Formation of Cerium and Neodymium Isocyanides in the Reactions of

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Formation of Cerium and Neodymium Isocyanides in the Reactions of Cyanogen with Ce and Nd Atoms in Argon Matrices Zongtang Fang, Monica Vasiliu, Xiuting Chen, Yu Gong, Lester Andrews, and David A Dixon J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b06026 • Publication Date (Web): 23 Aug 2019 Downloaded from pubs.acs.org on August 25, 2019

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The Journal of Physical Chemistry

Formation of Cerium and Neodymium Isocyanides in the Reactions of Cyanogen with Ce and Nd Atoms in Argon Matrices Zongtang Fang,a Monica Vasiliu,a Xiuting Chen,b Yu Gong,b,c,*,† Lester Andrews,c and David A. Dixona,*,† a

Department of Chemistry, The University of Alabama, Tuscaloosa, AL, 35487-0336, United

States b

Department of Radiochemistry, Shanghai Institute of Applied Physics, Chinese Academy of

Sciences, Shanghai 201800, China c

Department of Chemistry, University of Virginia, Charlottesville, VA, 22904-4319, United States

Abstract Laser ablation of metallic Ce and Nd reacting with cyanogen in excess argon during codeposition at 4 K form Ce(NC)x and Nd(NC)x for x = 1 to 3, which are identified from their matrix infrared spectra using cyanogen substituted with 13C and 15N. The electronic structure calculations were performed for isocyano and cyano Cd and Nd compounds for up to n = 4. The frequencies were calculated at the density functional theory (DFT) level with 3 different functionals as well as correlated molecular orbital theory (MP2) and are consistent with the experimental assignments and the corresponding 12C:13C isotopic frequency ratios for the isocyano species. The computed frequencies for the analogous cyanide complexes are significantly higher than those for the isocyano isomers and they are not observed in the spectra. The high spin isocyano complexes are the lowest energy structures. On the basis of the NPA results, the bonding in 4CeNC and 6NdNC

†

Corresponding authors: [email protected] (Y.G.), [email protected] (D.A.D.).

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are essentially purely ionic with the Ce/Nd in the +I oxidation state. The bonding for disocyano (3Ce(NC)2 and 5Nd(NC)2) and triisocyano (2Ce(NC)3 and 4Nd(NC)3) complexes are still quite ionic with the lanthanide in the +II and +III formal oxidation states, respectively. For 1Ce(NC)4 the oxidation state is best described as being between +III and +IV. Formation of 5Nd(NC)4 does not really change the electron configuration on the Nd from that in 4Nd(NC)3 and the oxidation state on the Nd remains at +III. Although Nd compounds with up to 3 NC- groups has more ionic binding than does the corresponding Ce compounds, Ce(NC)4 has more ionic binding than does Nd(NC)4. The ionic nature of isocyano Ce and Nd complexes decreases as the number of isocyano groups increases. The formation of the isocyano Ce and Nd complexes using cyanogen or CN radical are calculated to be mostly exothermic processes, with the exothermicity decreasing as the number of isocyano groups increases. Introduction The cyano group binds to transition metals at the carbon as a terminal ligand, as a bridging coordination ligand, 1,2,3,4 or as a terminal ligand in the isocyanide mode with the metal bonding to the nitrogen. 5 The cyano group binds to the actinides Th and U via the carbon in a number of complexes. 6, 7 Examples of actinide isocyanides with terminal -NC coordination include a Th complex generated by the reaction of an imido thorium metallocene and Me3SiCN/Ph2CO, 8 and the formation of a

IV

U complex with two axial U-NC linkages. 9 Polynuclear cyanide actinide

complexes with U-CN-U and Th-CN-Th bridging linkages are known.6,7 Thorium isocyanides have been synthesized in rare gas matrices from the reactions of laser ablated thorium atoms with HCN and CH3CN leading to HThNC and CH3ThNC, respectively. 10 , 11 Another approach to synthesize such complexes is to use the oxidative addition reagent dicyanogen (CN)2. 12 , 13 Reactions of (CN)2 with U and Th atoms have yielded U(NC)n for n = 1, 2, 4, 14,15 and Th(NC)n for


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n = 1, 2, 3, 4. 16 Additional examples of cyanogen reaction with metals include Al, 17 Group 3 metals including La, 18 Mn and Fe, 19 and alkaline earths. 20 In the current work, we react cyanogen with laser ablated lanthanide atoms Ce and Nd to examine the chemistry of the simple complexes of lanthanides with the CN group and observe the products by matrix infrared spectroscopy. Binary complexes in the form of Ln(NC)x were identified via their characteristic infrared absorptions and the structural interpretation was aided by calculated vibrational frequencies using electronic structure methods at the density functional theory and correlated molecular orbital theory levels. Experimental and Computational Methods The experimental apparatus and procedure for investigating laser-ablated Ce and Nd atom reactions with (CN)2 during condensation in excess argon at 4 K have been described previously. 21,22 The Nd:YAG laser fundamental (Continuum II, 1064 nm, 10 Hz repetition rate with 10 ns pulse width) was focused onto a freshly cleaned metal target mounted on a rotating rod. Laser-ablated cerium or neodymium atoms were codeposited with argon (research grade) containing 0.1, 0.2 or 2% (CN)2 gas. The (CN)2, (13CN)2, NC13CN and NCC15N samples were prepared and purified according to procedures reported previously.16,18 FTIR spectra were recorded at 0.5 cm-1 resolution on a Nicolet 750 FTIR instrument with a HgCdTe range B detector. Matrix samples were annealed at different temperatures and cooled back to 4 K for spectral acquisition. Selected samples were subjected to broadband photolysis by a medium-pressure mercury arc street lamp (Philips, 175W) with the outer globe removed. Geometry optimizations of Ln(NC)n and Ln(CN)n (n = 1 to 4) for Ln = Ce and Nd complexes were performed at the density functional theory23 level with the B3LYP 24,25 hybrid exchange-correlation functional and with the aug-cc-pVTZ basis set 26 on C and N and the Stuttgart



small core relativistic effective core potential (ECP) with its accompanying segmented basis set for the lanthanides 27 , 28 Vibrational frequencies were calculated to characterize the global minimum on the potential energy surface and to obtain the zero-point energy corrections (ZPEs). The unscaled calculated frequencies are reported. We do not use scaling factors from (CN)2 because this species differs from the formal CN- moiety in the lanthanide complexes and because of the different effects of the matrix on the different species. Multiple spin states were examined to obtain the spin state with the lowest energy. In order to test the functional, calculations were also performed with the PBE 29,30 and PW91 31,32 functionals, which gave the same energy ordering and similar frequencies. Additional geometry optimizations and frequency calculations starting from the DFT geometries were done at the second order Møller-Plesset (MP2) level 33,34 with the same basis set and ECP used in the DFT calculations for the Ln. We previously found that such MP2 calculations worked well for the corresponding LnFx compounds. 35 The optimized geometries obtained with the B3LYP functional were used for single point calculations at the CCSD(T) 36,37,38,39 level with the aug-cc-pVXZ basis set on C and N and the ccpVXZ-DK3 basis set on Ln (X = D, T, Q) and the energies were extrapolated to the complete basis set (CBS) limit.40 These basis sets are denoted as aX. The open-shell calculations were calculated with the R/UCCSD(T) approach where a restricted open shell Hartree-Fock (ROHF) calculation was initially performed and the spin constraint was then relaxed in the coupled cluster calculation. 41 , 42 The DFT calculations were done with the program Gaussian09 43 and the CCSD(T) calculations were done with the program MOLPRO. 44,45 Results and Discussion


