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J. Am. Chem. Soc. 1992, 114, 7879-7888
Transition Metal Imido Complexes
Thomas R. Cundari'
Contribution from The Department of Chemistry, Memphis State University, Memphis,
Tennessee 38152. Received February 13, 1992
Abstract: A wide range of transition metal imido (TMI) complexes is studied using ab initio molecular orbital (MO) calculations.
The main computational point of interest is the further testing of effective core potentials (ECPs) and valence basis sets to
allow for the accurate calculation of properties for reasonably sized transition metal complexes. On the chemical side, several
results from the study are to be noted. The agreement between geometries for calculated models and their experimental counterparts
ranges from very good to excellent, as found in previous studies of multiply bonded transition metal-group IVA complexes.
Taken together, these data suggest that the accuracy one has come to expect for the prediction of structural properties for
main-group compounds may yet become a reality for transition metal complexes. The correct prediction of molecular structure
also leads one to infer that the bonding in these complexes is also accurately described. The MC/LMO/CI (multiconfigu-
rational/localized MO/configuration interaction) technique shows that eight resonance structures are most significant in the
description of the metal-imido linkage. Three of these account for roughly two-thirds of the total contributions; two resonance
structures, both possessing a dative a bond, correspond to novel bonding descriptions for TMI complexes. The dative character
of the a bond has been largely unappreciated. The MC/LMO/CI results also indicate that the metal-imido bond is intermediate
between a double and a triple bond. Nucleophilic resonance structures decrease as one goes to the right in the transition series
while electrophilic ones show the reverse behavior, in agreement with observed reactivity.Introduction
The coordination of unstable and transient species to transition
metal complexes has been a successful technique for chemists
interested in studying them and harnessing their reactivity to carry
out synthetic and catalytic transformations. Perhaps the most
studied examples are the isovalent series: carbene (CR2), nitrene
(NR), and oxene (O).2- Having two unsatisfied valencies, these
moieties form double bonds to a transition metal when coordinated
as a terminal ligand and react similarly (donation of NR and O
lone pairs into vacant metal orbitals can be invoked to yield a triple
bond description).5 For example, the nitrene analogues of the
Wittig reaction6 and oxygen atom transfer' are known. Oxo
(LM=O) and carbene (LM=C(R)R') complexes have received
much attention due to their importance in catalysis, biochemistry,
and advanced materials.8'9
Transition metal imido (TMI) complexes have been relatively
ignored by theoreticians, despite being putative intermediates in
important processes such as ammoxidation and nitrogen fixation.5
Nitrenes were first proposed by Tiemann in 1892 as intermediates
in the Lossen rearrangement.i It seems appropriate that as
nitrenes and Werner's coordination theory approach their 100th
birthday that a detailed study of transition metal imido complexes
be undertaken.
(1) E-mail: cundarit@memstvxl.memst.edu
(2) The first carbene complex, W(CO)5(==C(OMe)(Ph), was reported by
Fischer and Maasbol in 1964. A discussion of various aspects of the chemistry
of the so-called Fischer-type carbene complexes can be found in ref 2a.
Schrock reported the first high-valent, alkylidene complex in 1975:
(Me3CCH2)3Ta(f==C(H)CMe3).2b (a) Transition Metal Carbene Complexes;
D6tz, K. H., Ed. Verlag: Weinheim, 1983, p 73. (b) Schrock, R. R. Ace.
Chem. Res. 1979, 12, 98.
(3) (a) Nugent, W. A.; Haymore, B. L. Coord. Chem. Rev. 1980, 31, 123.
(b) Cenini, S.; LaMonica, G. Inorg. Chim. Acta 1976, 18, 279. (c) Dehnicke,
K.; Strthle, J. Angew. Chem., Int. Ed. Engl. 1981, 20, 413. (d) Additional
reviews to the most recent imido literature, some of which are not referenced
here, can be found in footnote 1 of ref 19.
(4) The most complete reviews of metal-oxo complexes are as follows: (a)
Holm, R. H. Chem. Rev. 1987, 87, 1401. (b) Griffith, W. P. Coord. Chem.
Rev. 1970, 5, 459.
(5) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds; Wiley:
New York, 1989.
(6) Rocklage, S. M.; Schrock, R. R. J. Am. Chem. Soc. 1980, 102, 7808.
