The nature of M-PNR2 bonds in the electrophilic phosphinidene complexes [(L)(CO)3M{PNR2}]+ (L = PMe3, PPh3; M = Co, Rh, Ir; R = Me, iPr): Structure, bonding and 31P NMR study

Accepted Manuscript The nature of M-PNR2 bonds in the electrophilic phosphinidene complexes [(L) + i (CO)3M{PNR2}] (L = PMe3, PPh3; M = Co, Rh, Ir; R = Me, Pr): Structure, bonding 31 and P NMR study Krishna K. Pandey, Ravi Vishwakarma PII:

S0022-328X(16)30155-3

DOI:

10.1016/j.jorganchem.2016.04.012

Reference:

JOM 19468

To appear in:

Journal of Organometallic Chemistry

Received Date: 5 March 2016 Revised Date:

31 March 2016

Accepted Date: 12 April 2016

Please cite this article as: K.K. Pandey, R. Vishwakarma, The nature of M-PNR2 bonds in the + electrophilic phosphinidene complexes [(L)(CO)3M{PNR2}] (L = PMe3, PPh3; M = Co, Rh, Ir; R = Me, i 31 Pr): Structure, bonding and P NMR study, Journal of Organometallic Chemistry (2016), doi: 10.1016/ j.jorganchem.2016.04.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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The

nature

of

M-PNR2

bonds

in

the

electrophilic

phosphinidene

complexes

[(L)(CO)3M{PNR2}]+ (L= PMe3, PPh3; M = Co, Rh, Ir; R = Me, iPr): Structure, bonding

Krishna K. Pandey,* Ravi Vishwakarma

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and 31P NMR study

School of Chemical Sciences, Devi Ahilya University Indore, Khandwa Road Campus, Indore 452 001, India

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______________________________________________________________________________ ABSTRACT

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Theoretical insights into the structure and the nature of M-PNR2 bonding in the cationic electrophilic phosphinidene complexes [(L)(CO)3M{PNR2}]+ (M = Co, Rh, Ir; R = iPr, Me; L = PMe3, PPh3,) have been investigated at DFT level with emphasis on the density functional BP86, PBE, PW91 and TPSS and dispersion interactions, DFT-D3(BJ). Dispersion corrected functional

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yields accurate geometries. The geometry optimized with PBE-D3(BJ) functional is in excellent agreement with the experimental geometry of structurally characterized cobalt phosphinidene complex [(PPh3)(CO)3Co{PNiPr2}]+ (IV). The effects of metal atom, trans-influence of

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phosphine ligands (PMe3, PPh3) and substituent at nitrogen atom of PNR2 ligand on the M-PNR2 bond distances and M-P-N bond angles have been studied. The lengthening of M-PNR2 bonds

The

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trans to PMe3 ligand than those trans to PPh3 are due greater trans-influence of the PMe3 ligand. 31

P NMR chemical shifts of phosphinidene and phosphine ligands phosphorus in the

complexes I-XII have been calculated out at PBE-D3(BJ)/TZ2P/ZORA with scalar (SC) and spin orbit (SO) relativistic level of theory in solvent chloroform. The computed values of

31

P

NMR chemical shifts are within the range of experimental values. The Mulliken charge analysis shows that the overall charge flows from phosphinidene ligand to metal fragment. The energy

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decomposition analysis divulged that the contribution of the electrostatic interaction ∆Eelstat in all studied complexes is larger (54.5%-61.3%) than the orbital interactions ∆Eorb. The π-bonding contribution is much smaller than the σ-bonding (85.4%-87.0%).

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Keywords: DFT-D3(BJ); Phosphinidene; 31P NMR chemical shift; Bonding, Trans-effect ______________________________________________________________________________

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*Corresponding author. Tel.: 00 91 731 2460208; fax: 00 91 731 2762342 To whom correspondence should be addressed. Email: [email protected];[email protected]

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1.

Introduction The chemistry of phosphorus is undergoing a new renaissance. Transition metal

phosphinidene complexes LnM=PR have attracted much attention as powerful PR delivery

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reagents in organophosphorus chemistry [1-16]. Phosphinidenes (PR) are often introduced by comparison with isoelectronic nitrene (NR) and carbene (CR2) species. Like the metal carbene complexes, terminal phosphinidene complexes also display electrophilic (Fischer type) or

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nucleophilic (Schrock type) nature on the basis of reactivity at phosphorus atom. In 1987 Lappert and co-workers reported first stable transition metal phosphinidenes which are nucleophilic [1],

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while isolation of electrophilic phosphinidene complexes has proven to be more difficult. At starting, the terminal electrophilic phosphinidenes generated as transient species and trapped in situ [17-28]. Synthetic applications, structure, bonding and reactivity of electrophilic and nucleophilic terminal phosphinidenes complexes have blossomed greatly [29-57].

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In particular, a series of cationic, terminal electrophilic phosphinidene complexes including Group 6 metals [(η5-C5Me5)(CO)3M{PNiPr2}][AlCl4] (M = Mo, W), Group 8 metals [(η5-C5Me5)(CO)2M{PNiPr2}][AlCl4]

(M

=

Fe,

Ru,

Os),

the

rhenium

complex

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[(CO)5Re{PNiPr2}][AlCl4] and the cobalt complex [(PPh3)(CO)3Co{PNiPr2}][AlCl4] have been reported over the past few years [29-34]. Carty and co-workers reported the first examples of

i

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neutral electrophilic phosphinidene complexes of vanadium [(η5-C5H5)(CO)3V{P(NR2)}] (R = Pr, Cy) and their reactivity have also been described [35]. The bonding in terminal

phosphinidene complexes has been schematically presented in Fig. 1. They either contain a singlet ground state with two lone pairs and an empty pz-orbital on the P atom or a triplet state with one lone pair in sp hybrid orbital and two singly occupied-orbitals. Phosphinidene much prefer a triplet ground state.