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Experimental infrared spectra and product identifications: cerium isocyanides Figure 1 shows the infrared spectra from the reaction products of laser-ablated cerium atoms with 2% (CN)2 frozen in solid argon. Besides the absorptions of CNCN and CNNC arising from the photo-induced isomerization of (CN)2, 46,47 a new product absorption was observed at 2026.9 cm-1. Another three cerium product absorptions at 1994.5, 1978.2 and 2042.6 cm-1 appeared when the sample was subjected to λ > 220 nm irradiation (Figure 1, trace c). All of the cerium product bands were

Figure 1. Infrared spectra in the product absorption region from the reactions of laser-ablated cerium atoms with (CN)2 in solid argon at 4 K. (a) Ce and 2% (CN)2 co-deposited for 1 h, (b) after annealing to 25 K, (c) after λ>220 nm irradiation, (d) after annealing to 30 K.



sharpened when the sample was annealed to 30 K following UV-vis irradiation. Experiments were repeated by using isotopic labelling to help identify the reaction products (Figure 2). The vibrational frequencies of the new products are listed in Table 1. The 2026.9 cm-1 band shifted to 1985.5 cm-1 upon 13C substitution with a 12C/13C ratio of 1.0209 (Figure 2, trace c). Both the band position and isotopic ratio are characteristic of a terminal

Figure 2. Infrared spectra in the product absorption region from the reactions of laser-ablated cerium atoms with isotopically substituted (CN)2 in solid argon at 4 K. Spectra were taken after λ>220 nm irradiation followed by annealing to 30 K (a) 0.25% (CN)2 + 0.5% NCC15N + 0.25% (C15N)2, (b) 2% (CN)2, (c) 1% (13CN)2, (d) 0.25% (CN)2 + 0.5% NC13CN + 0.25% (13CN)2. The asterisks denote the absorptions of CNCN and its isotopomers. 6 ACS Paragon Plus Environment

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Table 1. Infrared Absorptions (cm−1) Observed for the Cerium and Neodymium Isocyanide Molecules in Solid Argon (CN)2

(13CN)2

(CN)2+NC13CN+(13CN)2

(CN)2+NCC15N+(C15N)2

2026.9a

1985.5

2026.9, 1985.5

2026.9, 1995.4

1994.5

1954.9

1994.5, 1988.1, 1954.9

1994.5, 1988.2, 1962.4

1978.2

1938.2

1978.2, 1945.0, 1938.2

1978.2, 1952.9, 1947.1

Ce(NC)3

2042.6

2001.3

2042.4,b , 2001.2

2042.4,b , 2010.4

NdNC

2036.6a

1995.0

2036.6, 1995.0

2036.6, 2005.1

1997.0c

b

b

1979.4

b

b

1979.4, 1954.2, 1948.2

2044.4

b

b

2044.4,b , 2013.3

CeNC Ce(NC)2

Nd(NC)2 Nd(NC)3 a

b

,1965.4

matrix site absorptions observed at 2026.1 (CeNC), 1984.8 (CeN13C), 2034.8 (NdNC) and

1993.1 (NdN13C) cm−1. b not observed. c overlap with the CNNC band.

C-N stretch of an isocyanide molecule. A doublet at 2026.9 and 1985.5 cm-1 was observed when cerium reacted with (CN)2 + NC13CN + (13CN)2, suggesting that only one CN moiety is involved in this mode. This is also consistent with the doublet at 2026.9 and 1995.4 cm-1 that appeared in the experiment using the (CN)2 + NCC15N + (C15N)2 sample. As a result, the 2026.9 cm-1 band is assigned to the triatomic CeNC molecule. The weak band at 1994.5 cm-1 and the intense band at 1978.2 cm-1 exhibit

12

C/13C ratios that are similar to that of CeNC. Two sets of triplets were

observed when (CN)2 + NC13CN + (13CN)2 was used (Figure 2, trace d), indicating the presence of two equivalent C-N moieties in the new product molecule. A similar pattern appeared in the spectrum from the reaction of cerium and (CN)2 + NCC15N + (C15N)2. We therefore assign the 1994.5 and 1978.2 cm-1 bands to symmetric and antisymmetric C-N stretches of the Ce(NC)2 molecule. The broad band at 2042.6 cm-1 belongs to a third product from the cerium and (CN)2 7 ACS Paragon Plus Environment


reaction. This band arises from a terminal C-N stretch of an isocyanide as well, but the intermediate bands are not resolved in either the (CN)2 + NC13CN + (13CN)2 or (CN)2 + NCC15N + (C15N)2 experiment. The most likely assignment for this band is Ce(NC)3. It should be noted that when the concentration of (CN)2 was reduced to 0.2%, the spectrum after sample annealing at 30 K following irradiation is dominated by the CeNC absorption (Figure S1). The intensity of the Ce(NC)3 band is significantly reduced, and the Ce(NC)2 bands almost disappear. Such a concentration dependence is in agreement with the fact that CeNC is favored at low (CN)2 concentration whereas the other two products with higher coordination numbers are not. Computational results: Ce compounds The optimized molecular structures for the isocyano and cyano Ce compounds are shown in Figure 3. The calculated vibrational frequencies for the Ce(NC)n

(n = 1 - 4) complexes are given in Table 2. Spin orbit corrections (Supporting

Information) for the relative energies were obtained at the BLYP/TZ2P level 48,49 using the ZORA + spin orbit (SO) approach 50,51,52,53,54 in ADF. 55,56 The SO corrections to the energy differences for the Ce species are all less than ±3.3 kcal/mol and do not change the ordering of the relative energies. The lower spin states for the compounds with two CN groups have higher SO corrections but these states are much higher in energy than the lowest energy structure. For 4CeNC, the MP2 and B3LYP approaches predict a NC stretch that is larger than experiment by 183 and 69 cm-1, respectively, and the PW91 and PBE functionals predict frequencies that are too low as compared to experiment by ~25 cm-1. Although, the NC stretches for the 2CeNC state are in better agreement with experiment with the B3LYP value 60 cm-1 too high, the MP2 value 16 cm-1 too high, and the PBE and PW91 values 17 and 13 cm-1 too low, respectively, a CCSD(T) calculation extrapolated to the CBS limit of the energy difference predicts


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4CeNC

3Ce(NC)

4CeCN

(C∞v)

2

(C2v)*

3Ce(CN)

3Ce(NC)(CN)

2Ce(NC)

1Ce(NC)

(C∞v)

2

(C2v)

(Cs)

3

(D3h)

2Ce(CN)

4

(Td)

1Ce(CN)

3

(C3v)

4

(Td)

Figure 3. Optimized B3LYP/aug-cc-pVTZ geometries for Ce complexes. C atoms shown in grey, N atoms shown in blue and Ce atoms shown in light yellow


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Table 2. Calculated Isocyanide N-C Stretches Stretches (cm-1) and Intensities (km/mol) for Ce(NC)n (n = 1 to 4). Modea

