , H 2 C = NH, and HCN, and the substituents X are H, F, and Cl. The main results of this study are summarised in the following points.
1. Intermolecular Be-F stretching frequencies in binary and ternary complexes depend on the substituent X. For each substituent, the Be-F stretching frequencies exhibit a linear dependence on the intermolecular Be-F distance.
2. P-F stretching frequencies show a third-order dependence on the P-F distance in binary and ternary complexes.
3. P chemical shieldings exhibit a second-order dependence on the N-P distance, while F chemical shieldings show a third-order dependence on the P-F distance. The graph illustrating the dependence of the F shieldings on the P-F distance bears a remarkable resemblance to the graph of the P-F stretching frequencies versus the P-F distance.
4. Values of 1p J(N-P) in the ternary complexes are always negative. At the longest N-P distances in complexes with HCN, 1p J(N-P) values indicate that the N ... P bond has some phosphorus-shared character. As the N-P distance shortens, 1p J(N-P) begins to decrease in absolute value as the pnicogen bond becomes a phosphorous-shared bond with some phosphoroustransferred character. At the shortest N-P distances in complexes with NH 3 and H 2 C = NH as the bases, 1p J(N-P) further decreases in absolute value as the N ... P bond becomes a phosphorous-transferred bond with ion-pair character.
5. Values of 1be J(Be-F) are positive in binary complexes and in ternary complexes with BeH 2 as the acid, independent of the nature of the nitrogen base and the structure of the isomer A or B. Values of this coupling constant are always negative when BeF 2 and BeCl 2 are the acids.
6. 1 J(P-F) values are large and negative in complexes with HCN as the nitrogen base. In the complexes with H 2 C = NH and NH 3 , 1 J(P-F) increases and becomes large and positive as the N-P distance increases. There is an excellent correlation between increasing N-P distance and increasing 1p J(N-P).