Binuclear heterometallic bonding between a first row transition metal and a second row transition metal: The cyclopentadienyliron molybdenum carbonyls Cp2FeMo(CO)n (n = 6, 5, 4, 3, 2)

Inorganica Chimica Acta 493 (2019) 102–111

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Research paper

Binuclear heterometallic bonding between a first row transition metal and a second row transition metal: The cyclopentadienyliron molybdenum carbonyls Cp2FeMo(CO)n (n = 6, 5, 4, 3, 2)

T

Xiuyuan Lia, Yuanhuai Zhua, Nan Lia, R. Bruce Kingb a b

State Key Laboratory of Explosion Science and Technology, School of Mechatronical Engineering, Beijing Institute of Technology, Beijing 100081, China Department of Chemistry and Center for Computational Chemistry, University of Georgia, Cedar Street, Athens, GA 30602, USA

ABSTRACT

The geometries and energetics of the heterometallic binuclear cyclopentadienyl iron-molybdenum carbonyls Cp2FeMo(CO)n (n = 6, 5, 4, 3, 2; Cp = η5-C5H5), including the experimentally known pentacarbonyl Cp2FeMo(CO)5 have been examined by density functional theory. The lowest energy structure for the pentacarbonyl Cp2FeMo(CO)5 by the mPWPW19 method corresponds to the unbridged Cp2FeMo(CO)5 isomer related to the experimental Cp2Mo2(CO)6 structure with exclusively terminal carbonyl groups. However, the doubly bridged structure, which is related to the experimental trans- and cis-Cp2Fe2(CO)2(µ-CO)2 structures of the homonuclear diiron derivative, lies only 1.4 kcal/mol in energy above the unbridged structure. The relative energies of the unbridged and doubly bridged Cp2FeMo (CO)5 are reversed using the BP86 method with the doubly bridged structure lying 2.0 kcal/mol below the unbridged structure. The closeness in energy of these two structures suggests a fluxional system for Cp2FeMo(CO)5. The low-energy structures for the tetracarbonyl Cp2FeMo(CO)4 are doubly bridged structures with formal Fe=Mo double bonds in accord with the 18-electron rule. However, Cp2FeMo(CO)4 is predicted not to be viable since its disproportionation into Cp2FeMo (CO)5 + Cp2FeMo(CO)3 is found to be highly exothermic. The tricarbonyl Cp2FeMo(CO)3 has an energetically favorable triply bridged structure with a formal Fe≡Mo triple bond analogous to the experimental valence isoelectronic Cp*2M2(µ-CO)3 (M = Mn, Re; Cp* = η5-Me5C5) structures. The low-energy structures of the dicarbonyl Cp2FeMo(CO)2 are doubly bridged structures which can be either singlets with an FecMo quadruple bond or triplets with an Fe≡Mo triple bond. The lowenergy structures of the hexacarbonyl Cp2FeMo(CO)6 can be interpreted as consisting of [CpFe(CO)3]+ cations linked to [CpMo(CO)3]− anions through one of the iron-bonded carbonyl groups with long ∼4 Å Fe⋯Mo distances indicating the absence of an iron-molybdenum bond.

1. Introduction The chemistry of binuclear metal carbonyls dates back to the early discoveries of the stable binary metal carbonyls [1,2] Fe2(CO)9 and Co2(CO)8 not long after the 1890 discovery [3] of the first binary metal carbonyl Ni(CO)4. The metal-metal bonds in Fe2(CO)9 and Co2(CO)8 were subsequently found by X-ray crystallography to be bridged by three [4] and two [5–7] carbonyl groups respectively. Later the homoleptic group 7 metal carbonyls M2(CO)10 (M = Mn [8,9], Tc, Re [9]) were synthesized and shown to have structures with exclusively terminal carbonyl groups and unbridged metal-metal bonds. These unbridged M2(CO)10 structures were particularly significant in showing unequivocally that a direct metal-metal bond between d-block transition metals was strong enough to hold together the two halves of a very stable molecule. The seminal 1951 discovery by two research groups [10,11] of the very stable ferrocene, Cp2Fe, having its iron atom sandwiched between two planar cyclopentadienyl (Cp) rings, led shortly thereafter to the discovery of cyclopentadienylmetal carbonyls for metals across the entire transition series from titanium to nickel and later even to copper

[12–14]. Among the cyclopentadienylmetal carbonyls first to be synthesized were the binuclear iron [15] and molybdenum [16] derivatives Cp2Fe2(CO)4 and Cp2Mo2(CO)6 since they could readily be obtained from thermal reactions of the readily available corresponding metal carbonyls, namely Fe(CO)5 and Mo(CO)6, respectively, with cyclopentadiene. The iron complex Cp2Fe2(CO)4 was shown to have a doubly bridged structure with two bridging carbonyl groups as well as a terminal carbonyl group bonded to each iron atom (Fig. 1). In fact, both trans and cis isomers of Cp2Fe2(CO)2(µ-CO)2 were separated and individually structurally characterized by X-ray crystallography [17–19]. The molybdenum complex Cp2Mo2(CO)6 was shown to have exclusively terminal carbonyl groups with only a direct Mo–Mo bond of length 3.235 Å holding together the two halves of the molecule [20]. In both Cp2Mo2(CO)6 and Cp2Fe2(CO)4 both metal atoms have the favored 18electron configuration [21–24]. Use of pentamethylcyclopentadiene instead of unsubstituted cyclopentadiene in the thermal reaction with Mo(CO)6 led to the discovery of the first binuclear metal carbonyl derivative with a metal-metal multiple bond, namely Cp*2Mo2(CO)4 (Cp* = η5-C5H5) with the formal Mo≡Mo triple bond to give both molybdenum atoms the favored 18-

E-mail address: [email protected] (R.B. King). https://doi.org/10.1016/j.ica.2019.04.044 Received 29 October 2018; Received in revised form 22 April 2019; Accepted 22 April 2019 Available online 23 April 2019 0020-1693/ © 2019 Elsevier B.V. All rights reserved.

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O C OC

Mo C O

O C Mo

O C

CO OC

C O

Cp2Mo2(CO)6

O C

Mo C O

Fe

Fe

C O

C O

O C C O

O C Fe

C O

C O

trans

Cp2FeMo(CO)5

O C

Fe

Fe C O

cis

Cp2Fe2(CO)4

Fig. 1. Structures of the experimentally known homobinuclear species Cp2Mo2(CO)6 and Cp2Fe2(CO)4 and the heterobinuclear species Cp2FeMo(CO)5.

OO C C Mo

Mo

C C O O

Cp2Mo2(CO)4 Singlet

interesting question whether the “hybrid” heterometallic MoFe systems will resemble more closely the Mo2 or the Fe2 systems. We report the use of density functional theory to explore this issue through the predicted low energy structures of the heterobimetallic cyclopentadienylmetal carbonyl systems Cp2FeMo(CO)n (n = 6, 5, 4, 3, 2).