Method

4

Asym

B3LYP

2096.0 (368)

Sym

B3LYP

Asym

PBE

Sym

PBE

Asym

PW91

Sym

PW91

Asym

MP2

CeNC (C∞v)

2002.0 (238) 2002.9 (168) 2210.4 (1006)

2

CeNC (C∞v)

2086.5 (512) 2009.9 (440) 2014.1 (416) 2042.6 (277)

3

Ce(NC)2 (C2v)

3

Ce(NC)(CN) (Cs)

2

Ce(NC)3(D3h)b

1

Ce(NC)4 (Td)

2081.1 (713)

2081.9 (481) (NC)

2072.4 (1721)

2033.5 (3027)

2084.6 (182)

2215.1 (52) (CN)

2096.3 (0)

2067.1 (0)

2001.9 (672)

1996.2 (302) (NC)

1992.2 (1669)

1958.9 (1174)

2006.2 (120)

2124.5 (41) (CN)

2014.6 (0)

1973.4 (0)

2007.6 (638)

1995.8 (396)(NC)

1999.2 (1662)

1964.0 (1158)

2010.0 (190)

2131.8 (71) (CN)

2021.9 (1)

1978.2 (0)

1960.9 (970),

1964.7 (2643)

1961.9 (231)

1971.0 (104) Sym

MP2

1971.4 (43)

2108.2(604)

2008.5(0)

a

In C2v symmetry asym = b2 and sym = a1. In D3h symmetry, asym = e˝ and sym = a˝1. In Td symmetry, asym = t2 and sym = a1.

b

MP2 has Cs symmetry.


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Table 3. Calculated Relative Energies for Ce(NC)n and Ce(CN)n (n = 1 to 4) Species at the DFTa and CCSD(T) levels.

a b

Molecule

PG

B3LYP

PBE

PW91 CCSD(T)/DK3/aD CCSD(T)/DK3/aT CCSD(T)/DK3/aQ CCSD(T)/DK3/CBS

4

CeNC

C∞v

0.0

0.0

0.0

0.0/0.0b

0.0/0.0b

0.0

0.0

4

CeCN

C∞v

6.4

12.5

11.4

3.4/4.0b

3.1/3.7b

3.1

3.1

2

CeNC

C∞v

-2.7

-2.2

-3.4

-3.3/10.3b

9.2/15.9b

11.5

12.8

2

CeCN

C∞v

4.9

3.7

2.2

17.719.8b

16.1/18.3b

15.3

14.8

3

Ce(NC)2

C2v

0.0

0.0

0.0

0.0

0.0

0.0

0.0

3

Ce(NC)(CN)

Cs

13.6

3.4

4.7

6.6

5.7

5.7

5.2

3

Ce(CN)2

C2v

14.0

7.7

8.0

9.2

6.8

6.5

6.4

1

Ce(NC)2

C2v

42.5

18.2

18.4

28.2

29.9

30.2

29.8

1

Ce(NC)(CN)

Cs

38.0

23.3

31.1

19.0

21.5

21.7

21.3

1

Ce(CN)2

C2v

41.9

34.8

34.9

16.6

19.1

20.1

20.1

2

Ce(NC)3

D3h

0.0

0.0

0.0

0.0

0.0

0.0

0.0

2

Ce(CN)3

C3v

31.1

22.6

24.6

17.6

15.5

15.2

15.1

1

Ce(NC)4

Td

0.0

0.0

0.0

0.0

0.0

0.0

0.0

1

Ce(CN)4

Td

30.4

22.1

22.8

28.7

27.3

27.1

26.9

DFT basis sets: Stuttgart for Ce and aug-cc-pVTZ for N and C. cc-pwCVnZ-DK for Ce and aug-cc-pwCVnZ-DK for N, C for CeNC and CeCN.



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Table 4. Calculated Reaction Energies in kcal/mol for NC bonded to Ce at the CCSD(T)-DK3, B3LYP, PBE, and PW91 Levels. Molecule

Rxn

CCSD(T)/CBS CCSD(T)/aQ CCSD(T)/aT CCSD(T)/aD

B3LYP

PBE

PW91

2 CN → (CN)2

1

-135.0

-134.6

-133.6

-127.5

-140.6

-140.1

-140.6

3

Ce + (CN)2 → 3Ce(NC)2

2

-98.9

-99.0

-99.3

-99.3

-92.1

-100.2

-101.9

3

Ce + (CN)2 → 4Ce(NC) + CN

3

37.6

37.6

36.9

33.8

27.6

15.7

16.1

4

Ce(NC) + (CN)2 → 2Ce(NC)3

4

-117.1

-116.7

-117.0

-117.4

-95.0

-82.2

-86.0

3

Ce(NC)2 + (CN)2 → 1Ce(NC)4

5

-41.2

-40.4

-40.9

-41.8

-3.3

-13.6

-13.9

3

Ce + CN → 4Ce(NC)

6

-97.3

-97.0

-96.8

-93.7

-1130

-124.4

-124.4

4

Ce(NC) + CN → 3Ce(NC)2

7

-136.6

-136.6

-136.1

-133.1

-119.8

-115.9

-118.0

3

Ce(NC)2 + CN → 2Ce(NC)3

8

-115.5

-114.7

-114.5

-111.8

-115.8

-106.4

-108.5

2

Ce(NC)3 + CN → 1Ce(NC)4

9

-60.7

-60.3

-60.0

-57.7

-28.1

-47.3

-45.9


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the quartet to be the ground state (Table 3) by 12.8 kcal/mol. The molecule 2CeNC is predicted to be lower in energy than the 4CeNC state at all of the DFT levels (Table 3). The B3LYP isotope ratio for

13

C substitution for 4CeNC of 1.0208 is in excellent agreement with the experimental

value of 1.0209. The triatomic 4CeCN is predicted to be only 3.1 kcal/mol above 4CeNC at the CCSD(T)/CBS level but the calculated DFT frequencies for the cyano compound (see Supporting Information (SI)) are much higher in energy and do not match experiment. We note that there is a significant basis set effect on the energy splitting of the different species and electronic states at the CCSD(T) level. The symmetric and asymmetric NC stretching combination bands for the bent Ce(NC)2 species are split by about 16 cm-1 in the experiment and the calculations predict a smaller splitting (Table 2). The calculated values always predict the asymmetric band to be at a lower frequency than the symmetric band. In this case, the calculated values are still about 100 cm-1 larger than experiment at the B3LYP level. At the PBE and PW91 levels, the bands are predicted to be higher by 10 to 30 cm-1 as compared to experiment. At the MP2 level, the calculated values are now ~ 25 cm-1 below experiment. The B3LYP isotope ratios for

