Bonding in the P(PH3)2 and N(PH3)2 Cations.
An ab initio Study
Wolfgang W. Schoeller
Fakultät für Chemie der U niversität, Postfach 8640, D-4800 Bielefeld 1, FR G
Z. Naturforsch. 40b, 1149-1151 (1985); received May 20, 1985
Bonding, Ab initio Double Z eta Calculations
Ab initio SCF calculations at a double-^ level were perform ed on the cations N (PH 3)2+ and
P(PH 3)2+. Although both structures are isoelectronic, their bonding situation is different. The
latter corresponds to a dipolar structure (phosphide) with considerable negative charge at the
bonding as polarization functions rather
central phosphorus atom. The d-orbitals contribute
than participate in hybridization.
While a wealth of structural data exist on the phosphonitrilic cations 1 [ 1 ], syntheses and structure
elucidations on the triphosphenyl cations 2 have
been reported only within recent years [2], In 1 the
average PN-bond distances are 1.57 Ä , i.e. consider­
ably shorter than a PN single bond. Similarly, in 2
the corresponding PP-bonds are shortened com pared
with a single bond.
1
In principle the cations 1 and 2 can be described by
the following canonical valence bond structures.
X = Nj P
2
In the present work we analyze the bonding situa­
tion in 1 and 2 with quantum chemical calculations at
an ab initio SCF level of double-^ quality. It will be
shown that both compounds, although isoelectronic,
differ in their bonding features. In contrast to 1, the
cation 2 accumulates negative charge at the central
phosphorus atom. Such an anion character is less
pronounced at the central nitrogen atom in 1 .
Results and D iscussion
Our analysis of bonding is based on the results of
ab initio SCF calculations. Details of the com puta­
tions, basis sets and their contractions are given in
the appendix. There the geometrical param eters for
the calculations are also summarized.
Verlag der Zeitschrift für Naturforschung, D -7400 Tübingen
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b
In a, a' resp., the positive charge is localized at the
term inal phosphorus atoms. Alternatively, in b nega­
tive charge is concentrated at the central atom and
com pensated for by an increase of positive charge
at the tetravalent phosphorus atoms. Hence the es­
sential difference between the two canonical struc­
tural alternatives, a (a') versus b is the negative
charge at X.
In order to reveal the relative importance of both
structural extrem es for a proper bonding description
for 1 and 2 , we perform ed quantum chemical calcula­
tions with inclusion of d-functions (polarization func­
tions) at the heavy atoms (P, N). It must be noted
that the chosen, s,p-basis set is fairly saturated, in
order to avoid overemphasis of d-orbital participa­
tion [3].
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1150
W. W. Schoeller • Bonding in the P(Ph3); and N (P H 3)2 Cations
The character (reactivity) of the PN (PP) bonds is
revealed by an inspection of the H O M O . In 2 the
1. 6 9
0. 06
(U8)
o.06
(0 .1 1 )
10
1- 0
(0 .1 1 )
charge, accompanied by an increase of positive
character of P(2), P(2') resp.
The energy of the H O M O in 2 is -1 3 .1 eV and in
1 —16.4 eV, as com pared with —10.2 eV in PH 3 and
—11.1 eV in N H 3, given a similar quality of the
wavefunction [4],
Conclusions
H3P------P ------- PH3
H2C----- CH ------CH2
(N)
electron density is almost perfectly localized at the
central phosphorus atom. Of the two electrons in this
m olecular orbital, 1.7 electrons (M ulliken gross
atomic population) are placed at the central position.
This refers to 85% of the electron density in the
frontier orbital placed at the central position. For 1
(values in parentheses) a slight delocalization of elec­
tron density to the peripheral PH 3 groups is ob­
served. H ere 64% of the electron density in the
H O M O are at N. For comparison in the nonbonding
m olecular orbital of an allyl system electron density
is exclusively concentrated at the term inal positions.
A detailed Mulliken population analysis of 1 and 2
is summarized in Table I. According to these data
the populations of the d-orbitals at P(2), P (2') resp.,
are negligible small. This indicates that these orbitals
act as polarization functions, instead of being in­
volved in hybridization. Since N is more electronega­
tive than P (l). it carries more negative overall
Table I. Mulliken gross atomic populations for the cations
1 and 2 , net charges are in parentheses.
Total density (net charges)
1
N(P)
P(2)
H(3)
H(4)
s
P
d
Z
s
P
d
I
2
3.63
4.42
0.02
8.06 (-1 .0 7 )
5.23
8.45
0.30
13.97 (1.03)
5.83
9.28
0.09
15.20 (-0 .2 0 )
Appendix
Gaussian basis sets were constructed from lobe
functions [10] and are collected in Table II. The basis
contains a 10s6p set for each phosphorus atom. The
orbital exponents for the polarization functions (d at
P. N; p at H) were chosen according to Ahlrichs etal.
[11]. Population analyses were perform ed according
to Mulliken [12] on the delocalized orbitals. The inTable II. Basis set for the ab initio calculations.
5.39
8.93
0.16
14.48 (0.52)
Atom
Primitive set
C ontraction
P
N
H
10s6 p
(4 .6 x 1 /3 .3 x 1 )
(4 .4 x 1 /2 .2 x 1 )
(3.1)
P
N
H
s
P
I
0.98
0.03
1.02 ( - 0 .0 2 )
0.96
0.03
0.99 (0.01)
s
P
0.96
0.03
0.99 (0.01)
0.93
0.03
0.97 (0.04)
2
In the traditional picture of bonding in phosphoranes a shortening of the bonds involving the
heavy atoms towards phosphorus is accounted for by
d-orbital participation, i.e. back donation towards
phosphorus. In 2 and to a lesser extent in 1, the dorbitals act as polarization functions rather than be­
ing involved in hybridization. In this respect our
analysis is in full accord with previous quantum
chemical studies of bonding in methylenephosphoranes [5], oxophosphoranes [6 ], and bisim inophosphoranes [7].