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π − bonding σ − bonding

P

M

P

M

R

(a) triplet, nucleophilic

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R

N-P π − bonding

σ − donation

P

R N R

M

P

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M

π − back donation

R

N R

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(b) singlet, electrophilic

Fig. 1. Schematic representation of the M-PR orbital interactions: (a) in nucleophilic phosphinidene complexes (triplet state) and (b) in electrophilic phosphinidene complexes (singlet state). The transition metal-phosphorus bonding in nucleophilic complexes has been described

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by electron sharing bonding model (covalent bonding) between phosphinidene and metal fragments in their triplet state. Substitution of a π-donor group on P stabilize the singlet state of free phosphinidene and result in the formation of electrophilic phosphinidene complexes that are

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analogous to Fischer type carbene complexes. The bonding in electrophilic phosphinidene complexes is best conceptualized by considering donor–acceptor interactions between

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phosphinidene fragment and metal fragment in singlet state. An earlier computational study has revealed that the degree of stabilization of the triplet state over the singlet state can vary depending on the particular substitution on phosphorus atom [58]. Ehlers, Baerends and Lammertsma investigated the factor governing the philicity of the terminal bent phosphinidene complexes [LnM=PR] (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Ru, Os, Co, Rh, Ir; L = CO, PH3, η5-C5H5) [59]. The philicity influenced by type of spectator ligand L rather than for the nature of transition metal M [59]. Substituents with strong σ-donor capabilities increase the 4

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electron density on the phosphorus atom, enhancing its nucleophilicity. Conversely, substituents with strong π-acceptor capabilities lower the charge concentration on phosphorus, causing electrophilic behavior.

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A number of theoretical studies on electrophilic Fischer type phosphinidene complexes have been performed [60-71]. The structure and bonding properties of the electrophilic phosphinidene complexes of iron, ruthenium, osmium [68], vanadium, niobium [69] and

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manganese, rhenium [70] have been investigated. Since the last ten years, there has seen a surge of interest in treating in the approximate density functional theory (DFT) methods with

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dispersion interactions [72-79]. Grimme and coworkers have developed DFT-D3(BJ) [80] method with Becke and Johnson (BJ) damping [81] for the computation of the dispersion interaction in molecules. Dispersion effects are important not only for an adequate description of non-covalent interaction, but also for obtaining accurate optimized geometries and bonding

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energies [78,79].

To the best of our knowledge structure and bonding energy analysis of the complexes [(L)(CO)3M{PNR2}]+ (M = Co, Rh, Ir; L = PPh3, PMe3; R = Me, iPr) have not been reported

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earlier. Theoretical insights into the structure and the nature of M-PNR2 bonding in the cationic electrophilic phosphinidene complexes [(L)(CO)3M{PNR2}]+ (M = Co, Rh, Ir; R = iPr, Me; L =

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PMe3, PPh3) have been investigated at DFT level with emphasis on the density functional BP86, PBE, PW91 and TPSS and dispersion interactions, DFT-D3(BJ). The effects of metal atom, trans-influence of phosphine ligands (PMe3, PPh3) and substituent at nitrogen atom of PNR2 ligand on the M-PNR2 bond distances and M-P-N bond angles have been studied. The 31P NMR chemical shifts of phosphinidene and phosphine ligands phosphorus in the complexes I-XII have been calculated out at PBE-D3(BJ)/TZ2P/ZORA with scalar (SC) and spin orbit (SO) relativistic

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level of theory in solvent chloroform. Our aim is to provide a quantitative description and rationalization of the general trends observed for electronic structures and M-PNR2 bonding across the studied complexes (I-XII). We would like to address the following questions: (i) what

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is the nature of M-PNR2 bonds in the cationic electrophilic phosphinidene complexes, (ii) what are the relative strengths of the covalent and electrostatic interactions, (iii) how the relative strength of M-PNR2 bond changes within the series of complexes [(L)(CO)3M{PNR2}]+ (I-XII),

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(iv) how the London dispersion interactions affect the geometries and M-PNR2 bonding in the studied complexes, (v) effect of metal, substituent groups on the phosphorus and of ancillary

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ligand on the nature of M-PNR2 bonding, and (vi) the effect of dispersion interactions (BeckeJohnson damping) on bonding energies. In order to further rationalize the nature of M-PNR2 bonding, we have computed

31

P NMR chemical shifts in the studied phosphinidene complexes

(I-XII). Computational methods Theoretical

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2.

calculations

of

the

transition

metal

phosphinidene

complexes

[(L)(CO)3M{PNR2}]+ (M = Co, Rh, Ir; L = PPh3, PMe3; R = Me, iPr) (I-XII) have been carried

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out by DFT and DFT-D3(BJ) methods. Density functionals BP86 [82,83], PBE [84], PW91 [85]

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and TPSS [86] were used for the geometry optimization. Grimme’s dispersion corrected DFT-D3 method [80] with BJ damping [81] was used to account the dispersion interactions. Scalar relativistic effects have been considered using the ZORA formalism [87]. Uncontracted Slatertype orbitals (STOs) using triple-ζ basis sets augmented by two sets of polarization functions were employed for the self consistent field (SCF) calculations [88]. The (1s)2 core electrons of the carbon and oxygen, (1s2s2p)10 core electrons of phosphorus and cobalt, (1s2s2p3s3p)18 core electrons of rhodium and (1s2s2p3s3p3d4s4p)36 core electrons of iridium were treated by the 6

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frozen-core approximation [89]. An auxiliary set of s, p, d, f, and g STOs was used to fit the molecular densities and to represent the Coulomb and exchange potentials accurately in each SCF cycle [88]. A numerical integration accuracy of INTEGRATION = 6 was used throughout.

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Frequency calculations were performed to determine whether the optimized geometries were minima on potential energy surface. The electronic structures of the complex (I-XII) were examined by the Mayer bond order [90] and Mulliken atomic charges [91]. The calculations

The

energy

decomposition

analysis

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were performed with the program package ADF-2014.01 [92]. (EDA)

has

been

carried

out

at

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DFT/PBE/TZ2P/ZORA level of theory. The bonding interactions between metal fragments [L(CO)3M]+ (M = Co, Rh, Ir, L = PPh3, PMe3) and ligand fragments PNR2 (R= Me, iPr) (singlet state) have been analyzed with Cs symmetry. The calculations have been performed with the energy decomposition scheme of the program package ADF [93], which is based on the work by

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Morokuma [94] and Ziegler and Rauk [95]. The bond dissociation energy (BDE) between the fragments is partitioned into several contributions which can be identified as physically meaningful quantities. First, ∆E is separated into two major components ∆Eint and ∆Eprep: ∆Eint + ∆Eprep

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∆E =

(1)

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∆Eprep is the energy, which is necessary to promote the fragments from their equilibrium geometry and electronic ground state to the geometry, and electronic state, which they have in the molecule. The instantaneous interaction energy ∆Eint is the focus of the bonding analysis and can be decomposed into three components: ∆Eint = ∆Eelstat + ∆EPauli + ∆Eorb