O C Fe

Fe

C O

C O

Cp2Fe2(µ-CO)3

2. Theoretical methods

Triplet

All of the calculations in this article were performed with the Gaussian 09 program package [32] using the mPW1PW91 and BP86 methods. The BP86 method is a pure DFT method combining Becke’s 1988 exchange functional (B) [33] with Perdew’s 1986 gradient correction related functional (P86) [34]. The vibrational scaling factors of BP86 for transition metal carbonyls are close to 1.00 [35] so that they directly can correspond to the experimental values. The mPW1PW91 method is a so-called second-generation functional, combining the modified Perdew-Wang exchange functional with the Perdew-Wang 91 gradient-correction functional [36]. The mPW1PW91 functional has been found to be more suitable for geometry optimization than the firstgeneration functional for second- and third-row transition metal systems, while the BP86 method usually provides better vibrational frequencies [37,38]. We used the Multiwfn [39] and the VMD [40] program package to calculate and exhibit some molecular orbitals and the results of NCI analysis [41]. The DZP and SDD basis sets were used for the light atoms and heavy atoms, respectively. For the carbon and oxygen atoms, the double-ζ plus polarization (DZP) basis sets were used. These consist of HuzinagaDunning’s contracted double-ζ sets plus a set of spherical harmonic d polarization functions with orbital exponents Rd(C) = 0.75 and Rd(O) = 0.85 designated as (9s5p1d/4s2p1d) [42]. For hydrogen, a set of p polarization functions (Rp(H) = 0.75) was added to the HuzinagaDunning DZ sets, which can be expressed as (4s1p/2s1p) [42,43]. The double-ζ Stuttgart-Dresden (SDD) effective core potentials (ECP) basis set [44] was used for molybdenum. The loosely contracted DZP basis set (14s11p6d/10s8p3d) used for iron augments the Wachters primitive set by two sets of p functions and one set of d functions followed by contractions according to Hood, Pitzer, and Schaefer [45]. Unless otherwise specified, the Gaussian 09 package fine grid (75,302) was the default for evaluating integrals numerically [46]. The finer (120,974) grid was used to check small imaginary vibrational frequencies possibly arising from DFT integration errors. The tight (10−8 Hartree) designation was the default for the self-consistent field (SCF) convergence. For the Cp2FeMo(CO)n (n = 6, 5, 4, 3, 2) systems each structure was optimized considering both singlet and triplet spin states. The Wiberg bond indices (WBIs) of the Fe-Mo bonds in the Cp2FeMo (CO)n (n = 6, 5, 4, 3, 2) derivatives were obtained by NBO analysis [47] using the BP86 method (Table 1). Previous studies on WBI values for bonds between two d-block transition metals suggest values in the 0.2–0.3 range for formal single bonds with proportionately higher values for formal multiple bonds [48]. The optimized Cp2FeMo(CO)n (n = 6, 5, 4, 3, 2) structures are designated as nCO-xX, where n is the number of carbonyl groups, x is the position of the structure in the relative energy sequence, and X refers to the spin state with S or T corresponding to singlet and triplet spin states,

Fig. 2. Structures of the unsaturated species singlet Cp2Mo2(CO)4 with a Mo≡Mo triple bond and of triplet Cp2Fe2(CO)3 with an Fe=Fe double bond.

electron configuration [21,22,23] corresponding to a singlet spin state structure (Fig. 2) [25]. Conditions were subsequently found [26] to give the corresponding unsubstituted Cp2Mo2(CO)4. The short Mo≡Mo distances of ∼2.5 Å in these tetracarbonyls [27] relative to the Mo–Mo single bond distance of 3.24 Å in the hexacarbonyl [20] Cp2Mo2(CO)6 are consistent with the formal triple bonds required to give each molybdenum atom the favored 18-electron configuration. The synthesis of unsaturated binuclear cyclopentadienyliron carbonyls is more delicate requiring photolysis of Cp2Fe2(CO)4 at or below room temperature. The resulting Cp2Fe2(CO)3 is a triply bridged triplet spin state structure with an Fe=Fe σ + 2/2 π double bond of length 2.265 Å similar to that in dioxygen (Fig. 2) [28]. Pyrolysis of the saturated Cp2Fe2(CO)4 leads to the tetranuclear derivative Cp4Fe4(µ3CO)4 possibly through a triply bonded Cp2Fe2(CO)2 intermediate (Fig. 3) [29]. The existence of Cp2Mo2(CO)6 and Cp2Fe2(CO)4 as stable species with the two halves joined by metal-metal single bonds suggested the possibility of an intermediate species Cp2FeMo(CO)5 with a heterometallic Fe–Mo single bond. This was achieved initially in a straightforward manner by the metathesis reaction of CpFe(CO)2I with NaMo (CO)3Cp [30] and later by the photolysis of a mixture of Cp2Fe2(CO)4 and Cp2Mo2(CO)6 [31]. Infrared spectroscopy of the resulting Cp2FeMo (CO)5 clearly indicated an unbridged structure like Cp2Mo2(CO)6 rather than a doubly bridged structure like Cp2Fe2(CO)4 (Fig. 1). These experimental results suggest that the binuclear heterometallic cyclopentadienyl MoFe carbonyl systems Cp2FeMo(CO)n might be a source of interesting unsaturated species likely to contain novel heterometallic metal-metal multiple bonds. Furthermore, the differences between the homometallic binuclear Mo2 and Fe2 systems, raise the

Fig. 3. The triply bonded Cp2Fe2(µ-CO)2 as an intermediate in the formation of the tetrahedral Cp4Fe4(µ3-CO)4 cluster upon pyrolysis of Cp2Fe2(CO)4. 103

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Table 1 Wiberg bond indices (WBIs) for the Fe–Mo bonds, formal bond orders, Fe–Mo distances, and the natural charges on the iron and molybdenum atoms in the optimized Cp2FeMo(CO)n (n = 6, 5, 4, 3, 2) structures from NBO analysis using the BP86 method. Structure

Fe-Mo WBI

Formal bond order

Distance (Å)

6CO-1S (C1) 6CO-2S (C1) 6CO-3S (C1) 6CO-4S (C1) 5CO-1S (Cs) 5CO-2S (Cs) 5CO-3S (Cs) 5CO-4S (Cs) 5CO-5S (Cs) 4CO-1S (Cs) 4CO-2S (Cs) 4CO-3S (C1) 4CO-4T (Cs) 4CO-5S (Cs) 3CO-1S (C3) 3CO-2T (C3) 3CO-3S (Cs) 3CO-4S (C1)

0.09 0.09 0.09 0.12 0.37 0.29 0.29 0.35 0.30 0.56 0.56 0.52 0.36 0.72 0.81 0.69 0.54 0.65

0 0 0 0 1 1 1 1 1 2 2 2 1 2 3 2 2 2

4.071 4.053 4.189 3.843 2.993 2.835 2.846 3.106 2.820 2.593 2.673 2.670 2.736 2.657 2.326 2.421 2.383 2.740

2CO-1S (Cs) 2CO-2T (C1) 2CO-3T (C1) 2CO-4S (C1) 2CO-5T (Cs)