The H O M O of 2 clearly establishes its character as
a phosphide, as described by valence bond structure
b. The delocalization of the p-orbital at the central
atom is small, but slightly more so in 1. On this basis
the observed shortening in 1 and 2 can be attributed
to the extra ionic resonance energies [8 ] of the PN
(PP) bonds. The results of our investigations are in
conformity with a X-ray photoelectron spectroscopic
study on bis(triphenylphosphine)iminium salts [9].
8 s4 p
4s
ld
ld
IP
Exponent Ref.
[a]
[a|
[bj
0.50
0.95
0.65
[a] S. H uzinaga, A pproxim ate Atomic Functions, U ni­
versity of A lberta. C anada 1971; [b] S. Huzinaga. J. Chem.
Phys. 42, 1293 (1965).
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W. W. Schoeller ■Bonding in the P (Ph3)2 and N (P H 3)2 Cations
1151
Table III. Structural param eters, bond lengths (in Ä) and bond angles (in °), for the N (PH 3) 2 and P(PH 3) 2 cations.
Structure
X( 1) P(2)
P(2)H (3)
P(2) H(4)
< P (2 ')X P (2 )
< H (3 )P (2 )X
< H (4 )P (2 )X
1
2
1.577
2.133
1.402
1.402
1.402
1.402
138.0
102.6
118.0
118.0
118.0
118.0
vestigated geometries and the coordinate system are
described in Fig. 1.
The geometrical param eters were chosen accord­
ing to the reported [lb ] structural values. Energy
optimization of the structures was not attem pted.
The structural param eters are sum m arized in
Table III. For the com putations of the parent com ­
pounds 1 and 2 C2v symmetry was imposed. The re­
sulting SCF energies are (in a. u .’s): for 1 —739.00262
and for 2 —1025.18315.
[1] a) R. A ppel and A. H auss, Z. Anorg. Allg. Chem.
311, 290 (1961); J. K. Ruff, Inorg. Chem . 7, 1821
(1968);
b) for a review on the structures see D. E. C. Corbridge. The Structural Chemistry of Phosphorus,
Elsevier, Scientific Publishing Company, A m sterdam
1974, pp. 356;
c) P. C. Steinhardt, W. L. G ladfelter, A. D. H arley, J.
R. Fox, and G. L. G eoffroy, Inorg. Chem . 19, 332
(1980); G. R. Steinmetz, A. D. H arley, and G. L.
G eoffroy, Inorg. Chem. 19, 2985 (1980); C. R. Eady,
P. F. Jackson, B. G. F. Johnson, J. Lewis, M. C.
M alatesta, M. M cPartlin, and W. J. H. Nelson, J.
Chem. Soc. Dalton Trans. 1980, 383; A. Schm idpeter,
K.-H. Zirzow, G. B ürget, G. H uttner, and I. Jibril,
Chem. Ber. 117, 1695 (1984);
d) for linear rather than bent PNP structures see R. D.
Wilson and R. Bau, J. Am. Chem. Soc. 96, 7601
(1974); S. W. Kirtley, J. P. Chanton, R. A . Love, D.
L. T ipton, T. N. Sorrell, and R. Bau, J. A m . Chem.
Soc. 102, 3451 (1980).
[2] A. Schmidpeter, S. Lochschmidt, and W. S. Sheldrick,
Angew. Chem. 97, 214 (1985); Angew. C hem ., Int.
Ed. Engl. 24, 226 (1985); A. Schm idpeter. S.
Lochschmidt, and W. S. Sheldrick, Angew. Chem. 94,
72 (1982); Angew. C hem ., Int. Ed. Engl. 21, 63
X = N, P
Fig. 1. Definition of geometry param eters for N (PH 3) 2 and
P (P H 3) 2 cations. C2v symmetry was assumed.
(1982); A. Schm idpeter, S. Lochschmidt, and A. Willhalm , Angew. Chem. 95, 561 (1983); Angew. C hem .,
Int. Ed. Engl. 22, 545 (1983); Angew. Chem. Suppl.
710 (1983).
[3] a) The role of d-orbitals in bonding at phosphorus is a
perpetual problem . A very detailed discussion on this
problem has been given by
b) H. W allm eier and W. Kutzelnigg, J. Am. Chem.
Soc. 101, 2804 (1979).
[4] Ref. [3b], Table XI.
[5] H. Lischka, J. Am. Chem. Soc. 99, 353 (1977).
[6 ] J. D em uynck and A. Veillard, Chem. Commun. 1970,
873; A. Serafini, J. F. Labarre, A. Veillard, and G.
V inot, Chem. Commun. 1971, 996; ref. [3b].
[7] W. W. Schoeller and C. Lerch, Inorg. C hem ., in
press.
[8 ] L. Pauling, The N ature of the Chemical Bond, Cornell
U niversity Press, Ithaca, N. Y. 1960.
[9] W. E. Swartz (Jr.), J. K. Ruff, and D. M. H ercules, J.
Am. Chem . Soc. 94, 5227 (1972).
[10] The com puter program is described by R. Ahlrichs,
T heor. Chim. A cta 33, 157 (1974).
[11] R. A hlrichs, H. Lischka, V. Staem m ler, and W. K ut­
zelnigg, J. Chem. Phys. 62, 1225 (1975).
[12] R. S. M ulliken, J. Chem. Phys. 23, 1833, 2343 (1955).
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