(2)

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The term ∆Eelstat gives the electrostatic interaction energy between the fragments which are calculated with a frozen density distribution in the geometry of the complex. The term ∆EPauli, which is called exchange repulsion or Pauli repulsion, takes into account the destabilizing

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two-orbital three- or four-electron interactions between occupied orbitals of both fragments. ∆EPauli is calculated by enforcing the Kohn-Sham determinant of the molecule, which results from superimposing both fragments, to obey the Pauli principle through antisymmetrization and

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renormalization. The last term ∆Eorb in equation 2 gives the stabilizing orbital interactions

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between occupied and virtual orbitals of the two fragments. ∆Eorb can be further partitioned into contributions by the orbitals that belong to different irreducible representations of the point group of the system. It has been shown that the results of EDA give a quantitative insight into the nature of metal-ligand interactions [96]. The calculations of the

31

P NMR chemical shifts were carried out at PBE-

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D3(BJ)/TZ2P/ZORA with scalar (SC) and spin orbit (SO) relativistic level of theory in solvent chloroform (ε = 4.8). Nuclear shieldings of

31

P were calculated with the ADF nmr property

module [97-100]. In the scalar-ZORA case, the isotropic shielding constant σ is given by the sum

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of diamagnetic and paramagnetic contributions (σ = σd + σp), whereas in the spinorbit-ZORA

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level, the spin-orbit contribution is also added to the isotropic shielding constant σ (σ = σd + σp + σSO). Computed chemical shifts are then determined by the difference of the shielding of the H3PO4 (δ = 0) with the shielding of 31P in the complexes [(L)(CO)3M{PNR2}]+ (M = Co, Rh, Ir; L = PPh3, PMe3; R = Me, iPr) (I-XII) from δ = σref – σ. 31P NMR chemical shifts were evaluated with the direct implementation of the gauge including atomic orbitals (GIAO) method [101]. 3.

Results and discussion

3.1

Geometries

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The important bond distances and angles of the cationic electrophilic phosphinidene complexes [(L)(CO)3M{PNR2}]+ (L= PMe3, PPh3; M = Co, Rh, Ir; R = Me, iPr) (I-XII) calculated using density functionals BP86, PBE, PW91, and TPSS are presented in Table S1

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(Supporting Information). The representative optimized geometries of the cobalt phosphinidene complexes I, IV, VII and X are shown in Fig. 2. The structures of other studied rhodium and iridium phosphinidene complexes are very similar to those presented in this Fig. 2 and are,

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therefore, not included.

Fig. 2. Optimized structures of the cationic electrophilic phosphinidene complexes of cobalt [(L)(CO)3Co(PNR2)]+ (L = PMe3; R = iPr (I), L = PPh3; R = iPr (IV), L = PMe3; R = Me (VII), L = PPh3; R = Me (X) at PBE-D3(BJ)/TZ2P level of theory. The performance of different density functionals has been examined for the M-PNR2 and M-P(PPh3) bond distances of structurally characterized cationic phosphinidene cobalt complex 9

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[(PPh3)(CO)3Co(PNR2)]+ (IV) (Table 1, Fig. 3). It can be inferred from the calculated bond distances and experimental bond distances that (a) dispersion corrected functional yields accurate geometries and (b) the geometry optimized with PBE-D3(BJ) functional is in excellent

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agreement with the experimental geometry of complex (IV). We expect the same accuracy in geometry optimized with PBE-D3(BJ) functional for the other studied complexes (I-III, V-XII)

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whose X-ray structural geometries are not available yet.

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Fig. 3. Performance of different density functionals and dispersion corrections for M-PNR2 and M-P(PPh3) bond distances in complex (IV).

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Table 1 Comparison of the structural parametersa of cobalt phosphinidene complex [(PPh3)(CO)3Co{PNiPr2}]+ (IV) obtained at DFT and DFT-D3(BJ) level of theory. _________________________________________________________________________________ Bond distances Bond angles Noncovalent distances

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Functional M-PNR2 M-P(L) P-N (L)P-M-P M-P-N CO(1)...P CO(2)…N M…N ____________________________________________________________________________________ BP86 2.203 2.310 1.651 179.8 115.6 2.767 3.357 3.274 BP86-D3(BJ) 2.190 2.258 1.648 176.7 113.0 2.820 3.283 3.213 PBE 2.196 2.300 1.651 179.8 115.4 2.757 3.351 3.266 PBE-D3(BJ) 2.188 2.270 1.650 178.1 114.0 2.787 3.306 3.230 2.187(8) 2.263(7) 1.626(2) 172.8(3) 115.4(8) Experimentalb TPSS 2.199 2.296 1.643 179.4 114.4 2.768 3.334 3.243 TPSS-D3(BJ) 2.192 2.258 1.642 176.5 112.0 2.814 3.272 3.194 _________________________________________________________________________________ a Bond distances are in angstrom (Å) and bond angles are in degree (º). b Geometrical parameters for the X-ray characterized complex of [(PPh3)(CO)3Co{PNiPr2}][AlCl4] [31].

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As PBE-D3(BJ) geometries are more accurate, we shall consider these geometries for further discussion. The important optimized geometrical parameters for the cationic electrophilic phosphinidene complexes of cobalt, rhodium and iridium (I-XII) are given in Table 2. All the studied complexes show trigonal bipyramidal geometries around the transition metal, in which

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carbonyl ligands occupy the equatorial positions.

