Werner Centennial

26 Aspects of Nuclear Magnetic Resonance and the Coordinate Covalent Bond G E O R G E F. SVATOS

Downloaded by UNIV LAVAL on October 19, 2015 | http://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/ba-1967-0062.ch026

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Chemical shielding and spin-spin coupling have been examined in a variety of trialkylphosphine-platinum(II) compounds and the results interpreted in terms of current bonding theory. Covalent bond energies have been calcu lated from coupling constants and correlated with transeffect phenomena. The data agree with chemical informa tion and show that trans-labilizing activity increases in the series: Cl-, Br-, I-, PR , CN-. Chemical shield ing is considered in terms of the interdependent effects of ionic contributions generated at phosphorus due to substituent atoms and temperature independent paramag netism. Cis- trans-isomers are easily differentiated. Changes in the Y substituent of Y P ligands are shown to be systematic in their influence on bond parameters. 3

3

/Coordinate covalent bond formation as we view it today is a result of information assembled over many years. Its general features were first outlined by Alfred Werner in 1893 {34). The electronic basis was established in 1916 (14, 16). From structural data made available by systematic chemistry, methods such as x-ray diffraction, and electronic information resulting from magnetic susceptibilities and dipole moments, our knowledge of this bonding process was greatly increased (22). In recent years nuclear magnetic resonance has been particularly useful in expanding information on the bonding process. M u c h work in high resolution, nuclear magnetic resonance has been devoted to establishing parameters which correlate chemical shielding and spin-spin coupling with particular bond structures. Shielding has been associated with diamagnetic and paramagnetic terms acting at the nucleus, with paramagnetic terms predominating (11,15,19, 27, 29, 35). Coupling 388 In Werner Centennial; Kauffman, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

26.

389

Nuclear Magnetic Resonance

SVATOS

of spins has been related primarily to the degree of s-hybridization and to ionic character contributions to the total wavefunction (12, 18, 20, 21, 25). In addition, useful relations between spin coupling and bond parameters, such as overlap integral and covalent bond energy, have been developed (33). Extension to the coordination process has been particularly useful. E a r l y P magnetic resonance studies of square planar [ N i ( R P ) 2 X ] complexes have shown that ligand field stabilization energies appear to be moderate and strongly influenced by secondary ligands (18, 32). Paramagnetic-diamagnetic equilibria have been observed in some instances (23). Generally, paramagnetic terms are an inverse function of singlettriplet transition energy and related to the degree of electron derealization in the molecule. A temperature and solvent dependence is not uncommon. Magnetic resonance spectra of platinum (II) compounds of the type [ P t ( R P ) X ] have been of particular interest (4, 24, 26, 31). Though phosphorus-31 has a natural abundance of 100% with a spin of one-half, platinum is composed of several isotopes, of which only P t has a nuclear spin (I = J , natural abundance is 33.7%). This gives rise to a doublet spectrum with a net relative area of 0.337 symmetrically superimposed upon a central absorption having a relative area of 0.643. Typical spectra are given in Figures 1 and 2. Phosphorus-31 magnetic resonance data for the [ P t ( R P ) X ] system are in general agreement with coordinate bond theory. Strong ligand field splitting and dir-dw back bonding predominate (1, 8, 6, 7, 24, 31). These compounds exhibit strong bonding and transactivating behavior (2, 28). Upon coordination, trialkylphosphines assume a nearly tetrahedral geometry (30).


3 1

3

2

3

2

1 9 5

3

2

2

H3PO4

Figure 1.

P

n

magnetic resonance in

[Pt((CJIi)iP)£l?\

In Werner Centennial; Kauffman, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

2

390

WERNER CENNENTIAL

J

P-Pt

=

6


S=

8

4

0

C

-P» *~ j 8

>

-68

J

-300

I

T— -200

Figure 2.

-100

P

3 1

P-P

=

1

5

5

100

0

C

s

P °

s

#

200

magnetic resonance in [Pt(PFz)2Ch]

Phosphorus-31 chemical shifts are not susceptible to simple interpre tation. M a n y molecular and bond properties determine the distribution and propagation of spin density throughout the molecule. These include: 1) ionic terms generated at the phosphorus atom due to substituents, 2) temperature independent paramagnetism arising from the central metal (9,10,17, 82), and 3) electron derealization giving rise to a charge balance. Generally, singlet-triplet excitation energies are large. In comparison with similar nickel complexes, which can have relatively large paramagnetic terms, the chemical shift in platinum compounds is small, ranging from — 19.3 to 8 ppm (Table I). For a given isomer where electronegativity of the secondary ligand, X ~ , increases, chemical shifts occur at a lower applied field. In addition, cis isomers have a smaller down-field shift than do trans isomers. The dependence of the chemical shift on alkyl sub stituents directly connected to phosphorus is somewhat variable. Coupling constants are usually large, ranging from 2100 to above 6800 cps. They are only slightly altered by changes i n electronegativity of the secondary ligand (becoming smaller as the electronegativity decreases)


26.

Svatos

391


and by changes in alkyl substituents. The difference between cis and trans isomers is striking. Where X ~ is a halide, coupling constants of isomers are in the ranges 3515 ± 15 cps and 2395 =t 25 cps, respectively. Table I.