1.27 1.01 1.00 1.50 0.40

4 3 3 4 1

2.290 2.315 2.314 2.231 2.809

Bridging groups

Natural charge Fe

Mo

μ-CO μ-CO μ-CO μ-CO None 2μ-CO 2μ-CO None 2μ-CO 2μ-CO 2μ-CO 2μ-CO 2μ-CO μ-CO 3μ-CO 3μ-CO None μ-CO, η2-μCO 2μ-CO 2μ-CO 2μ-CO None 2η2-μ-CO

−1.19 −1.19 −1.19 −1.23 −1.02 −1.06 −1.03 −0.98 −1.84 −0.52 −0.44 −0.96 −0.06 −1.46 −0.64 −0.06 −0.83 −0.12

−1.23 −1.26 −1.22 −1.09 −1.12 −1.17 −1.14 −1.09 −0.68 −1.06 −1.12 −0.69 −1.17 −0.40 −0.67 −0.83 −0.49 −0.98

−0.50 0.10 −0.78 −0.63 −0.92

−0.07 −0.45 0.18 0.01 0.14

Fig. 4. Four optimized Cp2FeMo(CO)6 structures (bond lengths in Å) with relative energies (kcal/mol) and symmetry point groups. The upper numbers were obtained from the BP86 method, while the lower numbers were obtained from the mPW1PW91 method. The data in the other figures have the same arrangement.

respectively. For example, the singlet energetically lowest-lying Cp2FeMo(CO)6 structure is designated as 6CO-1S.

Table 2 Relative energies (ΔE, in kcal/mol) and Fe-Mo bond distances (Å) for the Cp2FeMo(CO)6 structures. None of these structures has any imaginary vibrational frequencies.

3. Results 3.1. Cp2FeMo(CO)6 Various types of Cp2FeMo(CO)6 structures were optimized. Four singlet structures (Fig. 4 and Tables 2 and 3) were found for Cp2FeMo (CO)6 within 28 kcal/mol of the lowest energy structure 6CO-1S. This structure as well as the next two Cp2FeMo(CO)6 structures 6CO-2S and 6CO-3S, lying only 1.2 and 1.5 kcal/mol in energy above 6CO-1S, are similar by each having a two-electron donor weakly semibridging μ-CO group spanning a long non-bonding Fe⋯Mo distance of ∼4.1 Å as well as five terminal carbonyls. These semibridging μ-CO groups, representing the only connection between the iron atom and the molybdenum atom in the absence of a direct Fe–Mo bond, exhibit low ν(CO) frequencies of 1715, 1737, and 1737 cm−1 for 6CO-1S, 6CO-2S, and 6CO-3S, respectively. The main difference between the three structures 6CO-1S, 6CO-2S, and 6CO-3S are the different positions of the carbonyl groups. The Fe–Mo WBIs of 0.09 in 6CO-1S, 6CO-2S, and 6CO-3S suggest only weak metal-metal interactions. Structures 6CO1S, 6CO-2S, and 6CO-3S are best considered as ion pairs of the CpFe (CO)3+ cation [49] and the CpMo(CO)3– anion [15,50], both of which are known species with the favored 18-electron configuration. The semibridging carbonyl groups in these three structures originate from the CpFe(CO)3+ cation as indicated by the relatively long Mo–C distances of ∼2.5 Å to this carbonyl group. The much higher energy singlet Cp2FeMo(CO)6 structure 6CO-4S, lying 28.4 kcal/mol in energy above the global minimum structure 6CO-1S, resembles superficially the much lower energy structures 6CO1S, 6CO-2S, and 6CO-3S by having a long non-bonding Fe⋯Mo distance and a semibridging carbonyl group between the iron and molybdenum atoms (Fig. 4 and Tables 2 and 3). However, the Mo–C bond distance of 2.096 Å in 6CO-4S to the semibridging carbonyl group is

Structures

ΔE (BP86)

Fe–Mo

6CO-1S 6CO-2S 6CO-3S 6CO-4S

0.0 1.2 1.5 28.4

4.071 4.053 4.189 3.843

(C1) (C1) (C1) (C1)

∼0.4 Å shorter than the comparable Mo–C distances in the lower energy structures 6CO-1S, 6CO-2S, and 6CO-3S. Similarly the Fe⋯Mo distance of 3.843 Å is ∼0.2 Å shorter than the Fe⋯Mo distances in the three much lower energy structures. Also the ν(CO) frequency of 1561 cm−1 for the bridging carbonyl group in 6CO-4S lies ∼200 cm−1 below those for the bridging carbonyl groups in 6CO-1S, 6CO-2S, and 6CO-3S. These differences suggest formulations of the three 6CO-1S, 6CO-2S, and 6CO-3S structures lying within less than 2 kcal/mol as the {CpFe(CO)3+,CpMo(CO)3–} ion pairs in different relative orientations. However, the much higher energy isomer 6CO-4S with significantly different Mo–C(O) and Fe⋯Mo distances is better formulated as the ferraacyl derivative CpFe(CO)2–C(O)–Mo(CO)3Cp in which each metal atom has the favored 18-electron configuration like each metal atom in the much lower energy isomers. For comparison, the reported [51] acyl ν(CO) frequency for the benzoyl derivative of cyclopentadienyliron dicarbonyl, PhCOFe(CO)2Cp is 1603 cm−1. 3.2. Cp2FeMo(CO)5 Five singlet structures were found for Cp2FeMo(CO)5 within 23 kcal/mol of the global minimum (Fig. 5 and Tables 4 and 5). The two 104

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Table 3 Infrared ν (CO) vibrational frequencies (cm−1) predicted for the four lowest energy structures of Cp2FeMo (CO)6. Infrared intensities in parentheses are in km/mol. BP86 6CO-1S 6CO-2S 6CO-3S 6CO-4S

(C1) (C1) (C1) (C1)

1715(205), 1737(176), 1737(171), 1561(103),

1897(787), 1918(363), 1962(121), 1971(1130), 2010(737) 1894(668), 1917(554), 1957(552), 1968(1702), 2001(238) 1888(896), 1898(367), 1959(919), 1974(753), 2015(641) 1899(1471), 1939(453), 1976(562), 1991(799), 2020(689)

lowest energy structures 5CO-1S and 5CO-2S lie within ∼2 kcal/mol of each other suggesting a fluxional system. Furthermore, the relative energies of 5CO-1S and 5CO-2S depend upon the functional. The lowest energy Cp2FeMo(CO)5 structures 5CO-1S using the mPW1PW91 functional is the experimental structure with three terminal carbonyl groups bonded to molybdenum and two terminal carbonyl groups bonded to iron and a trans orientation of the Cp rings (Fig. 5 and Tables 4 and 5). However, 5CO-1S lies 2.0 kcal/mol in energy above 5CO-2S using the BP86 functional. The Fe-Mo distance in 5CO-1S of 2.993 Å corresponding to a WBI of 0.37 can be interpreted as a formal single bond thereby giving both metal atoms the favored 18-electron configuration. These predicted ν(CO) frequencies for 5CO-1S lie within 20 cm−1 of the experimental values for Cp2FeMo(CO)5 in CCl4 solution (Table 5). The second low-energy singlet Cp2FeMo(CO)5 structure 5CO-2S is a Cs structure with two bridging carbonyls, three terminal carbonyls, and a trans arrangement of the Cp rings (Fig. 5 and Tables 4 and 5). The two bridging µ-CO groups exhibit ν(CO) frequencies at 1784 and 1806 cm−1, whereas the three terminal CO groups exhibit ν(CO) frequencies at 1905, 1950, and 1977 cm−1. The predicted Fe-Mo distance of 2.835 Å with a WBI of 0.29 suggests the formal single bond needed to give each metal atom the favored 18-electron configuration. The Cp2FeMo(CO)5 structure 5CO-3S, lying 2.2 kcal/mol in energy above 5CO-1S, is similar to 5CO-2S except for the cis rather than trans orientation of the Cp rings. The Fe–Mo distances in the doubly bridged structures 5CO-2S and 5CO-3S are ∼0.16 Å shorter and the