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Table 2 Selected optimized geometrical parametersa for electrophilic phosphinidene complexes [(L)(CO)3M{PNR2}]+ (M = Co, Rh, Ir; L = PMe3, PPh3; R = Me, iPr) (I-XII). ___________________________________________________________________________________________

(L)P

M

P

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CO(1)

Bond distances

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NR2 CO(2) CO(2) ______________________________________________________________________________

Bond angles

2.196 2.257 1.647 2.789 2.333 2.400 1.647 2.967 2.360 2.415 1.647 2.937 2.188 2.270 1.650 2.787 2.187(8) 2.263(7) 1.626(2) ….. 2.322 2.418 1.647 2.789 2.349 2.432 1.649 2.909 2.180 2.264 1.653 2.794 2.318 2.407 1.652 2.963 2.343 2.420 1.652 2.932 2.172 2.277 1.654 2.775 2.306 2.425 1.653 2.924 2.331 2.438 1.653 2.900

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[(PMe3)(CO)3Co{PNiPr2}]+ (I) [(PMe3)(CO)3Rh{PNiPr2}]+ (II) [(PMe3)(CO)3Ir{PNiPr2}]+ (III) [(PPh3)(CO)3Co{PNiPr2}]+ (IV) Experimentalb [(PPh3)(CO)3Rh{PNiPr2}]+ (V) [(PPh3)(CO)3Ir{PNiPr2}]+ (VI) [(PMe3)(CO)3Co{PNMe2}]+ (VII) [(PMe3)(CO)3Rh{PNMe2}]+ (VIII) [(PMe3)(CO)3Ir{PNMe2}]+ (IX) [(PPh3)(CO)3Co{PNMe2}]+ (X) [(PPh3)(CO)3Rh{PNMe2}]+ (XI) [(PPh3)(CO)3Ir{PNMe2}]+ (XII)

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Complexes M-P M-P(L) P-N CO(1)---P CO(2)---N M---N (L)P-M-P M-P-N _________________________________________________________________________________________ 3.358 3.559 3.573 3.306 …. 3.358 3.550 3.331 3.562 3.562 3.283 3.495 3.529

3.247 3.361 3.382 3.230 ….. 3.247 3.365 3.225 3.361 3.361 3.210 3.324 3.348

176.6 114.6 174.0 114.1 172.4 113.9 178.1 114.0 172.8(3) 115.4(8) 176.6 114.6 176.2 113.5 176.4 113.9 173.9 113.4 172.3 113.4 179.4 113.3 177.4 113.1 175.7 113.2

_____________________________________________________________________________________________________________________________________________ a b

Bond distances are in angstrom (Å) and bond angles are in degree (º). Geometrical parameters for the X-ray characterized complex of [(PPh3)(CO)3Co{PNiPr2}][AlCl4] [31].

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We shall now discuss the effects of metal atom, trans-influence of phosphine ligands

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(PMe3, PPh3) and substituent at nitrogen atom of PNR2 ligand on the M-PNR2 bond distances and M-P-N bond angles. Trends of variation of M-PNR2 bond distances are shown in Fig. 4. As expected from periodic trends, the M-PNR2 as well as M-P(L) bond distances in the complexes (I-XII) increase from M = Co to M = Ir. Relatively shorter M-PNR2 bond distances as compared to M-P(L) bond distances clearly indicate that phosphinidene ligands bind more strongly to metal atom than the phosphine ligands.

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Fig. 4. Trends of variation of M-PNR2 and M-P(L) bond distances in the complexes [(L)(CO)3M{PNR2}]+ (M = Co, Rh, Ir; L = PMe3, PPh3; R = Me, iPr) (I-XII).

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Fig. 5. Trends of σ-bonding and π-bonding of M-L (L = PMe3, PPh3) bond in complexes I-XII. The strengths of M-L σ-bonding and π-back bonding for trimethylphosphine and ligands

are

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triphenylphosphine

computed

with

energy

decomposition

analysis

at

DFT/PBE/TZ2P/ZORA level of theory. Results are presented in Fig. 5. Results divulge that

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PMe3 ligand is a better σ-donor and poor π-acceptor than the PPh3 ligand and support the Tolman theory [102]. As a consequence, the M-PMe3 bonds are stronger than the M-PPh3 bonds. Relative shorter M-PMe3 bond distances are found as compared to M-PPh3 bond distances (see Fig. 4). Thus, the PMe3 ligand will exert greater trans-influence than the PPh3 ligand on the MPNR2 bonding. The lengthening of M-PNR2 bonds (Fig. 4) trans to PMe3 ligand than those trans to PPh3 are due greater trans-influence of the PMe3 ligand.

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Substituent at nitrogen atom of PNR2 ligand also affects the extent of M-PNR2 bonding. Such effect can be explained on the basis of donor ability of the NR2 groups. The NiPr2 group is better electron donor than the NMe2 ligand (Fig. 6). The better electron donor NiPr2 group in the

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complexes I−VI enhance the N → P π-donation and consequently, M → P π-back-donation is expected to reduce in these complexes. On the other hand, the relative poor electron donor NMe2 ligand reduce the N → P π-donation and enhance the M → P π-back-donation. Thereby, the

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M−PNR2 bond distances in the complexes VII−XII (R = Me) are shorter than that in the complexes I−VI (R = iPr). The shorter P-N bond distances in the complexes I-VI as compared to

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those in the complexes VII-XII (Table 2) may be explained likewise.

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Fig. 6. Mulliken charges on the NR2 group in the phosphinidene complexes I-XII. Moreover, The P−N bond distances of the complexes I−XII are markedly shorter than

those expected from single bond covalent radii prediction (P−N = 1.82 Å) (Table 2) [103]. These observations suggest that the P-N bond has nearly double bond character. As expected from bonding model (Fig. 1), the bent bonding at phosphorous (M-P-N bond angles in the range 112.4º-115.9º) is due to the presence of lone pair on phosphorus in these phosphinidene complexes. The important non-covalent distances, i.e., CO(1)---P, CO(2)---N and M---N are 15

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included in Table 1. The M---N distances in complexes (I-XII) increase on going from Co to Ir and vary with density functionals as BP86 > PBE > PW91 > TPSS. 3.2

Bonding analysis of M-PNR2 bond in the phosphinidene complexes I-XII

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We begin the analysis of M-PNR2 bonding in the phosphinidene complexes (I-XII), with a discussion of Mayer bond orders [90] and Mulliken atomic charges [91]. As shown in Table 3, the Mayer bond orders of M-PNR2 bond in phosphinidene complexes (I-XII) are 1.09 (I), 1.08

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(II), 1.10 (III), 1.11 (IV), 1.12 (V), 1.15 (VI), 1.12 (VII), 1.12 (VIII), 1.15 (IX), 1.15 (X), 1.16 (XI), 1.19 (XII), suggesting presence of multiple bond character of the M-PNR2 bond. It has

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been noted previously that the Mayer bond orders are usually lower than classical integer values [104]. Substituent at the nitrogen atom of the PNR2 ligands affects the M-PNR2 and P-N bond orders. Upon substitution of R = iPr with R = Me, the values of Mayer bond orders for M-PNR2 bonds slightly increases. The values of Mayer bond orders for P-N bonds (in the range 1.47-1.56)

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indicate that P-N bonds have essentially double bond characters.