13

C substitution for 3Ce(NC)2 of

1.0209/1.0210 are in excellent agreement with the experimental values of 1.0206/1.0203. The ground state is predicted to be the 3Ce(NC)2 structure followed by the mixed 3Ce(NC)(CN) isomer 5.2 kcal/mol higher in energy at the CCSD(T)/CBS level (Table 3). Although the calculated NC stretch for the mixed isomer matches experiment, the CN stretch certainly does not as it is predicted to be much higher than observed (Table 2). The triplet dicyano isomer is predicted to be 6.4 kcal/mol higher in energy at the CCSD(T)/CBS level. The three experimental bands for Ce(CN)3 are split by about 3 cm-1, likely due to matrix effects as the calculated splitting of the symmetric and asymmetric stretches is larger than that



value. At the DFT level, 2Ce(NC)3 is predicted to be planar with D3h symmetry. The asymmetric stretch is predicted to be 29 cm-1 too high at the B3LYP level, 51 cm-1 too low at the PBE level and 44 cm-1 too low at the PW91 level as compared to the average of the experimental values. The MP2 geometry is predicted not to be D3h and the average of the asymmetric stretch is predicted to be too low by 77 cm-1 as compared to experiment. The B3LYP isotope ratio for 13C substitution for 2Ce(NC)3 of 1.0209 is in excellent agreement with the experimental value of 1.0206. The 2

Ce(CN)3 isomer is predicted to be 15 kcal/mol higher in energy at the CCSD(T)/CBS level than

the corresponding 2Ce(NC)3. The tetraisocyano isomer is predicted to have an asymmetric stretch that is 2034 cm-1 at the DFT/B3LYP level with the values at the DFT/PW91 and DFT/PBE levels are 70 cm-1 to 75 lower, with the MP2 value even lower. The tetracyano isomer is predicted to be higher in energy as compare to the tetraisocyano isomer by ~ 27 kcal/mol at the CCSD(T)/CBS level. The energetics to produce the various species at the CCSD(T)/CBS level are given in Table 4. We note that many of the DFT energies are in poor agreement with the higher level correlated molecular orbital values. The calculated bond energy for cyanogen is in good agreement with the experimental value of 136.7 ± 1.6 kcal/mol. 57 The reaction of 3Ce with (CN)2 to produce the dimer (Reaction (2)) is almost -100 kcal/mol exothermic. The Ce-(NC) bond energy of 4Ce(NC) (Reaction (5)) is almost 100 kcal/mol. Addition of CN to 4Ce(NC) to form the dimer is even more exothermic, ca. -140 kcal/mol (Reaction (6)). Reactions (7) and (8) for further addition of CN become less exothermic with the final addition of CN to the trimer to form the tetramer being exothermic by -61 kcal/mol. The addition of (CN)2 to the monomer to form the trimer is exothermic by -117 kcal/mol (Reaction (3)) and the addition of (CN)2 to the dimer to form the tetramer is much less exothermic, -41 kcal/mol (Reaction (4)).


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The B3LYP optimized geometry parameters for the lowest energy isocyano and cyano compounds are reported in Table 5. The diisocyano and dicyano compounds have non-linear CeNC and Ce-CN moieties with bond angles of ~117° around the central Ce atoms and 162° and 170° for C-N-Ce angle and N-C-Ce angle. The mixed cyano-isocyano dimer also has nonlinear bond angles with a bond angle of 116° around Ce and much larger, 165° for C-N-Ce angle and 171° for N-C-Ce angle. The tricyano compound has a nonlinear Ce-CN group with an angle of 171°. The remaining compounds have linear Ce-CN and Ce-NC bonds. The Ce-N bond distances for isocyano compounds are up to 0.18 Å shorter than the corresponding Ce-C cyano, as less electron density is pulled away by a NC group than by a CN group. This gap becomes smaller as more CN and NC groups are present around Ce. The Ce-N(C) bond distances become shorter when more isocyano and cyano groups are around Ce atom. The NC bond distances are up to 0.02 Å shorter than the corresponding CN bond distance. Table 5. Optimized Geometry Parameters at B3LYP/aug-cc-pVTZ(C,N)/Stuttgart-ECP(Ce). Molecule

PG

Ce-N(C) (Å)

C-N (Å)

∠C(N)-N(C)-Ce (°)

4

CeNC

C∞v

2.331

1.177

180.0

4

CeCN

C∞v

2.509

1.162

180.0

2

CeNC

C∞v

2.291

1.178

180.0

2

CeCN

C∞v

2.448

1.161

180.0

3

Ce(NC)2

C2v

2.312

1.178

162.2

3

Ce(NC)(CN)

Cs

2.308 (NC)

1.178 (NC)

165.2 (NC)

2.478 (CN)

1.161 (CN)

170.5 (CN)

3

Ce(CN)2

C2v

2.440

1.161

170.1

117.1

2

Ce(NC)3

D3h

2.307

1.180

180.0

120.0

2

Ce(CN)3

C3v

2.438

1.160

171.4

114.1

1

Ce(NC)4

Td

2.224

1.184

180.0

109.5

1

Ce(CN)4

Td

2.361

1.161

180.0

109.5


∠(C)N-Ce-N(C) (°)

117.5 116.0


Page 16 of 40

The Natural Population Analysis (NPA) from the Natural Bond Orbitals (NBO) 58,59,60,61 for the Ce compounds are given in Table 6. For 4CeNC, the spin is found on the 4f (1.4), the 6s (0.9), and the 5d (0.7) so there is a loss of about one electron from the 6s as the molecule forms the +I oxidation state. The Ce atom has a 6s25d14f1 configuration so there is substantial charge transfer from the 5d to the 4f on addition of NC. The bonding in 4CeNC is essentially purely ionic with the Ce in the +I oxidation state and a charge transfer of only 0.15 electrons back to the Ce from the NC-. For 3Ce(NC)2, there is a small loss of 0.13 e electrons from the 4f as compared to 4

CeNC, coupled with a loss of 0.34 e from the 5d and

Table 6. NBO Population Analysis with the DFT with the B3LYP Functional Molecule

S2/2

Pop Ce

Ce Excess Spin

4

CeNC

3.75/3.75

4f1.425d0.816s 0.92

3

Ce(NC)2

2.00/2.00

3

Ce(NC)(CN)

2

Ce(NC)3

1

Ce(NC)4

4

CeCN

3.75/3.75

4f1.515d0.716s 0.94

4f1.515d0.656s0.90

3

Ce(CN)2

2.00/2.00

2

Ce(CN)3

0.75/0.75

1

Ce(NC)4

NPA C

NPA CN

4f1.415d 0.756s0.89 -1.07

0.20

-0.87

4f1.305d0.566s 0.53

4f1.285d 0.416s0.50 -1.05

0.23

-0.82

2.00/2.00

4f1.325d0.616s 0.53

4f1.305d0.456s0.47

-1.05/

0.34/

-0.71/

-0.38

-0.37

-0.75

0.75/0.75

4f1.125d0.536s 0.06

4f1.065d0.276s 0.03 -1.05

0.27

-0.78

4f0.655d0.986s 0.11

-0.94

0.36

-0.58

-0.43

-0.41

-0.84

4f1.135d0.776s 0.69

4f1.105d 0.576s0.61 -0.36

-0.36

-0.72

4f1.105d0.716s 0.19

4f1.055d0.366s0.09

-0.32

-0.36

-0.68

-0.25

-0.24

-0.49

4f0.655d1.236s 0.26

NPA N

a loss of almost 0.4 e from the 6s. The triplet spin is split between the 4f and the 5d/6s. The bonding is still quite ionic with the Ce in the +II oxidation state and back donation of 0.18 e from each CN group. Addition of another CN to form 2Ce(NC)3 leads to the spin localizing on the 4f and almost


