COvsCN LF Strength

8/19/2019 COvsCN LF Strength

http://slidepdf.com/reader/full/covscn-lf-strength 1/7

Sukaran S. Arora5099451

1

Ligand field strengths of carbon monoxide and cyanide in

octahedral coordination

Patrick Hummel, Jonas Oxgaard, William A. Goddard III, Harry Gray, J. Coord. Chem. 58 (2005) 41-45

Section 1: Brief Summary

This paper attempts at correcting one of the co-authors earlier experimental works 1 done to obtain

the ligand field splitting strength ( Δ 0) for metal hexacarbonyl and hexacyano complexes. The

absorption spectra of these complexes were obtained by Gray et al. over 45 years ago. In all

cases, the intense absorptions were assigned to allow metal to ligand charge transfer (MLCT)

and weaker energy bands were assigned to ligand field excitations (d-d transfers).

These assignments gave a ligand field splitting value of 35000 cm -1 (~4.35 eV) for CN -1 and 34000

cm -1 (~4.22 eV) for CO ligand.




2

This hinted towards the argument that CN -1 is slightly stronger ligand than CO. However, some

recent relativistic time-dependent density functional theory (TDDFT) calculations performed by

Baerends et al. 2 on M(CO) 6 complexes with M = Cr, Mo, W raised questions about the magnitudes

of these ligand field splittings, claiming that the stronger bands actually correspond to the LF

splittings and the orbitally-forbidden MLCT bands are much lower in energy. This leads to a

completely different proposal about the field strengths of carbonyl and cyanide ligands than what

the co-author had concluded earlier.

Thus, to reexamine the previous work and to study the effects systematically, the splittings for a

series of isoelectronic 3d6

complexes with CO, CN-1

, and CNH ligands were estimated from DFTcalculations using four different exchange correlation functionals. Using DFT, this issue is

resolved. DFT is used to calculate the ground-state energies of t 2g and e g orbitals for d 6 complexes

V(CO) 6-1, Cr(CO) 6, Mn(CO) 6

+, Fe(CO) 62+ , CO(CN) 6

3-, Fe(CN) 6+. LF splittings induced by iso-

cyanide ligand were also calculated for V(CNH) 6-1, Cr(CNH) 6, Mn(CNH) 6

+, and Fe(CNH) 62+ .




3

Computational Methods

TURBOMOLE program package was used for ab-initio electronic structure and energy

calculations. QZVP basis set was used for all atoms in calculating the properties of these

complexes. 4 different exchange correlation functionals (B3LYP, PBE, BP86, and BLYP) were

used to perform 4 separate DFT calculations for each complex. Octahedral symmetry was

assumed and m3 (triangular) grids were used. JOBEX program was used to optimize geometries

with generalized internal coordinates. Semi-direct self-consistent field (DSCF) module was used

to calculate ground state energies of the molecular orbitals.

Results

The obtained M-C bond distances in the complexes for the three ligands using the four different

functionals are listed in a tabular form as below:

The bond distances (C-X) in the ligands when complexed with the metal were also reported as

follows:




4

The computed N-H bond distances in the iso-cyanide complex are listed below. The final

calculated octahedral field splittings based on the energy differences of the d orbitals are also

reported.

Discussion

It is noted that for both Fe(CN) 64- and Co(CN) 6

3-, ligand field splitting ( Δ 0) from the experiments

conducted earlier by the co-author was around 4.3 eV, as stated earlier. The B3LYP functional is

observed to overestimate this value by 40% while other functionals underestimate the same by

10% for PBE and BP86, 20% for BLYP.




5

For metal carbonyl complexes, typical experimental estimate for Δ 0 in octahedral symmetry is ~

4.2eV. However, all the density functionals used here lead to larger values (20% more for BLYP

vs 70% more for B3LYP).

Based on these observations and taking into account their accuracy, Gray agrees with Baerend ’s

results that the original assignment of the weak bands for metal hexacarbonyl complexes

underestimates their field strength, possibly by more than 1 eV.

