This summary provides the key insights from the quantum mechanical analysis of donor-acceptor interactions in organometallic complexes: 1) Calculated parameters for bidentate phosphine ligands, such as the CO stretching frequency and H-H distance in nickel complexes, followed trends similar to unidentate phosphine ligands and provided insight into electronic and structural properties. 2) Electron donating phosphine ligands increased π-backbonding in nickel-dihydrogen complexes, lengthening the H-H bond, while electron withdrawing ligands increased σ-donation, shortening the H-H bond. 3) Calculations showed that a methane tautomer could be stabilized relative to a methyl hydride t

1. Quantum Mechanical
Analysis of Donor-Acceptor
Interactions in
Organometallic Complexes
Research Presentation to Borden Group
Denton, TX
Sept. 23rd, 2009
“Give me INSIGHT not numbers!”
- Richard Hamming
1

2. Outline of Talk
 Donor- acceptor properties of nickel bidendate
phosphine carbonyls
 Applications to transition metal - sigma complexes
 Nickel dihydrogen complexes
 Osmium methane complexes
 Are all M C close contacts agostic interactions?
Orbital analysis of a Titanium amide complex -
Poster
2

3. Calculation details
 Nickel carbonyl complexes
 RI-BP86 with def2-TZVP basis set in Turbomole 6.0
 Nickel dihydrogen complexes
 RI-BP86 with def2-TZVP basis set in Turbomole 6.0
 BP86 with Slater basis and ZORA (Nickel only) in ADF
 Osmium methyl hydride complexes
 BB1K and M05-2X with cc-pVTZ on Os-CH4 and
cc-pVDZ on other atoms in Gaussian03 Rev. E.01
 Douglas-Kroll (SDD) ECP on osmium
3

4. Donor- acceptor properties of nickel
bidendate phosphine complexes
 Electronic and steric properties of unidentate phosphines are
well known.
 Ni(PR3)(CO)3
 Tolman cone angle for sterics
 C0symm for electronic properties
 Computed parameters comparable
 Bidentate phosphines commonly used in synthetic studies
 Increased steric hindrance
 No thorough study of bidentate series
 Ni[(R2P(CH2)nPR2](CO)2
Tolman W. B.; Chem Rev., 77, 31.
Crabtree, R.H. et al; Inorg. Chem., 40, 5806. 4

5. Structures of carbonyl complexes used
for study
n=1 n=2 n=3
 Distorted tetrahedral
 P-Ni-P angle increases as number of backbone methylenes (n)
increases
 Cs symmetry for n =1,3
 C2 symmetry for n=2
5

6. Calculated stretching frequencies
show discernible trends for R and n
COsymm (cm-1) for Ni[(R2P(CH2)nPR2](CO)2
# Carbons tBu iPr Et Me Ph H OMe CF3 F
1 1984 1987 1994 2000 2001 2023 2027 2055 2057
2 1977 1983 1989 1996 1997 2020 2023 2053 2054
3 1972 1979 1984 1991 1992 2014 2018 2048 2048
More electron donating
2060
 Calculated frequency trends
generally agree with
Calculated CO stretching frequency
dcf3pe
2050
2040
R² = 0.942
experimental frequency trends
2030
 COsymm decreases as R
(cm-1)
2020
2010 becomes more electron
dppm
2000
depe dppe donating
1990 dppp
dmpe
1980  COsymm decreases ≈4 cm-1 for
every increase in ‘n’
6
1950
2000
2050
2100
Experimental CO stretching frequency
(cm-1)

7. Results from the DFT calculations can
be applied to other complexes bearing
bidentate phosphines
Nickel dihydrogen Osmium methane
complexes complexes
7

8. H-H distance in Ni(H2) (PˆP) complexes is
sensitive to donor/acceptor properties of the
bidentate phosphine
Decreasing H-H distance
 Non-linear trend for H-H distance between n=2 and n=1,3
8

9. Differences in H-H distance can be
explained
π
 When R is electron donating (tBu, iPr, etc), increased
shielding of nuclear charge on Ni raises energy of
d-orbitals, increases π-backbonding to H2 antibonding
orbital, increases H-H bond length.
 When R is electron withdrawing (F CF3), the effect is
,
exactly opposite, leading to shorter H-H bond length.
9