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no electron density on the 6s. There is about 0.5 e on the 5d but that density is mostly spin paired and represents backbonding from the ligands. The Ce is in the +III oxidation state with 0.22 electrons transferred back to the Ce from each NC-. Thus up to the tri-isocyano, there is change from 0.13 to 0.22 e transferred to the Ce from an individual NC-. Formation of 1Ce(NC)4 leads to a significant change in the character of the electron density on the metal with almost one spin paired electron on the 5d and 0.65 e on the 4f with only a very small amount on the 6s. This additional density has come from backbonding from the NC- to the Ce with each NC- contributing 0.42 e. Thus, there is about 1.7 e of backbonding from the isocyano ligands ligands to the Ce. Thus it is hard to know how best to describe the oxidation state and it is best described as being between +III and +IV. Note that for up to 3 isocyano groups, there is very little donation into the 4f but there is significant donation into the 5d. Only for the tetraisocyano is there really any donation into the 4f. Most of the negative charge is on the N in the isocyano compounds with the C having a positive charge. In the cyano compounds, the negative charge is about equally shared on the N and C and there is more charge transfer to the Ce. To provide additional insights into the orbital occupancies, single point complete active space self-consistent field (CASSCF) calculations were performed. The detailed results are given in the SI. In all cases, 13 orbitals (6s, 5d, 4f on Ce) were included in the CAS with the appropriate number of electrons based on the formal charges. Inclusion of the 2p orbitals on C and N did not change the results. The CASSCF for the isocyano compounds yields occupancies of 6s15d14f1, 6s14f1, (6s+5d)14f1 and 4f1 for CeNC, Ce(NC)2, Ce(NC)(CN), and Ce(NC)3. These results are completely consistent with the NBO results given that the CAS will not include back-binding from the ligands.



Experimental infrared spectra and product identifications: neodymium isocyanides The infrared spectra from the reactions of neodymium atoms with 2% (CN)2/Ar are shown in Figure 4. The

Figure 4. Infrared spectra in the product absorption region from the reactions of laser-ablated neodymium atoms with (CN)2 in solid argon at 4 K. (a) Nd and 2% (CN)2 co-deposited for 1 h, (b) after annealing to 25 K, (c) after λ>220 nm irradiation, (d) after annealing to 30 K.

CNCN and CNNC absorptions were observed after sample deposition,46,47 as in the case of cerium. Neodymium dependent product absorptions were observed at 2036.6, 1997.0, 1979.4 and 2044.4 cm-1. All of these new absorptions are due to the C-N stretches of neodymium isocyanides. The CN stretching band of NdNC was observed at 2036.6 cm-1 which shifted to 1995.0 cm-1 upon 13C substitution with a 12C/13C ratio of 1.0209. There is no intermediate absorption for this band in 18 ACS Paragon Plus Environment

Page 18 of 40

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both scrambled 13C and 15N experiments, consistent with the presence of single CN moiety in the NdNC molecule. The 1997.0 and 1979.4 cm-1 bands are assigned to the symmetric and antisymmetric C-N stretches of the Nd(NC)2 molecule. This is supported by the triplet that appears at 1979.4, 1954.2 and 1948.2 cm-1 by using the (CN)2+NCC15N+(C15N)2 sample. The broad band at 2044.4 cm-1 is most likely due to the Nd(NC)3 molecule. Similar to cerium, the infrared spectra from the reactions of neodymium and 0.1% (CN)2 (Figure S2) provide further support to the assignments of NdNC, Nd(NC)2 and Nd(NC)3. The absorption due to NdNC is predominant while Nd(NC)2 completely disappears, and the Nd(NC)3 band is much weaker compared with that in the 2% (CN)2 experiment. Computational results: Nd compounds The optimized molecular structures for the isocyano and cyano Nd compounds are shown in Figure 5. The calculated vibrational frequencies for the Nd(NC)n (n = 1 - 4) complexes are given in Table 7. The spin orbit corrections to the relative energies for Nd(NC) and Nd(CN) are similar to those for Ce(NC) and Ce(CN), obtained with the spin orbit approach 62 in NWChem. 63,64 We were unable to get any other states for larger numbers of cyano/isocyano ligands to converge. We expect the spin orbit corrections to the relative energies for the larger numbers of ligands to be similar to those for the Ce compounds, 65 as the spin orbit correction 66 for the ground state of the +I, +II, and +II ions for Nd are comparable, showing that the 4f orbitals are behaving in a similar manner. For 6NdNC, DFT/B3LYP predicts a NC stretch that is larger than experiment by 70 cm-1 and the corresponding value at the DFT/PBE and DFT/PW91 levels are also too high by 25 and 8 cm-1, respectively. The MP2 value is too low as compared to experiment by 30 cm-1. The B3LYP isotope ratio for

13

C substitution for 6NdNC of 1.0208 is in excellent agreement with the

experimental value of 1.0209, just as found for the corresponding Ce compound. The 4NdNC state



is predicted to be lower in energy than the 6NdNC at the B3LYP level, but higher in energy at the PBE and PW91 levels (Table 8). At the CCSD(T)/CBS level, the 6NdCN isomer is 5 kcal/mol higher in energy than the 6NdNC isomer.

6NdNC

(C∞v)

5Nd(NC) 2

(C2v)

5Nd(NC)(CN)

4Nd(NC) 3

(D3h)

5Nd(NC) 4

(D2d)

6NdCN

(C∞v)

5Nd(CN) 2

(C2v)

(Cs)

4Nd(CN) 3

(D3h)

5Nd(CN) 4

(D2d)

Figure 5. Optimized B3LYP/aug-cc-pVTZ geometries for Nd complexes. C atoms shown in grey, N atoms shown in blue and Ce atoms shown in light yellow 20 ACS Paragon Plus Environment

Page 20 of 40

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Table 7. Calculated Isocyanide N-C Stretches Stretches (cm-1) and Intensities (km/mol) for Nd(NC)n (n = 1 to 4). Modea

Method

6

Asym

B3LYP

2106.6 (313)

NdNC (C∞v)

5

Nd(NC)2 (C2v)

2089.7 (165)

5

Nd(NC)(CN) (Cs)

2088.5 (260) (NC)

4

Nd(NC)3 (D3h)

2072.3 (1777)

5

Nd(NC)4 (D2d)

2052.5 (e) (3432) 2055.4 (b2) (1482)

Sym

B3LYP

Asym

PBE

2011.4 (212)

2091.0 (15)

2211.8 (19) (CN)

2096.9 (0)

2138.0 (0)

1993.5 (19)

1993.1 (44) (NC)

1990.8 (924)

2007.0 (e) (582) 2008.3 (b2) (269)

Sym

PBE

Asym

PW91

2028.1 (271)

2001.4 (24)

2131.8 (32) (CN)

2002.9 (169)

2063.3 (0)

1998.8 (24)

1987.1 (29) NC)

1996.5 (942)

2012.4 (e) (585) 2013.6 (b2) (271)

a

Sym

PW91

Asym

MP2

Sym

MP2

2006.2 (24) 2006.8 (240)

2121.0 (16) (CN)

2008.8 (167)

1965.8 (33)

1992.0 (1210)

1964.9 (5)

2006.9 (0)

2068.8 (0)

In C2v symmetry asym = b2 and sym = a1. In D3h symmetry, asym = e˝ and sym = a˝1. In D2d symmetry, asym = b2, e and sym = a1