Table 4 Relative energies (ΔE, in kcal/mol) and Fe-Mo bond distances (Å) for the Cp2FeMo(CO)5 structures by both the BP86 and mPW1PW91 methods. None of these structures has any imaginary vibrational frequencies. Structures 5CO-1S 5CO-2S 5CO-3S 5CO-4S 5CO-5S

(Cs) (Cs) (Cs) (Cs) (Cs)

ΔE (BP86)

ΔE (mPW1PW91)

Fe–Mo

0.0 –2.0 2.2 14.3 21.0

0.0 1.4 5.4 14.3 29.2

2.993 2.835 2.846 3.106 2.820

Table 5 Infrared ν(CO) vibrational frequencies (cm−1) predicted for the two lowest energy structures of Cp2FeMo(CO)5. Infrared intensities in parentheses are in km/mol. BP86 5CO-1S (Cs) Cp2FeMo(CO)5 exp. (CCl4) 5CO-2S (Cs) 5CO-3S (Cs) 5CO-4S (Cs) 5CO-5S (Cs)

1887(638),1899(231),1937(906),1951(1434),1981(21) 1885 (s), 1887 (s), 1942 (vs), 1956 (vs), 1999 (w) 1784(684), 1777(663), 1854(795), 1771(831),

1806(81), 1905(779), 1950(1380), 1977(182) 1803(94), 1919(790), 1954(490), 1993(1123) 1865(590), 1917(190), 1970(693), 2008(720) 1793(73), 1937(963), 1958(502), 2010(1083)

Fig. 5. Five optimized Cp2FeMo(CO)5 structures with their relative energies (kcal/mol) by both the BP86 and mPW1PW91 methods.

105

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corresponding WBI is 0.08 lower than that in the unbridged structure 5CO-1S. This can be an effect of the two bridging CO groups in 5CO-2S and 5CO-3S relative to the lack of bridging CO groups in 5CO-1S. The Cp2FeMo(CO)5 structure 5CO-4S, lying 14.3 kcal/mol in energy above 5CO-1S, is an unbridged structure similar to 5CO-1S except for the arrangement of the Cp rings and CO groups. The unbridged Fe–Mo distance in 5CO-4S of 3.106 Å is ∼0.1 Å longer than that in 5CO-1S, but the Fe–Mo WBI in 5CO-4S of 0.35 is only 0.02 lower than that in 5CO-1S. The longer Fe–Mo distance in 5CO-4S relative to that in 5CO1S may relate to increased steric hindrance between the CpMo(CO)3 and CpFe(CO)2 subunits owing to the different arrangement of the Cp rings and CO groups. The increased steric hindrance might account for the considerably higher energy of 5CO-4S relative to 5CO-1S. This is supported by the NCI analysis of the steric hindrance in 5CO-1S and 5CO-4S (Fig. 3S in the Supporting Information). In this NCI analysis, the red area indicates repulsive interactions arising from steric hindrance so that a larger red area corresponds to increased repulsive energy from steric hindrance. In Fig. 3S the larger steric repulsion between iron, molybdenum, and the neighboring carbonyl groups in 5CO4S than that in 5CO-1S is indicated by the larger red area in the isosurface of 5CO-4S than that of 5CO-1S. The much higher energy Cp2FeMo(CO)5 structure 5CO-5S, lying 21.0 kcal/mol above 5CO-1S, is of a very different type than the lower energy singlet isomers discussed above (Fig. 5 and Tables 4 and 5). Thus 5CO-5S has the molybdenum atom bonded to both cyclopentadienyl groups as terminal ligands. Three of the five carbonyl groups are bonded to the iron atom as terminal ligands. The two remaining carbonyl groups in 5CO-5S bridge the Fe–Mo bond and exhibit the lower ν(CO) frequencies at 1771 and 1793 cm−1 characteristic of bridging carbonyl groups. The Mo→Fe distance in 5CO-5S of 2.820 Å suggests a formal single bond consistent with its WBI of 0.30 (Table 1). Considering this single bond as a dative Mo→Fe bond from molybdenum to iron gives both metal atoms in 5CO-5S the favored 18-electron configuration.