Further arguments are made to explain the trends observed in the value of field strengths based

on the metal carbon bond distances. For metals with same oxidation states, it is observed that

M(CO) 6 and M(CNH) 6 complexes have similar ground state geometries. However, M-C bond

distance in M(CN) 6 complex is much greater. A possible rationale is that the net negative charge

on the cyanide likely decreases the extent of metal back-bonding (M π *) compared to the other

complexes. MLCT is thought to be an important factor in the strength of these metal carbon bonds.

These greater bond distances further lead to lesser σ overlap, and the corresponding Δ 0 for

octahedral splitting is lower than CO and CNH; thus the claim that it is a weaker ligand than both

carbonyl and iso-cyanide.

It can also be argued by looking at the computed values that the induced octahedral splitting in

M(CO) 6 complexes has little dependence on the metal oxidation state. As the oxidation state goes

from negative [V(-I)] to positive [Fe(II)], M π * back-bonding decreases (because the nucleus

exerts more attractive forces on the electrons and thus they are less soft to move over to the

ligand orbitals); this destabilizes the t 2g orbitals. The e g orbitals are destabilized at the same time

with the increase in sigma bonding (explained by the decrease in the M-C bond distances). Thus

the effects are counter-balanced and there is no observed effect of the oxidation state. However,

for the iso-cyanide (CNH) complexes, a much stronger dependence of the ligand field splitting on

the metal oxidation state is observed.




6

For Fe 2+ , Fe(CO) 62+ and Fe(CNH) 6

2+ have comparable Δ 0 in O h field splitting. But for V 1-, Δ 0 is

much greater for V(CO) 61-. This indicates that CO is a much stronger ligand than CNH and that

the relative strength of the two ligands depends on the metal in consideration.

A more general rationale for the observed differences in the ligand field strength is that regardless

of the metal, cyanide derives most of its strength from the strong σ donation as there is little M π *

back-bonding. On the other hand, CO derives its strength from a combination of σ donation and

M π * back-bonding. The iso-cyanide (CNH) falls in between cyanide and carbonyl in terms of

its strength as a π acceptor, so M(CO) 6 will have more back-bonding for any metal than M(CNH) 6

complexes. For Fe(II) complexes, extent of back-bonding is small, so Δ 0 for CN -1 and CNH will be

similar; while for V -1, back-bonding is very important and since carbonyl is a better π acceptor

than iso-cyanide, CO has a larger field splitting than CNH for the vanadium complex.

Thus, an agreement is reached with Baerend ’s observations and a theoretical justification based

on physical arguments is provided to shed light on the relative strengths of CO and CN -1 ligands

in octahedral field splittings.




7

Section 2: Improved understanding

Contradicting their own work, Gray et al. have performed these DFT calculations to improve upon

and correct their mistake while assigning the bands observed in the absorption spectra of these

metal complexes 1. Baerends 2 in his work had noted that based on the TDDFT calculations

performed by his group, the lowest excited states in the spectra should not correspond to ligand

field excitations. Instead they should correspond to the charge transfer states. These low-energy

shoulders should be assigned to orbitally-forbidden MLCT states, which would be consistent with

the observed insensitivity of their position to the metal (as is clear from the data in Table 3). The

low intensity of these shoulders, thus, should be attributed to their forbidden nature, not to their

LF character.

The LF excitations should be calculated at much higher energy than what was done by Gray. This

paper and its observed results and reasoning help clear this contradiction and a better

understanding of the absorption spectra of transition metal complexes with carbonyl and cyanide

ligands is provided.

References(1) Gray, H. B.; Beach, N. A. J. Am. Chem. Soc. 1963 , 85 (19), 2922 – 2927.

(2) Rosa, A.; Baerends, E. J.; van Gisbergen, S. J. A.; van Lenthe, E.; Groeneveld, J. A.;

Snijders, J. G. J. Am. Chem. Soc. 1999 , 121 (44), 10356 – 10365.

Documents

COvsCN LF Strength