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Table 3 Mayer bond order and Mulliken atomic charges for terminal electrophilic phosphinidene complexes [(L)(CO)3M{PNR2}]+ (M = Co, Rh, Ir; L = PMe3, PPh3; R = Me, iPr). _________________________________________________________________ Mayer bond order Mulliken atomic charges

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Complex M-PNR2 P-N M L PNR2 _________________________________________________________________ 1.09 1.54 -0.73 0.72 0.80 [(PMe3)(CO)3Co{PNiPr2}]+ (I) 1.08 1.55 -0.89 0.81 0.84 [(PMe3)(CO)3Rh{PNiPr2}]+ (II) [(PMe3)(CO)3Ir{PNiPr2}]+ (III) 1.10 1.56 -0.78 0.77 0.79 [(PPh3)(CO)3Co{PNiPr2}]+ (IV) 1.11 1.52 -0.72 0.73 0.80 [(PPh3)(CO)3Rh{PNiPr2}]+ (V) 1.12 1.55 -0.98 0.82 0.82 [(PPh3)(CO)3Ir{PNiPr2}]+ (VI) 1.15 1.54 -0.80 0.78 0.74 [(PMe3)(CO)3Co{PNMe2}]+(VII) 1.12 1.48 -0.70 0.75 0.69 [(PMe3)(CO)3Rh{PNMe2}]+(VIII) 1.12 1.50 -0.83 0.79 0.74 [(PMe3)(CO)3Ir{PNMe2}]+ (IX) 1.15 1.50 -0.74 0.78 0.66 [(PPh3)(CO)3Co{PNMe2}]+ (X) 1.15 1.47 -0.70 0.77 0.65 [(PPh3)(CO)3Rh{PNMe2}]+ (XI) 1.16 1.50 -0.92 0.86 0.72 [(PPh3)(CO)3Ir{PNMe2}]+ (XII) 1.19 1.48 -0.75 0.80 0.63 ________________________________________________________________

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The values of Mulliken atomic charges reveal that (i) the PNiPr2 ligand is a better electron donor than the PNMe2 ligand, (ii) the electron flows from phosphinidene ligand and

rhodium atoms carry a more negative charge.

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phosphine ligand (L = PPh3, PMe3) to rhodium fragments are largest. As a consequence, the

To visualize the M-PNR2 bonding, envelope plots of some relevant molecular orbitals of the structurally known cobalt complex [(PPh3)(CO)3Co{PNiPr2}]+ IV are presented in Fig. 7.

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Fig. 7A (HOMO) depicts the lone pair at phosphinidene phosphorous atom along with the noncovalent interaction between metal and nitrogen. The P=N π-bonding is shown in the Fig. 7B

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(HOMO-9). The Fig. 7C (HOMO-53) shows the non-bonded non-covalent interactions between phosphinidene phosphorus and carbonyl carbon. The Co-PNR2 σ- bonding orbital is depicted in

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Fig. 7D (HOMO-59), which is polarized towards P atom.

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Fig. 7. Important molecular orbitals of the electrophilic phosphinidene complex [(PPh3)(CO)3Co(PNiPr2)]+ IV.

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3.3

Energy decomposition analysis of electrophilic phosphinidene complexes I-XII To further probe the nature of M-PNR2 bonds in the complexes (I-XII), we carried out an

energy decomposition analysis (EDA) as well as dispersion corrected DFT-D3(BJ) calculations.

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The energy decomposition analysis has been performed considering the fragments [(L)(CO)3M]+ and [PNR2] in their singlet state. The results are given in Table 4 and their trends are delineated

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in Fig. 8.

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Table 4 Energy decomposition analysisa of the M-PNR2 bonds in terminal electrophilic phosphinidene complexes [(L)(CO)3M{PNR2}]+ (M = Co, Rh, Ir; L = PMe3, PPh3; R = Me, iPr) (I-XII) at PBE/TZ2P level of theory.

∆Eorb ∆E(a’)c ∆E(a”) ∆Eprep ∆E(−BDE)

−90.2 171.8 −154.0 (58.9%) −108.0 −93.5 (86.6%) −14.5 8.1 −82.1

−98.4 201.6 −180.7 (60.2%) −119.2 −103.6 (86.9%) −15.7 8.8 −89.6

−90.3 181.4 −149.9 (55.2%) −121.8 −105.7 (86.8%) −16.1 5.7 −84.6

−84.8 186.8 −161.4 (59.4%) −110.3 −95.1 (86.2%) −15.2 7.9 −76.9

a

−92.8 219.5 −189.9 (60.8%) −122.4 −105.8 (86.3%) −16.5 8.6 −84.2

−94.7 171.4 −146.8 (55.1%) −119.3 −103.0 (86.3%) −16.3 5.2 −89.5

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−96.6 166.1 −143.3 (54.5%) −119.4 −103.9 (87.0%) −15.5 5.7 −90.9

[(PMe3)(CO)3M-{PNMe2}]+ VII VIII IX Co Rh Ir −88.5 177.9 −158.2 (59.4%) −108.2 −92.4 (85.4%) −15.3 7.7 −80.8

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∆Eint ∆Epauli ∆Eelest b

[(PPh3)(CO)3M-{PNiPr2}]+ IV V VI Co Rh Ir

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[(PMe3)(CO)3M-{PNiPr2}]+ I II III Co Rh Ir

−97.0 209.2 −186.2 (60.8%) −120.0 −103.3 (86.0%) −16.7 8.7 −88.3

[(PPh3)(CO)3M-{PNMe2}]+ X XI XII Co Rh Ir −88.6 187.3 −153.9 (55.8%) −121.9 −104.7 (85.8%) −17.2 5.1 −83.5

−83.6 −91.8 193.8 228.0 −166.3 −196.2 (59.9%) (61.3%) −111.0 −123.6 −94.8 −105.7 (85.4%) (85.5%) −16.3 −17.9 7.6 8.4 −76.0 −83.4

Energy contribution in kcal/mol. The values in parentheses are the percentage contribution to the total electrostatic interaction reflecting the ionic character of the bond. c The values in parentheses are the percentage of σ-contribution to the total orbital interaction, ∆Eorb.