Page 22 of 40

Table 8. Calculated Relative Energies for Nd(NC)n and Nd(CN)n (n = 1 to 4) Species with the DFT and CCSD(T). Molecules

B3LYP

PBE

PW91

CCSD(T)/DK3/aD

CCSD(T)/DK3/aT

CCSD(T)/DK3/aQ

CCSD(T)/DK3/CBS

0.0

0.0

0.0

0.0

5.4

4.8

4.8

4.9

6

NdNC

0.0

0.0

0.0

4

NdNC

-0.7

3.2

3.7

6

NdCN

5.1

0.4

3.7

4

NdCN

8.4

4.5

7.7

5

Nd(NC)2

0.0

0.0

0.0

0.0

0.0

0.0

0.0

5

Nd(NC)(CN)

4.9

1.9

0.4

3.9

3.0

3.0

3.9

3

Nd(NC)2

25.3

22.9

22.1

5

Nd(CN)2

9.5

5.4

7.2

9.6

8.1

8.2

8.2

3

Nd(CN)2

37.3

30.0

30.4

4

Nd(NC)3

0.0

0.0

0.0

0.0

0.0

0.0

0.0

2

Nd(NC)3

23.2

22.3

22.0

4

Nd(CN)3

19.1

17.4

18.3

18.6

17.1

17.2

17.3

2

Nd(CN)3

42.5

34.7

35.1

5

Nd(NC)4

0.0

0.0

0.0

0.0

0.0

0.0

0.0

3

Nd(NC)4

0.2

-1.7

-1.2

12.5

20.8

21.1

21.3

5

Nd(CN)4

52.9

42.2

43.8

36.1

36.2

36.8

37.1

3

Nd(CN)4

50.8

25.0

26.5

29.2

33.4

33.9

34.2


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Table 9. Calculated Reaction Energies in kcal/mol for NC bonded to Nd at the CCSD(T)-DK3, B3LYP, PBE, and PW91 Levels. Reaction

Rxn CCSD(T)/CBS CCSD(T)/aQ CCSD(T)/aT CCSD(T)/aD

B3LYP

PBE

PW91

5

Nd + (CN)2 → 5Nd(NC)2

10

-80.7

-79.1

-78.0

-77.9

-73.4

-69.7

-70.6

5

Nd + (CN)2 → 6Nd(NC) + CN

11

34.0

34.2

34.3

31.3

40.7

39.9

35.9

6

Nd(NC) + (CN)2 → 4Nd(NC)3

12

-89.5

-88.8

-89.9

-94.6

-54.8

-52.1

-49.9

5

Nd(NC)2 + (CN)2 → 5Nd(NC)4

13

15.4

14.8

12.7

6.3

20.2

17.8

16.0

5

Nd + CN → 6Nd(NC)

14

-101.0

-100.4

-99.3

-96.2

-99.9

-100.2

-104.7

6

Nd(NC) + CN → 5Nd(NC)2

15

-114.7

-113.2

-112.3

-109.2

-114.1

-109.6

-106.6

5

Nd(NC)2 + CN → 4Nd(NC)3

16

-109.7

-110.2

-111.3

-112.9

-81.3

-82.6

-83.9

4

Nd(NC)3 + CN → 5Nd(NC)4

17

-9.9

-9.6

-9.6

-8.3

-39.1

-39.7

-40.6



The symmetric and asymmetric NC stretching combination bands for bent Nd(NC)2 are split by about 18 cm-1 in the experiment and the calculations predict a smaller splitting (Table 7). The DFT calculated values predict the asymmetric band to be at a lower frequency than the symmetric band, whereas the MP2 calculations place the asymmetric stretch slightly above the symmetric stretch. In this case, the calculated values are still about 100 cm-1 larger than experiment at the B3LYP level. At the PBE and PW91 levels, the bands are predicted to be higher by up to 20 cm-1 as compared to experiment. At the MP2 level, the calculated values are as much as 32 cm-1 below experiment. The ground state is predicted to be the 5Nd(NC)2 structure followed by the mixed 5Nd(NC)(CN) isomer 4 kcal/mol higher in energy at the CCSD(T)/CBS level (Table 8). Although the calculated NC stretch for the mixed isomer is consistent with experiment, the CN stretch certainly does not as it is predicted to be much higher than observed (Table 7). The 5

Nd(CN)2 isomer is predicted to be 8 kcal/mol higher in energy at the CCSD(T)/CBS level. The calculated asymmetric band for planar 4Nd(NC)3 with D3h symmetry at the

DFT/B3LYP level is predicted to be too high by 28 cm-1 and the other methods all predict the frequency to be too low by ca. -50 cm-1 as compared to the experimental value. The 4Nd(CN)3 isomer is predicted to be higher in energy by 17 kcal/mol. The lowest energy tetraisocyano isomer is predicted to be 5Nd(NC)4 with D2d symmetry. There is no experimental data available for the vibrational frequencies. The DFT values follow the usual pattern with the B3LYP values larger than the PBE and PW91 values. The MP2 values are clearly in error so they are given in the Supporting Information only. The 3Nd(NC)4 state is 21 kcal/mol higher in energy than the 5Nd(NC)4 level. However, the DFT methods predict the quintettriplet splitting to be very small and, in some cases, predict the triplet to be more stable than the quintet. The 3Nd(CN)4 is predicted to be higher in energy than 5Nd(NC)4 by 34 kcal/mol at the


Page 24 of 40

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CCSD(T)/CBS level and the 5Nd(CN)4 isomer is predicted to be even higher at 37 kcal/mol at the same level. The energetics to produce the various Nd species at the CCSD(T)/CBS level are given in Table 9. Again, many of the DFT energies are in poor agreement with the higher level correlated molecular orbital values. The reaction of 5Nd with (CN)2 to produce the dimer (Reaction (9)) is almost -80 kcal/mol exothermic, significantly less than for the same reaction with Ce. The Ce(NC) bond energy of 6Nd(NC) (Reaction (12)) is almost -100 kcal/mol. Addition of CN to 6

Nd(NC) to form the dimer is less exothermic, -115 kcal/mol (Reaction (13)) than the

corresponding reaction (6) for Ce. Reactions (14) and (15) for further addition of CN become less exothermic with the final addition of CN to the trimer to form the tetramer being exothermic by only -10 kcal/mol. The addition of (CN)2 to the monomer to form the trimer is exothermic by -90 kcal/mol (Reaction (10)) but the addition of (CN)2 to the dimer to form the tetramer is now endothermic by 15 kcal/mol (Reaction (11)). Given the reaction energies, it is not surprising that the tetraisocyano Nd compound is not observed. The B3LYP optimized geometry parameters for the lowest energy isocyano and cyano compounds are shown in Table 10. The diisocyano and dicyano compounds have non-linear NdNC and Nd-CN moieties with bond angles around the central Nd atoms of 126° and 122°, respectively. These angles are larger than what was observed for the corresponding Ce diisocyano and dicyano compounds. The C-N-Nd angle and N-C-Nd angle are 108° and 166°, respectively, showing two very different geometries for diisocyano and dicyano compounds, with the Nd-NC far more bent than the Nd-CN compounds. The mixed cyano-isocyano dimer also has nonlinear bonds with a bond angle of 120° around Nd and 119° for C-N-Nd angle and 171° for N-C-Nd angle, again with a more bent Nd-NC bond. The tricyano and triisocyano, Nd-CN and Nd-NC



Page 26 of 40

bonds are linear. The tetra Nd-CN and Nd-NC bonds are slightly bent, in D2d symmetry, deviating from the Td symmetry predicted for the corresponding closed shell Ce compounds. For the bond distances, similar trends to Ce are observed for Nd. For up to triisocyano and tricyano compounds, the Nd-N (isocyano) and Nd-C (cyano) bond distances are comparable to the corresponding Ce compounds, and less than 0.05 Å shorter. For the tetraisocyano and tetracyano Nd compounds, the Nd-N and Nd-C bond distances are up to 0.1 Å shorter than the corresponding Ce compounds. The NC (CN) bond distances and trends for Nd isocyano and cyano compounds are the same as the corresponding Ce compounds.