minimum (Fig. 6 and Tables 6 and 7). There is relatively little spin contamination in 4CO-4T, i.e., S 2 = 2.06. Structure 4CO-4T has two bridging carbonyl groups and two terminal carbonyl groups bonded to the molybdenum atom. The Fe–Mo distance in 4CO-4T of 2.736 Å, corresponding to a formal Fe-Mo single bond. This gives the molybdenum atom in 4CO-4T the favored 18-electron configuration but the iron atom only a 16-electron configuration; the latter can be responsible for the triplet spin state. The much higher energy singlet Cp2FeMo(CO)4 structure 4CO-5S, lying 22.6 kcal/mol above the lowest energy structure 4CO-1S, is similar to the high-energy Cp2FeMo(CO)5 structure 5CO-5S by having both cyclopentadienyl rings as terminal ligands bonded exclusively to the molybdenum atom, one carbonyl group bridging the Fe=Mo bond, and the iron atom bonded only to carbonyl groups (Fig. 6 and Tables 6 and 7). The Fe=Mo distance in 4CO-5S of 2.657 Å with a WBI value of 0.72 suggests a formal double bond. This gives each metal atom the favored 18-electron configuration with a formal negative charge on the iron atom and a compensating formal positive charge on the molybdenum atom. The Cp2Mo(µ-CO)Fe(CO)3 structure 4CO-5S can be generated by removing a bridging µ-CO group from the Cp2Mo(µCO)2Fe(CO)3 structure 5CO-5S. 3.4. Cp2FeMo(CO)3 Three singlet structures and one triplet structure (Fig. 7 and Tables 8 and 9) were found for Cp2FeMo(CO)3 (Fig. 7 and Tables 8 and 9). The lowest-energy structure 3CO-1S is the triply bridged isomer Cp2FeMo (µ-CO)3 with local C3 symmetry of the central Fe(µ-CO)3Mo unit. The three bridging carbonyl groups in 3CO-1S are nearly symmetrical with Fe–C distances of ∼2.07 Å and Mo–C distances of ∼2.03 Å and exhibit ν(CO) frequencies at 1807, 1807, and 1847 cm−1 (average 1820 cm−1). The relatively short Fe≡Mo distance of 2.326 Å in 3CO-1S with a WBI of 0.81 suggests a formal triple bond. This gives the both metal atoms in 3CO-1S the favored 18-electron configuration provided that the iron atom bears a formal negative charge and the molybdenum atom bears a formal positive charge. The Cp2FeMo(µ-CO)3 structure 3CO-1S is obviously a favorable structure since the next lowest energy Cp2FeMo(CO)3 isomer, namely 3CO-2T, lies 17.5 kcal/mol in energy above 3CO-1S (Fig. 7 and Tables 8 and 9). Structure 3CO-2T, like 3CO-1S, is a triply bridged structure but in a triplet spin state. The carbonyl bridges in 3CO-2T are less symmetrical than those in 3CO-1S with relatively long Fe–C distances of ∼2.40 Å and Mo–C distances of ∼1.97 A. They thus are closer to terminal carbonyl groups than the three µ-CO groups in 3CO-1S and correspondingly exhibit somewhat higher ν(CO) frequencies of 1784, 1843, and 1887 cm−1 (averaging 1838 cm−1). The Fe=Mo distance of 2.421 Å in 3CO-2T is ∼0.1 Å longer than that in 3CO-1S suggesting a formal double bond. Suitable distribution of the electrons from the bridging carbonyl groups and/or compensating formal changes on the metal atoms can give each metal atom in 3CO-2T a 17-electron configuration consistent with a binuclear triplet. The second singlet Cp2FeMo(CO)3 structure 3CO-3S, lying 22.9 kcal/mol above the lowest energy structure 3CO-1S, has two semibridging carbonyl groups with short Mo–C distances of ∼1.97 Å and long Fe–C distances of ∼2.36 Å and a terminal group bonded to the iron atom (Fig. 7 and Tables 8 and 9). The bridging carbonyl groups in 3CO-3S exhibit ν(CO) frequencies at 1848 and 1873 cm−1 and the terminal carbonyl group exhibits a ν(CO) frequency at 1965 cm−1 (Table 9). The Fe≡Mo distance of 2.383 Å in 3CO-3S is similar to that in 3CO-1S and can likewise be interpreted as a formal triple bond. This gives each metal atom in 3CO-3S, like those in 3CO-1S, the favored 18electron configuration. Using the BP86 method gives a very small imaginary vibrational frequency of 16i cm−1 for 3CO-3S. However, this can be removed using a finer grid (120, 974), indicating that this small imaginary frequency is an artefact arising from numerical integration error.

3.3. Cp2FeMo(CO)4 Four singlet structures and one triplet structure were found for Cp2FeMo(CO)4 (Fig. 6 and Tables 6 and 7). The lowest energy Cp2FeMo (CO)4 structure 4CO-1S as well as 4CO-3S, lying only 0.6 kcal/mol in energy above 4CO-1S, have two bridging carbonyl groups as well as a terminal carbonyl group bonded to each metal atom. The Fe=Mo distances in 4CO-1S and 4CO-3S of 2.593 and 2.670 Å, respectively, are ∼0.4 Å shorter than that in 5CO-1S. This suggests the formal Fe=Mo double bond required to give each metal atom the favored 18-electron configuration with a formal positive charge on the iron atom and a formal negative charge on the molybdenum atom. This is consistent with the Fe=Mo WBIs of 0.56 in 4CO-1S and 0.52 in 4CO-3S, which are close to 1.5 times the WBI values expected for formal single bonds (Table 1) The Cp2FeMo(CO)4 structure 4CO-1S can be derived from the likewise doubly bridged Cp2FeMo(CO)5 structure 5CO-2S by removing a terminal group from the molybdenum atom. The singlet Cp2FeMo (CO)4 structures 4CO-3S and 4CO-1S differ only in the orientations of the carbonyl and Cp ligands. The second low-energy singlet Cp2FeMo(CO)4 structure 4CO-2S, lying only 0.2 kcal/mol in energy above 4CO-1S, has two carbonyl groups bridging the Fe=Mo bond and two terminal carbonyl groups bonded to the molybdenum atom (Fig. 6 and Tables 6 and 7). The Fe=Mo distance in 4CO-2S of 2.673 Å corresponding to a WBI of 0.56 (Table 1) suggests the formal double bond required to give each metal atom the favored 18-electron configuration with a positive charge on the molybdenum atom and a negative charge on the iron atom. The lowest energy triplet Cp2FeMo(CO)4 structure 4CO-4T, lying 4.9 kcal/mol above 4CO-1S, has a very small imaginary vibrational frequency of 16i cm−1 implying a transition state rather than a local 106

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Fig. 6. Five optimized Cp2FeMo(CO)4 structures with their relative energies in kcal/mol.

semibridging CO group, which is a two-electron donor with a long Fe⋯O distance rather than a four-electron donor with a short Fe⋯O distance. The Fe-Mo distance in 3CO-4S of 2.740 Å with a WBI of 0.65 suggests the formal double bond required to give each metal atom the favored 18-electron configuration.

Table 6 Relative energies (ΔE, in kcal/mol), Fe-Mo bond distances (Å), numbers of imaginary vibrational frequencies (Nimag), and spin contaminations S 2 for the Cp2FeMo(CO)4 structures. Structures

ΔE

Fe-Mo

Nimag

S2

4CO-1S (Cs) 4CO-2S (Cs) 4CO-3S (C1) 4CO-4T (Cs) 4CO-5S (Cs)

0.0 0.2 0.6 4.9 22.6

2.593 2.673 2.670 2.736 2.657

0 0 0 1(16i) 0

0.00 0.00 0.00 2.06 0.00

3.5. Cp2FeMo(CO)2 Two singlet and three triplet structures (Fig. 8 and Tables 10 and 11) were found for Cp2FeMo(CO)2 within 32 kcal/mol (Fig. 8 and Tables 10 and 11). The two carbonyl groups in the lowest energy Cp2FeMo(CO)2 structure 2CO-1S are nearly symmetrical bridges with Mo-C distances of 2.036 Å and Fe-C distances of 1.954 Å and exhibit the expected relatively low ʋ(CO) frequencies, namely 1774 and 1791 cm−1 (Table 11). The short FecMo distance in 2CO-1S of 2.290 Å corresponding to a relatively high WBI of 1.27 suggests the formal quadruple bond required to give each metal atom the favored 18electron configuration. The MO analysis of this quadruple bond indicates one σ component, two π component, and one δ component (Fig. 1S in the Supporting Information). The doubly bridged Cp2FeMo (CO)2 structure 2CO-1S may be derived from the triply bridged Cp2FeMo(CO)3 structure 3CO-1S by removing one of the bridging carbonyl groups. The two lowest energy triplet Cp2FeMo(CO)2 structures 2CO-2T and 2CO-3T, lying 6.5 and 8.4 kcal/mol, respectively, above 2CO-1S, are doubly bridged structures with similar overall geometries except for the Fe-C and Mo-C distances to the bridging carbonyl groups (Fig. 8 and Tables 10 and 11). In 2CO-2T both Mo-C distances are relatively short at ∼1.97 Å and both Fe-C distances are relatively long at ∼2.28 Å. However, 2CO-3T has one short Mo-C distance at 2.026 Å and one long Mo-C distance at 2.373 Å as well as one short Fe-C distance at 1.778 Å and one long Fe-C distance at 2.024 Å. Both 2CO-2T and 2CO-3T have Fe≡Mo distances of ∼2.31 Å corresponding to WBIs of ∼1.0. These