P

31

N M R Data on the Trialkylphosphine-Platinum(n) System (30)

cJs-[Pt(RzP)2X ] 2


R

C n-C n-C n-C n-C

2 3 4 4 4

H H H H H

X~

r 7 9 9 9

CI CI CI I CN

-11 - 1 - 2.4 8 - 7

3500 3530 3510 3500 2100

12.0 12.0 12.0 12.0 10.1

CI CI CI Br Br I

-12.4 - 3.8 - 5.9 - 7.8 0 4

2400 2420 2410 2390 2380 2370

10.6 10.6 10.6 10.5 10.5 10.5

2240

10.3

trans-[Pt(R P ) X ] 3

2

2

C2H5

n-C n-C C n-C n-C

3 4 2 4 4

H H H H H

7 9 6 9 9

[Pt((C H ) P) ]Cl 2

0 6 c

6

3

4

-19.3

2

Chemical shift given in ppm with reference to 8 5 % H P 0 . Spin-coupling constant given in cps. Bond energy given in kcal./mole. 3

4

Employing the relation, D = 0.79 J !.p (33), it is possible to deter mine approximate bond energies for P - P t complexes (Table I). For cisand trans-halide complexes bond energies are 10.5-10.6 and 12.0 k c a l . / mole, respectively. The difference in energy is somewhat smaller than the 2.5-5 kcal./mole suggested in earlier work (5). A n examination of the data shows that as trans-labilizing activity of the secondary ligand increases in the series, C l ~ , B r ~ , I~, R P , C N , P - P t , coupling decreases. This is in agreement with established literature (1, 2, 28). The role of 7r interactions in trans-labilizing activity of ligands is emphasized by the relatively small dependence of coupling constants on ligand size and elec tronegativity. For example, strong 7r-electron acceptor ligands, such as C N ~ in the trans position, greatly decrease the coupling constant between alkylphosphine and platinum. A somewhat low value for the coupling constant of [ P t ( ( C H ) 3 P ) 4 ] may be attributed to additional terms gen erated in orbitals of B symmetry and to an increase in net positive charge on the complex, thus reducing T back bonding. P

-

3

2

t

3 1

1 9 5

+2

5

2 g

A n examination of czs-[Pt(Y P) Cl ] and £rans-[Pt(Y P)Cl ] com plexes (8), with respect to changes in the Y substituent, shows several trends: 1) A s the electronegativity of the Y group decreases, the chemical 3

2

2

3

2


2

392

WERNER

CENNENTIAL

shift (with reference to the parent phosphine) increases linearly until ionic terms at phosphorus approach zero, where a marked change in paramag netic terms is observed (Figure 3). 2) A plot of P - P t coupling data 3 1

1 9 5


120

100

-

80

'

60

\

-

C1

ei£-[Pt(T P) Cl^] 3

\

-"

2

OR

P.

-

-20 •

R i

-40

3.0

2.5

Figure 3.

i

i

3.5

4.0

Changes in P chemical shift with electronegativity of substituent Y n

against electronegativity of Y shows a simple linear relationship (Figure 4). Bond energies increase to 15 kcal./mole (Tables II and III). Bond multiplicity may be estimated using the expression: M

~ D* ~

\J*/

where M , D , D * J , and J * are the multiplicity, bond energy, bond energy of the single bond, spin-coupling constant, and spin-coupling constant for the single bond, respectively. Assuming that the P - P t bond in cis-


26.

SVATOS


8000 -

393

• c-[Pt(Y P) 01 '] 2

3

2

.t-[Pt(Y P)01 ] 2

3

2

7000

"?

6000 -

00


e O

5000 -

•P

P-4 •-a

4000-

3000-

2000

Electronegativity o f Y Figure 4-

Table II.

Relation of electronegativity of alkylphosphine substituent to coupling constant

P

31

Chemical Shifts and P - P t cw-[Pt(Y P) Cl ] 31

3

Y

-11 -70 -71 -112 -60

6

71-C4H9O

CI F 6 c

Coupling Constants in

2>

a

C2H5

0

195

2

B HzPOA

n-C H 0 2

2

3500 5675 5650 4650 6840

Chemical shift given in ppm with reference to 8 5 % Spin-coupling constant given in cps. Bond energy given in kcal./mole.

12.0 14.0 14.0 13.2 15.0

H3PO4.

[ P t ( ( n - C H ) 3 P ) ( C N ) ] is essentially a single bond, multiplicities can be estimated for remaining compounds. These range from 1 to 1.49. 4

9

2

2


394

WERNER CENTENNIAL

Table III.

P

31

Chemical Shifts and P - P t in [Pt(Y P)Cl ] 31

3

Y

a

9

4

b

A

-30 -45 —

6

4

9

195

Coupling Constants

2

2>

J P*-Pt

d HzPO

C H 0 n-C H 0 n-C H 2

2

196

6562 6537 3810(23)

14.7 14.7 12.3

° Chemical shift given in ppm with reference to 85% H P 0 . Spin-coupling constant given in cps. Bond energy given in kcal./mole. 3

4

6

e

Platinum (II) d-orbital splitting for cis- and frans-[Pt(Y P) X ] com plexes is given i n Figure 5. Orbital splitting is generally largest where P - P t bond multiplicity is greatest—i.e., i n cis isomers. It is usually difficult to prepare trans isomers when Y P ligands participate in bonds of high multiplicity; cis, trans equilibria strongly favor the formation of cis compounds. Trans-labilizing ability of Y P ligands increases with electronegativity of the substituent, Y . A n examination of spin coupling and bond energy data for [ P t ( ( C H ) P ) ] , m - [ P t ( ( C H ) P ) X ] , and

Werner Centennial

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