Table 10. Optimized Geometry Parameters at B3LYP/aug-cc- pVTZ(C,N)/Stuttgart-ECP(Nd). Molecule

PG

Nd-N(C) (Å)

C-N (Å)

∠C(N)-N(C)-Nd (°)

6

NdNC

C∞v

2.355

1.176

180.0

4

NdNC

C∞v

2.349

1.176

180.0

6

NdCN

C∞v

2.524

1.161

180.0

4

NdCN

C∞v

2.527

1.161

180.0

5

Nd(NC)2

C2v

2.384

1.176

108.2

5

Nd(NC)(CN)

Cs

2.365 (NC)

1.176 (NC)

119.5 (NC)

2.528 (CN)

1.161 (CN)

170.9 (CN)

5

Nd(CN)2

C2v

2.519

1.161

166.2

121.5

4

Nd(NC)3

D3h

2.269

1.180

180.0

120.0

4

Nd(CN)3

D3h

2.411

1.160

180.0

120.0

5

Nd(NC)4

D2d

2.339

1.169

173.2

107.5/110.4

5

Nd(CN)4

D2d

2.472

1.163

167.3

104.7/111.9

∠(C)N-Nd-N(C) (°)

126.0 120.1

The NPA populations from the NBO analysis for the Nd compounds are given in Table 11. The Nd atom has a 6s24f4 configuration. For 6NdNC, there are 4 spins in the 4f and one spin on the


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6s with very little 5d occupation as expected for a +I formal oxidation state on the Nd. The bonding in 6NdNC is ionic with only 0.08 e transferred from the NC- to the Nd. There is a small amount of backbonding into the 5d. For 5Nd(NC)2, there are 4 unpaired 4f electrons and only a small amount of population in the 6s and 5d as expected for a +II formal oxidation state on the Nd. The bonding

Table 11. NBO6 Population Analysis with the B3LYP Functional for the Lowest Energy States of Nd(NC)n and Nd(CN)n for n = 1 - 4. Molecule

S2/2

Pop

Nd Excess Spin

NPA N

NPA C

NPA CN

6

NdNC

8.75/8.75 4f3.985d0.156s0.95

4f3.985d0.126s0.92

-1.08

0.17

-0.91

5

Nd(NC)2

6.00/6.00 4f3.955d0.226s0.11

4f3.935d0.136s0.08

-1.03

0.15

-0.88

5

Nd(NC)(CN)

6.00/6.00

4f3.845d0.386s0.09

4f3.835d0.286s0.05

-0.94/

0.08/

-0.86/

-0.43

-0.39

-0.82

4

Nd(NC)3

3.76/3.75 4f3.135d0.526s0.06

4f3.075d0.276s0.03

-1.05

0.28

-0.77

5

Nd(NC)4

6.00/6.00 4f3.065d0.606s0.12

4f3.025d0.316s0.06

-0.92

0.36

-0.56

6

NdCN

8.75/8.75 4f3.995d0.176s0.98

4f3.985d0.136s0.92

-0.44

-0.42

-0.86

5

Nd(CN)2

6.00/6.00 4f3.995d0.256s0.13

4f3.985d0.146s0.06

-0.43

-0.41

-0.84

4

Nd(CN)3

3.76/3.75 4f3.125d0.656s0.17

4f3.085d0.346s0.09

-0.32

-0.38

-0.70

5

Nd(CN)4

6.00/6.00 4f3.065d0.806s0.32

4f3.025d0.426s0.16

-0.28

-0.19

-0.47

is still ionic with only 0.12 e per NC- group transferred to the Nd. The backbonding is into the 5d and 6s. Addition of another CN to form 4Nd(NC)3 leads to 3 unpaired electrons in the 4f and about 0.5 spin paired electron density in the 5d from backbonding from the ligands. Again, the bonding is very ionic with 0.23 e per NC- group transferred to the Nd with a +III oxidation state on the Nd. Formation of 5Nd(NC)4 does not really change the electron configuration on the Nd from that in 4

Nd(NC)3. In fact, there is now a spin unpaired α electron delocalized over the 4 carbon atoms.

Thus the oxidation state on the Nd remains at +III and there is 0.44 e transferred form the NC- to 27 ACS Paragon Plus Environment


the Nd. Although Nd with up to 3 NC- groups has more ionic biding than does the corresponding Ce compounds, Ce(NC)4 has more ionic binding than does Nd(NC)4. This differs from the tetrafluorides where NdF4 is found to have a +IV oxidation state as does CeF4.35 This is consistent with the higher electronegativity of the F ligand. As for the Ce cyano compounds, the negative charge on C and N is about equally shared and the bonding is less ionic. The same delocalization of an α spin is found for the higher energy 5Nd(CN)4. As for the Ce compounds, single point complete CASSCF calculations were performed. The detailed results are given in the SI. In all cases, 13 orbitals (6s, 5d, 4f on Ce) were included in the CAS with the appropriate number of electrons based on the formal charges. Inclusion of the 2p orbitals on C and N did not change the results for up to the tetra isocyano compound. The CASSCF for the isocyano compounds yields occupancies of 6s15d14f3, (6s+5d)14f3, (6s+5d)14f3, and 4f3 for NdNC, Nd(NC)2, Nd(NC)(CN), and Nd(NC)3. These results differ somewhat from the NBO results which give approximately 6s14f4, 4f4, and 4f4 for NdNC, Nd(NC)2, and Nd(NC)(CN). The DFT/NBO results favor 4f occupancy over 6s and 5d occupancy for Nd. For Nd(NC)3, the results are the same between the CASSCF and NBOs with a 4f3 occupancy. We note that the CASCF calculations for Ce and Nd result in very similar occupancies with the additional electrons going into the 4f. For Nd(NC)4, the isocyano 2p orbitals were included as there is the potential for spin on the NC group as shown by the NBOs. This is exactly what the CASSCF predicts with a 4f3 occupancy for Nd with the extra spin on the isocyano groups just as found with the NBOs. Comparison to U(NC)n and Th(NC)n. The Ce(NC)n and Nd(NC)n compounds can be compared to the corresponding U(NC)n14,15 and Th(NC)n16 compounds for n = 1 to 4 (see comparison table in the SI). Laser-ablated U atoms react with (CN)2 leading to the formation of major products of UNC, U(NC)2, and U(NC)4 and a minor product of U(NC)3. Similarly, the reactions of (CN)2 with