Table 7 Infrared ν(CO) vibrational frequencies (cm−1) predicted for the two lowest energy structures of Cp2FeMo(CO)4. Infrared intensities in parentheses are in km/mol. BP86 4CO-1S (Cs) 4CO-2S (Cs) 4CO-3S (C1) 4CO-4T (Cs) 4CO-5S (Cs)

1845(994), 1766(651), 1783(533), 1764(700), 1820(624),

1859(80), 1918(1359), 1946(301) 1790(198), 1928(848), 1968(912) 1828(482), 1895(907), 1968(651) 1794(219), 1921(824), 1965(866) 1906(1023), 1928(242), 1987(1261)

The next singlet Cp2FeMo(CO)3 structure 3CO-4S, lying 26.6 kcal/mol in energy above 3CO-1S, has two bridging carbonyl groups as well as a terminal carbonyl group bonded to the molybdenum atom (Fig. 7 and Tables 8 and 9). The short Fe–O distance of 2.136 Å to one of the bridging carbonyl groups indicates a four-electron donor η2-μ-CO group. This is consistent with the extremely low ν(CO) frequency of 1676 cm−1 and relatively long C–O distance of 1.226 Å indicating a very low C–O bond order for this η2-µ-CO group. The ν(CO) frequency in 3CO-4S of 1784 cm−1 is assigned to the second 107

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Fig. 7. Four optimized Cp2FeMo(CO)3 structures.

(CO)n (n = 6, 5, 4, 3) derivatives according to the following equation:

Table 8 Relative energies (ΔE, in kcal/mol), Fe-Mo bond distances (Å), numbers of imaginary vibrational frequencies, and spin contaminations S 2 for the Cp2FeMo(CO)3 structures. Structures

ΔE

Fe-Mo

Nimag

S2

3CO-1S (C3) 3CO-2T (C3) 3CO-3S (Cs) 3CO-4S (C1)

0.0 17.5 22.9 26.6

2.326 2.421 2.383 2.740

0 0 1(16i) 0

0.00 2.07 0.00 0.00

Cp2 FeMo (CO )n

BP86 1807(945), 1784(729), 1848(900), 1676(330),

1807(936), 1843(835), 1873(200), 1784(436),

1

+ CO

(1)

The predicted dissociation energy of one CO group from Cp2FeMo (CO)6 to give Cp2FeMo(CO)5 (Table 12) is very small at 5.4 kcal/mol. This suggests that Cp2FeMo(CO)6 is not likely to be a viable species consistent with the experimental observation of Cp2FeMo(CO)5 rather than Cp2FeMo(CO)6. Further dissociation of a CO ligand from Cp2FeMo (CO)5 to give Cp2FeMo(CO)4 requires a relatively high energy of 45.8 kcal/mol, again consistent with the experimental observation of Cp2FeMo(CO)5 as a stable species. The low CO dissociation energy of 15.2 kcal/mol for the tetracarbonyl Cp2FeMo(CO)4 relative to the high CO dissociation energy of 67.0 kcal/mol for the tricarbonyl Cp2FeMo (CO)3 suggest the tricarbonyl to be a viable species and a reasonable synthetic objective. Thus the disproportionation reaction 2Cp2FeMo (CO)4 → Cp2FeMo(CO)5 + Cp2FeMo(CO)3 is predicted to be very exothermic by 67.0–15.2 = 51.8 kcal/mol.

Table 9 Infrared ν(CO) vibrational frequencies (cm−1) predicted for the two lowest energy Cp2FeMo(CO)3 structures. Infrared intensities in parentheses are in km/mol.

3CO-1S (C3) 3CO-2T (C3) 3CO-3S (Cs) 3CO-4S (C1)

Cp2 FeMo (CO )n

1847(0.8) 1887(365) 1965(1110) 1933(864)

4. Discussion The experimentally known Cp2FeMo(CO)5 structure can be considered as a “hybrid” of the doubly bridged Cp2Fe2(CO)2(µ-CO)2 structure and the unbridged Cp2Mo2(CO)6 (Fig. 1) and thus conceivably might have either a doubly bridged structure like the former or an unbridged structure like the latter. The experimental Cp2FeMo(CO)5 structure is clearly shown by its ν(CO) frequencies (Table 5) to be the lowest energy unbridged structure 5CO-1S analogous to Cp2Mo2(CO)6 rather than the slightly higher energy doubly bridged structure 5CO-2S analogous to Cp2Fe2(CO)2(µ-CO)2 (Fig. 5). The small energy difference between the unbridged 5CO-1S structure and the doubly bridged 5CO2S Cp2FeMo(CO)5 structure suggests a fluxional system involving interchange of bridging and terminal carbonyl groups. The preferred unbridged heterometallic Cp2FeMo(CO)5 structure 5CO-1S as well as its slightly higher energy doubly bridged 5CO-2S isomer (Fig. 5) can also be compared with the isoelectronic homometallic Cp2M2(CO)4(µ-CO) (M = Mn [52], Re [53]) structures, which have a single bridging carbonyl group and two terminal carbonyl groups on each metal atom (Fig. 9). The distribution of the carbonyl groups is different in the heterometallic FeMo and homometallic M2 (M = Mn, Re) systems. This reflects the fact that in order to achieve the favored 18-electron configuration the central molybdenum atom in Cp2FeMo(CO)5 requires 12 electrons from external ligands whereas the central iron atom requires only 10 electrons from external ligands. The three low-energy singlet structures of the tetracarbonyl Cp2FeMo(CO)4 are all doubly bridged structures obtained by removal of terminal carbonyl groups from the doubly bridged structures 5CO-2S or 5CO-3S of the pentacarbonyl Cp2FeMo(CO)5 (Fig. 6). Isomeric Cp2FeMo(CO)2(µ-CO)2 structures with both terminal carbonyl groups on molybdenum (4CO-2S) or with one on molybdenum and one on iron

can be interpreted as the formal triple bonds needed to give each metal atom a 17-electron configuration for a binuclear triplet. The singlet Cp2FeMo(CO)2 structure 2CO-4S, lying 19.5 kcal/mol in energy above 2CO-1S, is an unbridged structure with single terminal groups bonded to each metal atom (Fig. 8 and Tables 10 and 11). The predicted FecMo distance of 2.231 Å with a relatively high WBI of 1.50 can be interpreted as the formal quadruple bond required to give each metal atom the favored 18-electron configuration. The FecMo quadruple bond in 2CO-4S is confirmed by MO analysis indicating four components of this chemical bond (Fig. 2S in the Supporting Information). The relatively high energy triplet Cp2FeMo(CO)2 structure 2CO-5T at 31.5 kcal/mol above 2CO-1S has two four-electron donor η2-μ-CO bridging carbonyls with non-equivalent Fe–C and Mo–C distances of 1.750 Å and 2.288 Å, respectively, as well as bonding Mo-O distances of 2.400 Å (Fig. 8 and Tables 10 and 11). These η2-µ-CO groups exhibit very low ν(CO) frequencies of 1676 and 1722 cm−1 in accord with expectation. Structure 2CO-5T has a significant imaginary vibrational frequency of 52i cm−1 but only a relatively small degree of spin contamination, i.e., S 2 = 2.02. The Fe–Mo distance in 2CO-5T of 2.809 Å with a WBI of 0.40, can be interpreted as a formal single bond. This gives each metal atom the 17-electron configuration for a binuclear triplet after considering the four electrons donated to the central Fe–Mo unit by each of the η2-µ-CO groups. 3.6. Thermochemistry of the Cp2FeMo(CO)n (n = 6, 5, 4, 3, 2) structures Table 12 lists the carbonyl dissociation energies for the Cp2FeMo 108