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b

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Fig. 8. Trends of absolute values of different energy terms (in kcal/mol) for the complexes I-XII. The calculated bond dissociation energies (BDEs) display a V-like trends in all four sets

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(I-III, IV-VI, VII-IX, X-XII) (Fig. 8), with a minimum at the rhodium complexes. Such V-like trend in bond energies, with a minimum at the second-row transition metal complexes are common [105,106]. This trends is largely caused by the relativistic effects, which becomes very

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important for the 5d elements. The interaction energies, ∆Eint show the same trends as calculated BDEs, with the discrepancies between two values (i.e. ∆Eprep), ranges between the 5.1 – 8.8

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kcal/mol. In order to estimate the contribution of dispersion interactions to the bond dissociation energies (BDEs) between the metal fragments and PNR2 ligands in complexes I-XII, we performed single point energy calculations on the optimized structures of I-XII using Grimme’s DFT-D3 method with Becke-Johnson damping. The calculated BDEs with DFT-D3(BJ) method is presented in Table 5. Fig. 9 shows the variation of BDEs as a function of density functionals. The BDEs are smallest for the functional BP86 and larger for the functional PW91. The bond

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dissociation energies of M-PNR2 bond are relatively smaller for trans PPh3 ligand than those for trans PMe3 ligand. The DFT-D3(BJ) dispersion correction are in the range 13.1–9.3 kcal/mol

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(BP86), 7.2–4.2 kcal/mol (PBE), 9.8–6.7 kcal/mol (TPSS).

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Table 5 Bond dissociation energies (BDE) (kcal/mol) of the M-PNR2 bond in [(L)(CO)3M{PNR2}]+ (M = Co, Rh, Ir) (L = PMe3, PPh3) (R = Me, iPr) complexes calculated using different DFT methods. __________________________________________________________________________________________________ BDE Dispersion BDE + Dispersion Complexes BP86 PBE TPSS PW91 BP86 PBE TPSS BP86 PBE TPSS ___________________________________________________________________________________________________ [(PMe3)(CO)3Co-PNiPr2]+(I) 87.5 90.9 88.8 91.4 11.6 6.4 8.8 99.1 97.3 97.6 [(PMe3)(CO)3Rh-PNiPr2]+(II) 79.3 82.1 79.8 82.8 11.2 6.1 8.5 90.5 88.2 88.3 [(PMe3)(CO)3Ir-PNiPr2]+(III) 86.4 89.6 88.0 90.2 11.6 6.2 8.6 98.0 95.8 96.6 [(PPh3)(CO)3Co-PNiPr2]+(IV) 80.5 84.6 82.0 84.9 13.1 7.2 9.8 93.2 91.9 91.8 [(PPh3)(CO)3Rh-PNiPr2]+(V) 73.8 76.9 73.8 77.5 12.1 6.7 9.1 85.9 83.6 82.9 80.7 84.2 82.2 84.8 12.5 6.8 9.2 93.2 91.0 91.4 [(PPh3)(CO)3Ir-PNiPr2]+(VI) [(PMe3)(CO)3Co-PNMe2]+(VII) 86.4 89.5 87.3 90.5 9.7 5.2 7.2 96.1 94.8 94.5 [(PMe3)(CO)3Rh-PNMe2]+(VIII) 78.3 80.8 78.4 81.3 9.3 4.9 6.7 87.6 85.7 85.1 85.6 88.3 86.7 88.9 9.4 5.0 6.9 95.1 93.3 93.6 [(PMe3)(CO)3Ir-PNMe2]+(IX) [(PPh3)(CO)3Co-PNMe2]+(X) 80.5 83.5 80.9 84.0 10.4 6.1 7.9 91.0 89.6 88.8 73.2 76.0 72.6 76.5 9.9 5.3 7.1 83.1 81.3 79.7 [(PPh3)(CO)3Rh-PNMe2]+(XI) [(PPh3)(CO)3Ir-PNMe2]+(XII) 80.2 83.4 81.1 83.8 10.1 5.4 7.3 90.4 88.5 88.4 _________________________________________________________________________________________________

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Fig. 9. Trends of the absolute value of the bond dissociation energies (BDEs) (kcal/mol) of the M-PNR2 bond of complexes I-XII by using different functionals. The breakdown of the interaction energy, ∆Eint, into the repulsive term ∆EPauli, and the attractive terms ∆Eorb and ∆Eelstat, presented in Table 4, shows that the repulsive interaction ∆EPauli, has the largest absolute values. The electrostatic interaction ∆Eelstat, in all complexes, (I-

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XII) is significantly larger than the orbital interaction ∆Eorb which means, the M-PNR2 bonds in these complexes have a greater degree of ionic character than the covalent contributions. The

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values of electrostatic interaction ∆Eelstat contribute nearly 54.5%-61.3% to the total attractive interactions. Table 4 also gives the breakdown of the orbital interaction into the M ← PNR2 σ-

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bonding and M → PNR2 π back donation components. In all four sets, [(PPh3)(CO)3Rh]+ ← [PNMe2] σ-donor and [(PPh3)(CO)3Rh]+ → [PNMe2] π-acceptor bondings are smaller to those for cobalt and iridium complexes. The M-PNR2 bonding interactions have smaller M → PNR2 πbonding contribution to the total orbital interactions. The EDA data, therefore, suggest that the phosphinidene ligands behave predominantly as σ-donors.

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3.4

31

P NMR chemical shift

The

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P NMR spectroscopy is one of the most important techniques available for the

characterization of compounds containing phosphorus. In particular,

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P NMR chemical shifts

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are of primary importance in providing evidence for a specific functional group. The isotropic P chemical shifts of phosphinidene ligands generally indicate significant deshielding but vary

over an extremely wide range from 66 to 1362 ppm [107]. The

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P NMR chemical shifts of

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electrophilic phosphinidene phosphorous are ranged in between 1111.0 – 664.0 ppm (see Table 6) [29-37]. Calculated chemical shifts have been referenced with respect to phosphoric acid

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(H3PO4) in chloroform (CHCl3). The calculated 31P NMR shielding constant for H3PO4 is σSC = 289.3 ppm, σSO = 290.9 ppm. The absolute shielding for 85% aqueous solution is 328.4 ppm at 298 K [108]. The

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P NMR spectra of the structurally characterized cobalt phosphinidene

complex [(PPh3)(CO)3Co-PNiPr2][AlCl4] has been recorded in CDCl3. Therefore, we have 31

P NMR chemical shifts

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performed calculation using solvent chloroform (ε = 4.8). Computed

of phosphinidene ligands and phosphine ligands of studied complexes I-XII has been collected in Table 7. Our calculated values of 31P NMR chemical shifts of PNR2 ligands (876.4 – 797.3 for

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SC-ZORA, 872.7 – 775.2 for SO-ZORA) in the complexes I-XII are within the range of

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experimental values [29-37] (see Tables 6 and 7).