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Th produce mostly Th(NC)2, and Th(NC)4 with small amounts of ThNC and Th(NC) 3. In contrast, the tetra-isocyano isomers for Ce and Nd were not observed under similar reaction conditions. It is important to note that the isocyano isomers are more stable than cyano isomers for Ce, Nd, Th, and U. The calculated Ln-NC and An-NC15,16 BDEs for the mono-isocyano isomers range from 97 to 140 kcal/mol and follow the orders of Ce < Nd < Th < U at the CCSD(T)/CBS level. For the di-isocyano isomers, the Ln-NC and An-NC BDEs are comparable to that for the mono-isocyano isomers and follow the order of Ce > Th > Nd ≈ U. With respect to the tri-isocyano isomers, the Ln-NC and An-NC BDEs are also comparable and fall in the range of 110 to 125 kcal/mol with the order of Th > Ce ≈ U > Nd. Significant differences in the CN-M (M = Ln or An) BDEs are predicted for the tetra-isocyano isomer. The dissociation of NC from Ln(NC)4 and An(NC)4 is calculated to be 61, 10, 121, and 81 kcal/mol respectively for Ce, Nd, Th, and U. Thus, tetraisocyano complexes for Ce and Nd are less stable than that for Th and U. This is consistent with the result that the tetra-isocyano Ce and Nd compounds are not observed and the tetra-isocyano Th and U compounds are observed. The NPA charges show that Th and U have the formal +IV oxidation state in Th(NC)4 and U(NC)4 compounds. However, Nd(NC)4 has +III oxidation state and Ce(NC)4 has a mixed +III/+IV oxidation state. The NBO analysis shows that the Nd(NC)n and Th(NC)n compounds are more transition-metal-like than the uranium compounds with very little backbonding to the 4f and 5f orbitals. Backbonding to 4f orbitals is only predicted for Ce(NC)4 and the backbonding to 4d orbitals is dominant for Ce(NC)n (n=1 to 3 ).



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Conclusions The infrared spectra clearly show the presence of isocyanide stretching frequencies in the 1975 to 2045 cm-1 region for Ce(NC)x and Nd(NC)x for x = 1 to 3 isolated in solid argon for molecules generated by the laser ablation of metallic Ce and Nd reacting with cyanogen. The calculated electronic structure frequencies with DFT with 3 functionals and correlated molecular orbital theory are consistent with the experimental assignments as are the corresponding

12

C:13C

isotopic frequency ratios. As expected, our DFT and MP2 computed frequencies for the analogous cyanide complexes are significantly higher than those for the isocyano isomers and are not in the observed spectral region. The Ln atoms react with (CN)2 under laser ablation or uv irradiation leading to the formation of the oxidative addition product Ln(NC)2 with Ln in the formal +II oxidation state in a highly exothermic reaction. The di-isocyanide is predicted to have an open shell triplet ground state for Ce and a quartet ground state for Nd with both the NLnN and LnNC bond angles being non-linear. The dicyano isomer is predicted to be 6.4 and 8.2 kcal/mol higher in energy than the diisocyanide for Ce and Nd, respectively. The mixed isocyano/cyano dimer isomer is energetically in between the diisocyano and dicyano isomer. The reaction of Ce(NC)2 with another (CN)2 is predicted to lead to the formation of the unobserved tetrahedral Ce(NC)4 with the Ce in a mixed formal +III/+IV oxidation state in an exothermic reaction with the tetra-cyano isomer is predicted to be 27 kcal/mol higher in energy. The corresponding reaction of (CN)2 with Nd(NC)2 to form Nd(NC)4 with the Nd in the +III oxidation state is endothermic. The reaction of (CN)2 with Ln to form LnNC and a CN radical is predicted to be endothermic. Dissociation of (CN)2 leads to the formation of the CN radical which can react with Ln and Ln(CN)2, respectively, leading to the respective formation of LnNC and Ln(NC)3, again by highly exothermic reactions. The Ce-NC bond dissociation energies range from


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137 kcal/mol for Ce(NC)2 to 61 kcal/mol for Ce(NC)4. The corresponding Nd-NC bond dissociation energies range from 115 kcal/mol for Nd(NC)2 to only 10 kcal/mol for Nd(NC)4. The NPA charges show that the monoisocyano to triisocyano compounds for both Ce and Nd are highly ionic. The first electron to be lost to generate Ln(I) is the 6s for LnNC for Ce and Nd. The Ln(II) in Ln(NC)2 for Ce is generated by loss of 6s and 5d character whereas for Nd, it is loss another 5s electron. The formation of Ln(III) in Ln(NC)3 is by loss of the remaining 5s and 6d electron character and for Nd it is for loss of a 4f electron. In contrast to the corresponding tetrafluorides with the Ln in the +IV oxidation state,35 Ce(NC)4 has a mixed +III/+IV oxidation state and Nd(NC)4 has +III oxidation state. In contrast to the uranium isocyanides,14,15 there is no backbonding into the tight 4f orbitals and backbonding is dominated by backbonding into the 5d. The Ln-NC bond dissociation energies do not show a large variation from r n =1 to n = 3 for each Ln, consistent with the dominance of ionic bonding. Formation of the tetra-isocyano compounds shows significantly less ionic bonding as it becomes harder to oxidize the Ln, and, for Nd(NC)4, it is not possible to attain the +IV oxidation state on the Nd. The negative charge is localized on the N in the isocyanides and is delocalized over the C and N in the cyano compounds. Thus, the isocyanides have a more ionic interaction between the Ln and the NC than the corresponding cyanides leading to stronger Ln-NC bonds in the isocyanides than Ln-CN bonds in the cyanides. The Ln-N bond distances are shorter in the isocyanides than are the Ln-C bond distances in the cyanides consistent with the increased ionic bonding in the former. As a consequence, the CN bond distances are shorter in the cyanides than in the isocyanides with the CN stretch at higher energy in the former than in the latter.



Supporting Information Complete citation references for 43 and 45, and 63. Infrared spectra from the reactions of Ce/Nd with low concentrations of (CN)2 in solid argon. Additional calculated vibrational frequencies and electronic structure properties. Optimized Cartesian coordinates. The Supporting Information is available free of charge on the ACS Publications website at DOI. ORCID Yu Gong: 0000-0002-8847-1047 Lester Andrews: 0000-0001-6306-0340 David A. Dixon: 0000-0002-9492-0056 Zongtang Fang: 0000-0001-6034-1083 Monica Vasiliu: 0000-0001-7573-4787 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The computational work at UA was supported by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, U.S. Department of Energy (DOE) under the Heavy Element Program grant number DE-SC0018921. DAD also thanks the Robert Ramsay Chair Fund of The University of Alabama for support. The experimental work at UVa was supported by the University of Virginia (LA). YG thanks the NSFC (21771189), the Strategic Priority Research Program and Frontier Science Key Program of the Chinese Academy of Sciences (Grant Nos. XDA02030000 and QYZDYSSW-JSC016), and the Young Thousand Talented Program for support.


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TOC Graphic

+ CN

+ (CN)2

3Ce(NC)

-41.2

-117.1

- CN

(CN)2 + Ce

-136.6

4CeNC

+ (CN)2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-60.7 + CN 2Ce(NC)

1Ce(NC)

3


4

2

Formation of Cerium and Neodymium Isocyanides in the Reactions of

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