10. Orbital analysis shows correlation between H-H distances
and type of orbital contributions to Ni-H2 bond
 Electron donating phosphines have large H-H distances
and a larger π-orbital contribution to the Ni-H2 bond.
 Electron withdrawing phosphines have large -orbital
contributions. 10

11. Orbital analysis of Ni-H2 complexes
complements with CO
 Complexes with larger COsymm have large component.
 Complexes with smaller COsymm have large π component.
11

12. Search for stable methane coordination complexes
+
+
+
+
 Barrier (energy of 1‡) can be measured experimentally.
 It is known that when R=Me, E2 > E1 but E2 < E3+CH4
 Alter ancillary ligands to lower energy of methane tautomer
(2) relative to methyl hydride (1) and dissociation (3 +CH4)
Girolami, G.S.; JACS, 1999, 111, 4114. 12

13. Calculations of Energy of 1‡ relative to 1
are in good agreement with experiment
C5Me5 C5Me4H* C5H4Me* C 5H 5
ΔG‡, kcal mol-1 (-100°C) 8.1 ± 0.1 8.6 ± 0.1 8.4 ± 0.1 8.1 ± 0.1
BB1K 6.4 6.4 7.2 6.7
M05-2X 7.1 7.6 7.2 7.2
 Changing the substituents on the
Cp ring has little effect on the +
energetics.
 What about changing the
substituents on the bidentate
phosphine?
* Two rotameric isomers calculated for C5H4Me where the most sterically hindered isomer is 0.5
kcal/mol higher than the less sterically hindered isomer. We expect similar trends for C5Me4H

14. Electronic information can be gained
from CO frequencies
 For electronic comparisons, +
calculate the vibrational
frequency of a CO complex
with the ligand set
2060
 CO from M05-2X
CF3
COsymm for Ni(R2PCH2PR2)(CO)2
2050
R² = 0.97
corresponds closely
2040
2030
(cm-1) H
to CO for Ni with 2020
some bidentate
2010
Me
2000 Ph
complexes 1990
2080 2100 2120 2140 2160 2180
COsymm for Os(C5H5)(R2PCH2PR2) (cm-1)
14

15. Methane tautomer (2) becomes more stable
than methyl hydride tautomer (1) as
phosphine becomes less donating
6.0 M05-2X BB1K + +
Me2PCH2PMe2 Me2PCH2PMe2
4.0
Ph2PCH2PPh2
E1-E2 (kcal mol-1)
Me2PCF2PMe2
2.0 Me2PCF2PMe2
Ph2PCH2PPh2
H2PCH2PH2 H2PCH2PH2
0.0
H2PCF2PH2 H2PCF2PH2
-2.0 (CF3)2PCH2P(CF3)2
(CF3)2PCH2P(CF3)2
(CF3)2PCF2P(CF3)2
-4.0 (CF3)2PCF2P(CF3)2
R² = 0.86 R² = 0.99
-6.0
2025 2050 2075 2100 2125 2150 2175 2200 2225 2250
COsymm (cm-1)
 Electron poor ligands (F CF3) favor the methane
,
tautomer (2) while electron rich ligands favor the
methyl hydride (1)
15

16. Summary
 Computed Tolman parameters for bidentate phosphines show
trends generallly similar to unidentate phosphines.
 The carbonyl stretching frequency of Ni(CO)(P^P) is strongly
affected by the number of carbons in the backbone while the
H-H distance in Ni(H2)(P^P) is weakly affected.
 Energy decomposition analysis showed that the H-H distance
increased as the π contribution to the Ni-H2 bond increased.
 Electron poor R groups such as CF3 favor -donation into the
H-H bond, thus stabilizing the H2 complex.
 A methane tautomer can be stabilized relative to a methyl
hydride tautomer with bidentate phosphines bearing electron
withdrawing CF3 groups.
16

17. Acknowledgments
Frenking Group Girolami Group
Dunning Group (sorry no photo)
Computational Support
$$$$
German - American Fulbright Hochschulrechenzentrum
Commission of Phillips Universität