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Fig. 8. Five optimized Cp2FeMo(CO)2 structures with their relative energies in kcal/mol. Table 10 Relative energies (ΔE, in kcal/mol), Fe-Mo bond distances (Å), numbers of imaginary vibrational frequencies, and spin contaminations S 2 for the Cp2FeMo(CO)2 structures. Structures

ΔE

Fe-Mo

Nimag

S2

2CO-1S (Cs) 2CO-2T (C1) 2CO-3T (C1) 2CO-4S (C1) 2CO-5T (Cs)

0.0 6.5 8.4 19.5 31.5

2.290 2.315 2.314 2.231 2.809

0 0 0 0 1(52i)

0.00 2.12 2.03 0.00 2.02

Fig. 9. Comparison of the unbridged and doubly bridged low-energy Cp2FeMo (CO)5 structures with the experimental singly bridged valence isoelectronic Cp2Re2(CO)4(µ-CO) structure.

Table 11 Infrared ν(CO) frequencies (cm−1) predicted for the five lowest energy structures of Cp2FeMo(CO)2. Infrared intensities in parentheses are in km/mol.

(4CO-1S and 4CO-3S) are of comparable energies. The Fe=Mo distances in these doubly bridged Cp2FeMo(CO)4 structures suggest the formal double bonds needed to give each metal atom the favored 18electron configuration. However, these Cp2FeMo(CO)4 structures with Fe=Mo double bonds are strongly energetically disfavored by ∼52 kcal/mol with regard to disproportionation into Cp2FeMo(CO)5 with a formal Fe–Mo single bond and Cp2FeMo(CO)3 with a formal Fe≡Mo triple bond. Such an energetically favored disproportionation of a metal=metal double bond into a metal–metal single bond plus a metal≡metal triple bond was previously observed, even experimentally, in the disproportionation of the unstable Cp2Cr2(CO)5 with a Cr=Cr double bond into Cp2Cr2(CO)6 with a Cr–Cr single bond and Cp2Cr2(CO)4 with a Cr≡Cr triple bond [54]. For the tricarbonyl Cp2FeMo(CO)3 a triply bridged structure 3CO-1S with a formal Fe≡Mo triple bond is very favorable with the next lowest energy isomer lying ∼17 kcal/mol above 3CO-1S (Fig. 7). Closely related triply bridged valence isoelectronic group 7 homometallic structures Cp2M2(µ-CO)3 (M = Mn [55,56], Re [57]) have been synthesized and characterized structurally. The lowest energy structures for the dicarbonyl Cp2FeMo(CO)2 are

BP86 2CO-1S (Cs) 2CO-2T (C1) 2CO-3T (C1) 2CO-4S (C1) 2CO-5T (Cs)

1774(1043), 1791(149) 1819(873), 1860(528) 1791(672), 1854(472) 1857(722), 1938(926) 1676(290), 1722(336)

Table 12 Carbonyl dissociation energies (ΔE, in kcal/mol) for Cp2FeMo(CO)n using the BP86 method. Structures Cp2FeMo(CO)6 Cp2FeMo(CO)5 Cp2FeMo(CO)4 Cp2FeMo(CO)3

ΔE (6CO-1S) → Cp2FeMo(CO)5 (5CO-1S) → Cp2FeMo(CO)4 (4CO-1S) → Cp2FeMo(CO)3 (3CO-1S) → Cp2FeMo(CO)2

(5CO-1S) + CO (4CO-1S) + CO (3CO-1S) + CO (2CO-1S) + CO

5.4 43.8 15.2 67.0

109

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all doubly bridged structures with short Fe-Mo distances suggesting multiple bonds (Fig. 8). The singlet structure 2CO-1S requires an FecMo quadruple bond for the metal atoms to have the favored 18electron configuration whereas the triplet structures 2CO-2T and 2CO3T lying respectively ∼6 kcal/mol and ∼8 kcal/mol above 2CO-1S require only formal triple Fe≡Mo bonds for the 17-electron metal configurations for the binuclear triplet structures. This difference in formal bond order has little effect on the Fe-Mo distances with 2.29 Å for the FecMo quadruple bond in 2CO-1S and 2.31 Å for the Fe≡Mo triple bonds in 2CO-2T and 2CO-3T. However, the difference in formal bond order in these lowest energy Cp2FeMo(µ-CO)2 structures is reflected in the higher FeMo quadruple bond WBI of 1.27 for 2CO-1S relative to the Fe≡Mo triple bond WBIs of 1.01 and 1.00 for 2CO-2T and 2CO-3T, respectively. The lowest energy structures of the valence isoelectronic Cp2M2(CO)2 (M = Mn [58], Re [59]) are also singlet and triplet doubly bridged structures. All of the lowest energy Cp2FeMo(CO)n (n = 5, 4, 3, 2) structures have one terminal Cp ring bonded to each metal. The significantly higher energy structures 5CO-5S and 4CO-5S are of the type Cp2Mo(µCO)nFe(CO)3 (n = 2, 1) with both Cp rings bonded to the molybdenum atom and only carbonyl groups bonded to the iron atom. The two structures 5CO-5S and 4CO-5S are related since removal of one of the bridging carbonyl groups in 5CO-5S generates 4CO-5S. The 18-electron rule appears to apply to these Cp2Mo(µ-CO)nFe(CO)3 derivatives. Thus the Fe–Mo distance of 2.820 Å with a WBI of 0.30 clearly corresponds to the formal single bond required to give each metal atom the favored 18electron configuration. Loss of a bridging carbonyl group in 5CO-5S to give the Cp2Mo(µ-CO)Fe(CO)3 structure 4CO-5S appears to preserve the favored 18-electron metal configurations by an increase in the metal-metal formal bond order. Thus the decrease of the Fe=Mo distance from 2.820 in 5CO-5S to 2.657 Å in 4CO-5S coupled with an Fe=Mo WBI increase to 0.72 suggests an increase in the metal–metal bond order to a double bond thereby preserving the 18-electron configuration for both metal atoms in 4CO-5S. The hexacarbonyl Cp2FeMo(CO)6 was also investigated. Since both the cation [49] [CpFe(CO)3]+ and the anion [15,50] [CpMo(CO)3]– are stable species with 18-electron metal configurations, the hexacarbonyl could simply be the salt [CpFe(CO)3]+[CpMo(CO)3]−. In the three lowest energy Cp2FeMo(CO)6 structures within predicted energies of 1.5 kcal/mol, namely 6CO-1S, 6CO-2S, and 6CO-3S, the CpFe(CO)3 unit is connected to the CpMo(CO)3 unit through one of the carbonyl groups of the CpFe(CO)3 unit in a CpFe(CO)2(µ-CO)Mo(CO)3Cp structure (Fig. 4). The Fe⋯Mo distances in these structures are long at ∼4.0 Å indicating the expected absence of a direct iron-molybdenum bond. Even in the absence of a metal–metal bond the ν(CO) frequency of this bridging carbonyl group of ∼1730 cm−1 lies at least 150 cm−1 below the next lowest ν(CO) frequency for all three structures. The carbonyl dissociation energy of the lowest energy Cp2FeMo(CO)6 structure 6CO-1S to form a heteronuclear Fe–Mo bond in 5CO-1S is only slightly exothermic at ∼ 5 kcal/mol as opposed to much higher CO dissociation energies for the other Cp2FeMo(CO)n (n = 5, 4, 3) species (Table 12). The energetic gain from forming an Fe–Mo bond in 5CO-1S can balance most of the typical energies of 20 kcal/mol or greater for carbonyl dissociation from a typical d-block transition metal carbonyl derivative.

