Coordination of cyano-ligands to copper through nitrogen: An electron spin resonance study

INORG.

NUCL.

CHEM.

LETTERS

COORDINATION NITROGEN~ Martyn C.R. Department

Vol.

10,

pp.

995-998,

1974.

Pergamon

Press.

Printed

in

Great

Britain.

OF CYANO-LIGANDS TO COPPER THROUGH AN E L E C T R O N SPIN RESONANCE STUDY.

Symons,

Douglas X. West + and James G. W i l k i n s o n

of Chemistry,

The University,

Leicester

LE1 7RH.

+On Sabbatical leave from Central Michigan University, Mount Pleasant, Michigan, U.S.A. ~ e c e i v ~ 18 ~ n e 1974)

Although

coordination

have been studied

for many years(l),

the e.s.r,

spectroscopic

electronic

structures.

the most useful spectroscopy,

atoms(2). e.s.r,

technique

have one metal

their

and ultraviolet that compounds ion octahedrally

to six carbon atoms and the other to six nitrogen Thus such systems would appear

studies

nitrogen atoms dope a range Cu 2+ ions,

type

to X-ray crystallography,

have been infrared

type MIM2(CN)6

blue

little use has been made

in elucidating

In addition

techniques

of the Prussian

and it now seems fairly certain

of the general coordinated

polymers

of transition-metal of cyano-ligands.

of diamagnetic

to be suitable

ions coordinated We therefore

for

to the

attempted

MIIhexacyanoferrate(II)

to

salts with

but met with no success.

Recently,

however,

MII[Fe(CN)sNO].2H20

Inoue et al.(3)

salts are analogues

have confirmed

that

of Prussian blue(h),

and in these cases we were able to dope the zinc,cadmium and mercury difference

salts with Cu 2+ quite in cupric

satisfactorily.

ion solubility may well be related

need to induce a linear distortion which would resisted

This

by the highly

symmetrical

995

to its

be greatly

hexacyanides.

of

996

An Electron Spin Resonance Study

Experimental a typical

e.s.r,

results

are summarised

powder spectrum

Vol, 10, No. 11

in the Table,

and

is shown in the Figure.

3212G

500G~ ~H

* 3/2//(63Cu.65Cu)

Figure

÷1/28

First

-1/2/~

derivative

pentacyanonitrosylferrate The large positive that there

is considerable

After allowance

hyperfine example,

for copper

spectrum

for zinc

orbital magnetism

for gll means

in this direction

coupling

is made for this effect,

to 63Cu and the copper

very similar to that found,

in copper phthalocyanine,

for

which exhibits

small gl/ shift(5,6).

We conclude orbital

JV

~-xOPPH

II doped with Cu 2+ in low concentration.

to the hyperfine

coupling becomes

a relatively

e.s.r,

//

shift from the free-spin value

which will contribute 65Cu.

X-band

- ~2/~

on copper,

that the unpaired

electron

w i t h some delocalisation

is in the dx2_y 2 onto four in-plane

Vol. 10, No. 11

An Electron Spin Resonance Study

nitrogen atoms.

997

The large shift in gl/ can be understood in

terms of strong in-plane ~-bonding coupled with weak in-plane ~-bonding. 1

The shift stems from a magnetic coupling between ~

2

~

dx2_y2(b2g ) and dxy(blg ).

In-plane ~-bonding involves dxy

and this destabilises the =

orbital.

Such bonding is not

possible for the phthalocyanines since the ligands provide no in-plane =-orbitals.

Again,

~-bonding occurs via dx2_y2, and

this is weak because the ligands are already involved in coordination to iron, weakly donating.

and

the nitrogen ~-eleotrons are only

Hence the energy gap between the orbitals

involved is small a n d

the g-shift large.

The presence of four coupled nitrogen nuclei supports the concept that the z-distortion is along the metal-NO direction, as anticipated.

The I~N parameters (Table) c a n be

processed in the usual manner(7) to give approximate 2s and 2p character on nitrogen, provided the true All and AI values are obtained. AI(~N)

These powder spectra give good values for

on the parallel g-features, but the splitting detected

on the perpendicular features is really the average, i.e.

(All

+ AI)/2(6 ) •

Hence we find a~s $ •026 and a 2 2p

$ .I0 "

The total spin-density on nitrogen accords reasonably well with the results from the copper hyperfine coupling and is comparable

with results for the phthalocyanine complex.

The

p:s ratio is close to ~, whilst that estimated for the phthalocyanine complex is ca. 2.

Although no weight should be

placed on the numerical significance of these results, they do indicate that more 2pz and less 2s orbital on nitrogen is utilised in the

~

orbital for these "isocyanide" ligands.

998

An Electron Spin Resonance Study

Vol. 10, No. 11

TABLE Magnetic data for copper(II) ions in zinc and cadmium pentacyanonitrosylferrate(ll) A~3Cu

(G) c

AI~N (G) c

Zn[Fe(CN)sN0]

lh6

21

12.5

15

2.337

2.060

Cd[Fe(CN)sNO]

137

2O

12.5

15

2.38O

2.O6O

s

'~N coupling on Cu(;l) lines,

b

On Cu(l) lines,

c

G = 10 -4 T.

m

therefore

g-values

therefore A('LN)~.

(I] + 4)/2 •

w

REFERENCES (i)

D.F. SHRIVER,

(2)

See, for example, S.E. ANDERSON,

Struct. Bonding

(Berlin), ~, 32 (1966).

D.F. SHRIVER,

S.A. SHRIVER and

Inorg. Chem., 2, 725 (1965) and references

therein. (3)

H. I N O ~ ,

H. IWASE and S. YANAGISAWN,

Inorg. Chim. Acts,

Z, 259 (1973). (4)

P.G. SALVADEO,

Gazz. Chim. Ital., 8_.9.9,218h (1959).

(5)

S.E. HARRISON and J.M. ACSOUR,

J. Chem. Phys., 40, 365

(196a). (6)

C.M. GUZY, J.B. RAYNOR and M.C.R.

SYMONS,

J. Chem. Soc.(A),

2299 (1969). (7)

P.W. ATKINS and M.C.R. SYMONS, Radicals,

Elsevier,

Amsterdam

The Structure (1967).

of Inorganic