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Table 6 An overview of the experimentally recorded 31P-NMR chemical shift (ppm) for phosphinidene phosphorous in terminal electrophilic phosphinidene complexes. ___________________________________________________________ References Complexes Chemical shift (δ) ___________________________________________________________ [(η5-C5Me5)2(CO)3Mo(PNiPr2)][AlCl4] 1007.5 [29] [(η5-C5Me5)2(CO)3W(PNiPr2)][AlCl4] 939.4 [29] [(η5-C5Me5)(CO)2Ru(PNiPr2)][AlCl4] 932.0 [29,30] [(η5-C5Me5)(CO)2Fe(PNiPr2)][AlCl4] 965.0 [30] [(η5-C5Me5)(CO)2Os(PNiPr2)][AlCl4] 838.0 [30] [(PPh3)(CO)3Co(PNiPr2)][AlCl4] 861.2 [31] [(CO)5Re(PNiPr2)][AlCl4] 956.0 [32] [(η5-C5Me5)(CO)2(PEt3)Mo(PNiPr2)][AlCl4] 957.0 [34] [(η5-C5Me5)(CO)2(PEt3)W(PNiPr2)][AlCl4] 878.0 [34] [(η5-C5H5)(CO)3V(PNiPr2)] 1100.0 [35] [(η5-C5H5)(CO)3V(PNCy2)] 1111.0 [35] [(η5-C5Me5)(PPh3)Ir(PMes*)] 687.0 [36] [(η5-C5Me5)(PPh3)Ir(PIs)] 664.0 [36] [(η5-C5Me5)(PPh3)Ir(PMes)] 652.0 [36] [(η5-C5Me5)(PH2Mes*)Ir(PMes*)] 715.0 [36] [(η5-C5Me5)(PMe3)Ir(PMes*)] 727.0 [36] [(η5-C5Me5)(P(OMe)3)Ir(PMes*)] 797.0 [36] 655.0 [36] [(η5-C5Me5)(AsPh3)Ir(PMes*)] [(η5-C5Me5)(t-BuNC)Ir(PMes*)] 740.0 [36] [(η5-C5Me5)(XyNC)Ir(PMes*)] 757.0 [36] [(η5-C5Me5)(CO)Ir(PMes*)] 805.0 [36] 867.0 [37] [(η5-C5H5)(PPh3)Co(PMes*)] [(η5-C5Me5)(PPh3)Rh(PMes*)] 868.0 [37] ____________________________________________________________ Ph = phenyl; Me = methyl; Xy = 2,6-dimethylphenyl; Mes = 2,4,6-trimethylphenyl; Is = 2,4,6-tri-isopropylphenyl; Mes* = 2,4,6-tri-tert-butylphenyl.

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Table 7 Theoretical 31P-NMR chemical shift (ppm) for phosphinidene phosphorous and phosphine phosphorous in complexes I-XII calculated at DFT/PBED3(BJ)/TZ2P/ZORA. _________________________________________________________________________________________________________________________ Phosphinidene ligand Phosphine ligand __________________________________________ ________________________________________ σp σd σ σSO δ σp σd σ σSO δ

_______________________________________________________________________________________________________________ ____ -671.4 -707.6 -689.3 -683.1 -732.9 -720.2 -713.1 -706 .7 -688.1 -682.1 -731.0 -718.6 -711.6

960.6 958.0 957.9 958.1 964.6 964.5 964.6 957.9 957.7 958.1 964.5 964.4 964.5

-671.4 -707.6 -689.3 -682.3 -732.8 -720.2 -712.8 -706.7 -688.1 -681.8 -731.1 -718.5 -7114

960.6 958.0 957.9 958.1 964.6 964.5 964.5 957.9 957.7 958.1 964.5 964.4 964.5

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0.0 839.2 865.4 803.2 842.3 858.5 797.3 845.2 876.4 810.5 840.5 865.1 801.5

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290.9a 1.7 -549.7 0.0 -570.2 5.5 -485.0 27.2 -553.0 -0.2 -563.4 5.3 -479.4 26.9 -556.0 -0.3 -581.8 4.9 -492.7 26.7 -551.5 -0.5 -570.6 4.8 -484.4 26.2

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960.6 962.5 962.7 963.1 962.7 963.1 963.6 962.9 963.3 963.7 963.1 963.6 964.2

289.3a -549.9 -576.1 -573.9 -553.0 -569.2 -508.0 -555.9 -587.1 -521.2 -551.2 -575.8 -512.2

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960.6 962.5 962.7 963.1 962.7 963.1 963.6 962.9 963.3 963.7 963.1 963.6 964.2

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ZORA scalar (CHCl3) [H3PO4] -671.4 [(PMe3)(CO)3Co{PNiPr2}]+ (I) -1512.4 [(PMe3)(CO)3Rh{PNiPr2}]+ (II) -1538.8 [(PMe3)(CO)3Ir{PNiPr2}]+ (III) -1477.0 -1515.7 [(PPh3)(CO)3Co{PNiPr2}]+ (IV) [(PPh3)(CO)3Rh{PNiPr2}]+ (V) -1532.3 [(PPh3)(CO)3Ir{PNiPr2}]+ (VI) -1471.6 [(PMe3)(CO)3Co{PNMe2}]+ (VII) -1518.8 [(PMe3)(CO)3Rh{PNMe2}]+ (VIII) -1550.4 [(PMe3)(CO)3Ir{PNMe2}]+ (IX) -1484.9 [(PPh3)(CO)3Co{PNMe2}]+ (X) -1514.3 [(PPh3)(CO)3Rh{PNMe2}]+ (XI) -1539.4 [(PPh3)(CO)3Ir{PNMe2}]+ (XII) -1476.4 ZORA spin-orbit (CHCl3) [H3PO4] -671.4 [(PMe3)(CO)3Co{PNiPr2}]+ (I) -1512.2 [(PMe3)(CO)3Rh{PNiPr2}]+ (II) -1538.4 [(PMe3)(CO)3Ir{PNiPr2}]+ (III) -1475.3 -1515.6 [(PPh3)(CO)3Co{PNiPr2}]+ (IV) [(PPh3)(CO)3Rh{PNiPr2}]+ (V) -1531.8 -1469.9 [(PPh3)(CO)3Ir{PNiPr2}]+ (VI) [(PMe3)(CO)3Co{PNMe2}]+ (VII) -1518.6 [(PMe3)(CO)3Rh{PNMe2}]+ (VIII) -1550.0 [(PMe3)(CO)3Ir{PNMe2}]+ (IX) -1483.1 [(PPh3)(CO)3Co{PNMe2}]+ (X) -1514.1 + [(PPh3)(CO)3Rh{PNMe2}] (XI) -1539.0 [(PPh3)(CO)3Ir{PNMe2}]+ (XII) -1474.7 a Values of shielding constant for reference H3PO4.

