Chemistry Letters 2001
1069
In the structure of cis-[1]BF4 (Figure 2(a)), two phenyl
rings of PHPh2 are oriented in the way that the intramolecular
steric interaction between two adjacent PHPh2 ligands is mini-
mized. The stacking interactions between the dtc plane and one
of the phenyl rings of PHPh2 were also indicated. The average
Co–P bond length in cis-[1]BF4 is 2.230 Å, which is shorter
7
than that (2.272 Å) in cis-[Co(dtc)2(PMe2Ph)2]PF6 having
PMe2Ph with a smaller steric requirement (Tolman’s cone
angle, θ = 122°)10 and a stronger σ-donicity (χd = 10.60) than
PHPh2 (θ = 126°, χd = 17.35). The corresponding Co–P bond
lengths in the analogous PMe3 (θ = 118°, χd = 8.55) and
P(OMe)Ph2 (θ = 132°, χd = 14.82) complexes are 2.200 and
2.245 Å, respectively.7 Furthermore, the P(1)–Co–P(2) angle in
cis-[1]BF4, 90.51(2)°, is significantly smaller than those in the
above cis-[Co(dtc)2(P-ligand)2]+-type complexes: 96.8(1)° for
PMe3, 95.14(2)° for PMe2Ph, and 92.68(4)° for P(OMe)Ph2. It
appears that there is no reasonably simple relationship between
these structural parameters (Co–P bond length and P–Co–P
angle) and either the Tolman’s cone angle or the σ-donicity of
P-ligands. Probably, on the basis of the observed conformation
of three substituents of PHPh2, the negligibly small steric
requirement of the H substituent reduces steric congestion
around the Co center more effectively than that expected from
the Tolman’s cone angle.
PMe3, PMe2Ph, PMePh2, and PPh3 complexes, respectively.7 It
seems that the ligand-field perturbation energy of PHPh2 is not
as small as the one expected from the σ-donicity, but correlates
to the steric bulkiness (Tolman’s cone angle) of the phosphines:
PMe3 < PMe2Ph ≤ PHPh2 < PMePh2 < PPh3.
For trans-[Co(dtc)2(P-ligand)2]+, the above-mentioned
complexity arised from steric congestion would be ignored
because of the mutual trans configuration of two P-ligands.
The electronic trans influence, in addition to the steric require-
ment (termed by the cone angle), of P-ligand must be taken into
consideration for comparison of the Co–P bond lengths.11 In
fact, the comparable Co–P bond lengths in trans-[Co(dtc)2-
(PMe3)2]BF4 (2.287(1) Å) and trans-[Co(dtc)2(PMe2Ph)2]BF4
(2.2843(8) Å) are resulted from competition of the mutual elec-
tronic trans influence with the steric requirement.7 Despite a
larger cone angle of PHPh2 than those of PMe3 and PMe2Ph,
the Co–P bond length in trans-[1]BF4, 2.276(1) Å, is also com-
parable to (or even slightly shorter than) those in the above
PMe3 and PMe2Ph complexes, which is also indicative of a sig-
nificant contribution of electronic trans influence to the Co–P
bond lengths. The influence of PHPh2 would be not as small as
expected from the very weak σ-donicity of PHPh2, since the
Co–P bond in trans-[1]BF4 is appreciably longer (by 0.046 Å)
than that in cis-[1]BF4.
In summary, the Co(III)-PHPh2 complexes of cis- and
trans-[1]BF4 exhibit unexpectedly high stabilities, short Co–P
bond lengths, and strong ligand-field strengths from a very
weak σ-donicity of PHPh2.
References and Notes
1
H.-Y. Liu, K. Eriks, A. Prock, and W. P. Giering, Organometallics, 9,
1758 (1990); Md. M. Rahman, H.-Y. Liu, K. Eriks, A. Prock, and W.
P. Giering, Organometallics, 8, 1 (1989).
2
3
P. Rigo, M. Bressan, and A. Morvillo, J. Organomet. Chem., 93, C34
(1975); P. Rigo and M. Bressan, Inorg. Chim. Acta, 33, 39 (1979).
For example: M. A. Zhuravel, N. S. Grewal, D. S. Glueck, K.-C. Lam,
and A. L. Rheingold, Organometallics, 19, 2882 (2000); E. Alonso, J.