(7) Groves, J. T.; Takahashi, T. J. Am. Chem. Soc. 1983, 105, 2073.
(8) In addition to experimental and theoretical references in the above
reviews,2-5 more theoretical references can be found in references 8 (carbene
complexes) and 9 (oxo complexes). Cundari, T. R.; Gordon, M. S. J. Am.
Chem. Soc. 1991, 113, 5231.
(9) Cundari, T. R.; Drago, R. S. Inorg. Chem. 1990, 29, 2303.
(10) Tiemann, F. Ber. Dtsch. Chem. Ges. 1891, 24, 4162.Several molecular orbital (MO) analyses of imido complexes
have been reported, many as adjuncts to experiment. Nugent et
al." use Xa calculations and NMR data to probe the changes
in nucleophilicity/electrophilicity as a function of position in the
transition series. They conclude that the imido N becomes in-
creasingly electrophilic (less nucleophilic) as one proceeds
downward and to the right in the transition series. Maatta et al.
use extended Heckel calculations of V-imido dimers to help
interpret 51V NMR and electronic spectral data.'2 Rothwell et
al. employ extended Heckel theory (EHT) to assess competition
for metal dir AOs between the amido and imido ligands in Zr-
(NH)(NH2)2(NH3)2, a model of their characterized complex.'3
Trogler and co-workers'4 combine density functional calculations,
EPR, and X-ray crystal data to analyze the electronic structure
of Cp2VN(2,6-Me2-Ph). An extended Htlckel analysis of the
reaction of Os-oxo(imido) complexes with olefins has been pub-
lished.5 Complexes pertinent to nitrogen fixation (N2, NNH,
NNH2, etc.) were studied by Dubois and Hoffmann'6 using EHT
and Fukui et al. using CNDO/217 and minimal basis set ab initio
calculations.'8 Schrock et al. have published Xa calculations of
Os(NH)3 and related d2-Os imidos in conjunction with experi-
ment.19
Each TMI complex has a description of the metal-imido bond
suggested by its observed reactivity. Early transition metal imido
complexes are typically envisioned as containing a metal-imido
triple (or double) bond and a nucleophilic imido nitrogen, 1 (aMN
+ 21rm) or 2 (CMN + MN + 1PN). A bonding scheme ostensibly
identical to 2, i.e., a triple bond with dative character for one 7r
component, 3, has also been forwarded.3c Late TMIs have
electrophilic imido nitrogens with a metal-imido linkage which
has been described in terms of a MN d bond, a N lone pair, and
a filled metal dir orbital for back-bonding to the imido ligand,
4.20
(11) Nugent, W. A.; McKinney, R. J.; Kasowski, R. V.; Van-Catledge, F.
A. Inorg. Chim. Acta 1982, 65, L91.
(12) Devore, D. D.; Lichtenhan, J. D.; Takusagawa, F.; Maatta, E. A. J.
Am. Chem. Soc. 1987, 109, 7408.
(13) Profilet, R. D.; Zambrano, C. H.; Fanwick, P. E.; Nash, J. J.;
Rothwell, I. P. Inorg. Chem. 1990, 29, 4364.
(14) Osborne, J. H.; Rheingold, J. H.; Trogler, W. C. J. Am. Chem. Soc.
1985, 107, 7945.
(15) Jorgensen, K. A.; Hoffmann, R. J. Am. Chem. Soc. 1986, 108, 1867.
(16) Hoffmann, R.; Dubois, D. L. Nouv. J. Chim. 1977, 1, 479.
(17) Yamabe, T.; Hori, K.; Minato, T.; Fukui, K. Inorg. Chem. 1980, 19,
2154.
(18) Yamabe, T.; Hori, K.; Fukui, K. Inorg. Chem. 1982, 21, 2046, 2816.
(19) Schofield, M. H.; Kee, T. P.; Anhaus, J. T.; Schrock, R. R.; Johnson,
K. H.; Davis, W. M. Inorg. Chem. 1991, 30, 3595.0002-7863/92/1514-7879$03.00/0 1992 American Chemical Society
7879
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Cundari, Thomas R., 1964-. Transition Metal Imido Complexes, article, September 1, 1992; [Washington, D.C.]. (https://digital.library.unt.edu/ark:/67531/metadc107775/m1/1/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT College of Arts and Sciences.