doubly bridged structure lying 2.0 kcal/mol below the unbridged structure. The closeness in energy of these two structures suggests a fluxional system for Cp2FeMo(CO)5. The low-energy structures for the tetracarbonyl Cp2FeMo(CO)4 are doubly bridged structures with formal Fe=Mo double bonds in accord with the 18-electron rule. However, the tetracarbonyl Cp2FeMo(CO)4 with an Fe=Mo double bond is predicted not to be viable since its disproportionation into Cp2FeMo(CO)5 with an Fe–Mo triple bond and Cp2FeMo(CO)3 with an Fe≡Mo triple bond is found to be highly exothermic. The tricarbonyl Cp2FeMo(CO)3 has an energetically favorable triply bridged structure with a formal Fe≡Mo triple bond analogous to the experimental valence isoelectronic Cp*2M2(µ-CO)3 (M = Mn, Re; Cp* = η5-Me5C5) structures. The lowenergy structures of the dicarbonyl Cp2FeMo(CO)2 are doubly bridged structures which can be either a singlet with an Fe=Mo double bond or a triplet with an Fe≡Mo triple bond. The low-energy structures of the hexacarbonyl Cp2FeMo(CO)6 can be interpreted as consisting of [CpFe (CO)3]+ cations linked to [CpMo(CO)3]– anions through one of the iron-bonded carbonyl groups with long ∼4 Å Fe⋯Mo distances indicating the absence of a direct iron-molybdenum bond. Acknowledgements We are indebted to the National Natural Science Foundation of China (grant U1530262) and the Excellent Young Scholars Research Fund of the Beijing Institute of Technology (grant GZ2018025101) for support of this research. Appendix A. Supplementary data Tables S1–S4: Cartesian coordinates of the optimized Cp2FeMo (CO)6 structures using the BP86 and mPW1PW91 methods; Tables S5–S9: Cartesian coordinates of the optimized Cp2FeMo(CO)5 structures using the BP86 and mPW1PW91 methods; Tables S10–S14: Cartesian coordinates of the optimized Cp2FeMo(CO)4 structures using the BP86 and mPW1PW91 methods; Tables S15–S18: Cartesian coordinates of the optimized Cp2FeMo(CO)3 structures using the BP86 and mPW1PW91 methods; Tables S19–S23: Cartesian coordinates of the optimized Cp2FeMo(CO)2 structures using the BP86 and mPW1PW91 methods; Tables S24–S28: Harmonic vibrational frequencies and corresponding infrared intensities predicted for the Cp2FeMo(CO)n (n = 6, 5, 4, 3, 2) structures using the BP86 method; Tables S29–S33. Total energies (E, in hartree), relative energies (ΔE, in kcal/mol), numbers of imaginary vibrational frequencies (Nimg) and Fe-Mo bond distances (Å) for the Cp2FeMo(CO)n (n = 6, 5, 4, 3, 2) structures; complete Gaussian reference (Ref. [32]). Supplementary data to this article can be found online at https://doi.org/10.1016/j.ica.2019.04.044. References [1] J. Dewar, H.O. Jones, The physical and chemical properties of iron carbonyl, Proc. R. Soc. Lond. 76 (1905) 558–577. [2] L. Mond, H. Hirtz, M.D. Cowap, Some new metallic carbonyls, J. Chem. Soc. 798–809 (1910). [3] L. Mond, C. Langer, F. Quincke, Action of carbon monoxide on nickel, J. Chem. Soc. 749–753 (1890). [4] F.A. Cotton, J.M. Troup, Accurate determination of a classic structure in metalcarbonyl field – nonacarbonyldi-iron, J. Chem. Soc., Dalton Trans. (1974) 800–802. [5] G.G. Sumner, H.P. Klug, L.E. Alexander, Crystal structure of dicobalt octacarbonyl, Acta Crystallogr. 17 (1964) 732. [6] P.C. Leung, P. Coppens, Experimental charge-density study of dicobalt octacarbonyl-star and comparison with theory, Acta Crystallogr. B 39 (1983) 535–542. [7] D. Braga, F. Grepioni, P. Sabatino, A. Gavezzotti, Molecular organization in crystalline [Co2(CO)8] and [Fe2(CO)9] and a search for alternative packings for [Co2(CO)8], J. Chem. Soc., Dalton Trans. (1992) 1185–1191. [8] E.O. Brimm, M.A.J. Lynch, W.J. Sesny, Preparation and properties of manganese carbonyl, J. Am. Chem. Soc. 92 (1954) 3831–3835. [9] L.F. Dahl, E. Ishishi, R.E. Rundle, Structures of Mn2(CO)10 and Re2(CO)10, J. Chem. Phys. 26 (1957) 1750–1751. [10] T.J. Kealy, P.L. Pauson, A new type of organo-iron compound, Nature 168 (1961) 1039–1040. [11] S.A. Miller, J.A. Tebboth, J.F. Tremaine, Dicyclopentadienyliron, J. Chem. Soc.

5. Summary The lowest energy structure for the pentacarbonyl Cp2FeMo(CO)5 by the mPW1PW91 method corresponds to the experimentally known unbridged Cp2FeMo(CO)5 isomer. However, a doubly bridged Cp2FeMo (CO)5 isomer related to the experimental trans- and cis-Cp2Fe2(CO)2(µCO)2 structures of the homonuclear diiron derivative, lies only ∼1.4 kcal/mol in energy above the undbridged structure suggesting a fluxional system. The relative energies of the unbridged and doubly bridged Cp2FeMo(CO)5 are reversed using the BP86 method with the 110

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