Forniés, C. Fortuño, A. Martín, and A. G. Orpen, Organometallics, 19,
2690 (2000); C. Mealli, A. Ienco, A. Galindo, and E. P. Carreño,
Inorg. Chem., 38, 4620 (1999).
4
5
P. Leoni, M. Pasquali, M. Sommovigo, F. Lashi, P. Zanello, A.
Albatini, F. Lianza, P. S. Pregosin, and H. Rueegger, Organometallics,
12, 1702 (1993).
For example: A. J. Blake, N. R. Champness, R. J. Forder, C. S.
Frampton, C. A. Forst, G. Reid, and R. H. Simpson, J. Chem. Soc.,
Dalton Trans., 1994, 3377; R. B. Forder and G. Reid, Polyhedron, 15,
3249 (1996); A. J. Carty, F. Hartstock, and N. J. Toylor, Inorg. Chem.,
21, 1349 (1982).
The UV–vis absorption spectrum of cis-[1]BF4 in CH2Cl2 is
similar to that of cis-[Co(dtc)2(PMe3 or PMe2Ph)2](BF4 or PF6)
(Figure 3(a)).7 The PHPh2 complex, cis-[1]+, shows two bands at
18430 and 23430 cm–1, which are assignable as the first and the
second d–d transition bands, respectively.6,7 The ligand-field
strength, ∆, and the Racah’s interelectronic repulsion parameter,
B, of cis-[1]+ are estimated12 as 19680 and 313 cm–1, and those of
the PMe3 and PMe2Ph complexes as 19580 and 311; 19170 and
316 cm–1, respectively, indicating that the ligand-field strength of
PHPh2 is a little stronger than those of PMe3 and PMe2Ph, in
contrast to the much weaker σ-donicity of PHPh2.
The absorption spectrum of trans-[1]BF4 (Figure 3(b))
gives the a1Eg component of the first d–d transition band at
17300 cm–1 and the lowest energy LMCT transition band at
24920 cm–1. The corresponding bands of trans-[Co(dtc)2(P-lig-
and)2]BF4 were observed at 17900 and 27380; 17170 and
25570; 16580 and 24770; and 15740 and 23080 cm–1 for the
6
7
8
H. Matsui, M. Kita, K. Kashiwabara, and J. Fujita, Bull. Chem. Soc.
Jpn., 66, 1140 (1993).
T. Suzuki, S. Kashiwamura, and K. Kashiwabara, Bull. Chem. Soc.
Jpn., in press.
Found for cis-[1]BF4: C, 47.20; H, 4.49; N, 4.03%. Found for trans-
[1]BF4: C, 47.09; H, 4.41; N, 3.79%. Calcd for C30H34BCoF4N2P2S4:
C, 47.50; H, 4.52; N, 3.69%.
cis-[1]BF4·CH3CN·0.5Et2O; fw = 836.63, Rigaku Raxis-rapid (23 °C,
λ(Mo Kα) = 0.71073 Å), monoclinic, P2/n (no. 13), a = 19.359(1), b =
11.1101(7), c = 20.642(1) Å, β = 115.830(2)°, U = 2996.3(4) Å3, Z =
4, Dx = 1.391 Mg m–3, 9084 independent reflns (2θ ≤ 55°), R1(F2: F2 >
2σ(F2)) = 0.040, wR2(F2: all reflns) = 0.117. trans-[1]BF4; fw =
758.51, Rigaku AFC-5R (23 °C, λ(Mo Kα) = 0.71073 Å), monoclinic,
C2/c (no. 15), a = 19.507(4), b = 14.529(5), c = 14.445(3) Å, β =
122.20(1)°, U = 3464(1) Å3, Z = 4, Dx = 1.454 Mg m–3, 5070 inde-
pendent reflns (2θ ≤ 60°), R1(F2: F2 > 2σ(F2)) = 0.055, wR2(F2: all
reflns) = 0.147.
9
10 C. A. Tolman, Chem. Rev., 77, 313 (1977).
11 T. Suzuki, S. Kaizaki, and K. Kashiwabara, Inorg. Chim. Acta, 298,
131 (2000).
12 C = 4B is assumed.