0.0 840.6 861.1 775.9 843.3 854.3 770.3 846.9 872.7 783.6 842.4 861.5 775.2

289.3a 250.4 268.6 275.0 231.7 244.3 251.5 251.2 269.6 276.0 233.4 245.8 252.9

290.9a 259.5 286.2 332.1 238.5 256.1 290.5 260.2 287.0 332.8 240.1 257.5 291.4

0.0 38.9 20.7 14.3 57.6 45.0 37.8 38.1 19.7 13.3 56.5 43.5 36.4 1.7 9.1 17.6 56.8 6.8 11.8 38.8 9.0 17.4 56.5 6.7 11.6 38.3

0.0 31.4 4.7 -41.2 52.4 34.8 0.4 30.7 3.9 -41.9 50.8 33.4 -0.5

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Concerning the scalar relativistic results, the diamagnetic contribution to the shielding constant (σd) is essentially constant (about 4 ppm variation) through the series, whereas the paramagnetic contribution (σp), spanning over 79 ppm, is sensitive to the metal fragments. The

for iridium complexes. As a consequence, the

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absolute values of paramagnetic shielding (σp) are largest for rhodium complexes and smallest P NMR chemical shifts for phosphorus of

phosphinidene ligands are largest for rhodium complexes and smallest for iridium complexes

the values of

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(for examples: 842.3 ppm in IV, 858.5 ppm in V, 797.3 ppm in VI). Only a small variations in P NMR chemical shifts for phosphorus of phosphinidene ligands for cobalt and

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rhodium complexes have been observed using SO-ZORA method instead of SC-ZORA method, while for iridium complexes, a significant upfield shift (∆δP(PNR2) = -27.3 in III, -27.0 in VI, 26.9 in IX, -26.3 in XII) are found. Shielding of the positive core charges by the full inner d- and f-shells is stronger in iridium than in rhodium. 31

P NMR chemical shifts for the phosphine ligands (PMe3,

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We found large downfield

PPh3) phosphorus atoms (see Table 7). PMe3 ligand has a upfield chemical shift value compared to PPh3 ligand δP(PMe3): 38.9 ppm in I, 20.7 ppm in II, 14.3 ppm in III, 38.1 ppm in VII, 19.7

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ppm in VIII, 13.3 ppm in IX; δP(PPh3): 57.6 ppm in IV, 45.0 ppm in V, 37.8 ppm in VI, 56.5

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ppm in X, 43.5 ppm in XI, 36.4 ppm in XII).

Conclusions

The following conclusions may be drawn on the basis of theoretical calculations of the

structure and dispersion corrected bonding energy analysis in the electrophilic phosphinidene complexes [(L)(CO)3M{PNR2}]+ (M = Co, Rh, Ir; L = PPh3, PMe3; R = Me, iPr) (I-XII) at the DFT and DFT-D3(BJ) methods using density functionals BP86, PBE, PW91 and TPSS.

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1)

Dispersion corrected functional yields accurate geometries. The geometry optimized with PBE-D3(BJ) functional is in excellent agreement with the experimental geometry of structurally characterized cobalt phosphinidene complex [(PPh3)(CO)3Co{PNiPr2}]+ (IV). Relatively shorter M-PNR2 bond distances as compared to M-P(L) bond distances clearly

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indicate that phosphinidene ligands bind more strongly to metal atom than the phosphine

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ligands.

Mulliken charge analysis shows that the overall charge flow from phosphinidene ligand

4)

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to metal fragment.

The values of bond dissociation energies of all studied complexes, I-XII, exhibit a V-like trend with the minimum at the rhodium.

5)

The NiPr2 group is better electron donor than the NMe2 ligand. The better electron donor

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NiPr2 group in the complexes I−VI enhance the N → P π-donation and consequently, M → P π-back-donation is expected to reduce in these complexes. 6)

The 31P NMR chemical shifts of phosphinidene and phosphine ligands phosphorus in the

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complexes I-XII have been calculated. The computed values of 31P NMR chemical shifts

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of PNR2 ligands (876.4 – 797.3 for SC-ZORA, 872.7 – 775.2 for SO-ZORA) in the complexes I-XII are within the range of experimental values.

7)

The absolute values of paramagnetic shielding (σp) are largest for rhodium complexes

and smallest for iridium complexes. As a consequence, the 31P NMR chemical shifts for

phosphorus of phosphinidene ligands are largest for rhodium complexes and smallest for iridium complexes.

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TOC The

nature

of

M-PNR2

bonds

in

the

electrophilic

phosphinidene

complexes

[(L)(CO)3M{PNR2}]+ (L= PMe3, PPh3; M = Co, Rh, Ir; R = Me, iPr): Structure, bonding

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and 31P NMR study Krishna K. Pandey,* Ravi Vishwakarma

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School of Chemical Sciences, Devi Ahilya University Indore,Khandwa Road Campus, Indore 452 001, India

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The nature of M-PNR2 bonds in the terminal, cationic electrophilic phosphinidene complexes of cobalt, rhodium and iridium have been investigated at the DFT, DFT-D3(BJ) methods using density functionals BP86, PBE, PW91 and TPSS. The effects of metal atom,

EP

trans-influence of phosphine ligands (PMe3, PPh3) and substituent at nitrogen atom of PNR2 31

P NMR chemical shifts have

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ligand on the M-PNR2 bond distances, M-P-N bond angles and been studied.

38

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Highlights Nature of M-PNR2 bonds in the electrophilic phosphinidene complexes is investigated.

•

The M-PNR2 bonds trans to PPh3 ligand are shorter.

•

The electrostatic interactions ∆Eelstat are larger than the orbital interactions ∆Eorb.

•

The π-bonding contribution is much smaller than the σ-bonding.

•

31

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•

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P NMR chemical shifts of phosphinidene ligands are largest for rhodium complexes.