Indian Journal of ChemistryVol. 22A, June 1983,pp. 529-530

Novel Cyano Bridging NitrosylComplexes of Chromium(I)

RAM CHARITRA MAURYAChemicalLaboratories, Atarra P.G. College,Atarra 210201

Received 17August 1982;revised and accepted 4 February 1983

Pentacyanonitrosylchromate(I) reacts with nitrogen donorligands like 2,2'-bipyridyl (bipy), o-phenanthroline (o-phen),pyridine (py) and quinoline (Qu) to give complexesof the types[CrNO(CN)2L2](L=pyor Qu) and [CrNO(CN)2L](L=bipyor 0-phen). A tetramericcyano bridgingstructure hasbeenproposed forthecomplexes.Chromium(I)hasa low-spindS configurationin thesecomplexes.

Pentacyanonitrosylchromate(I) has attracted muchattention! -4 due to the controversy regarding themode of bonding between chromium and nitrosylligand. Raynor and coworkerss have studied thestepwise aquation of this anion in acetic acid mediumand observed that the third aquated speciespresumably has the composition [CrNO(CN)2(H20h]' We have recently observed" that acetyl-acetone reacts with the anion to give pentacoordinatedspecies, [CrNo(acac)2]. This observation prompt-ed the author to characterise the acidic mediumaquation species of the anion using some nitrogendonor ligands and the present note describes the resultsof this study.

Preparation of the complexesA filtered aqueous solution of the potassium salt of

pentacyanonitrosylchromate(I) hydrate I was addedslowly to an aqueous acetic acid solution (1:1) of thecorresponding ligand with shaking when a co louredsolid started to precipitate on warming in the cases of

dipyridyl and o-phenanthroline complexes and onwarming and dilution with a large volume of water(100 ml) in the cases of pyridine and quinolinecomplexes. The resulting mixture was freed fromliberated HCN by passing a current of CO2 throughthe mixture for a few hours. The precipitate wasfiltered off, washed with water, ethanol and ether anddried in vacuo to constant weight and ana lysed for Cr,N, C and H. The analytical data are given in Table I.

All the reagents used were of AR grade. Deaeratedwater was used in all the operations. Chromium wasdetermined as chromium oxide after decomposing thecomplexes by heating with alkali followed bydissolving in HN03 and precipitating by ammonia.Carbon, hydrogen and nitrogen were determinedmicroanalytically. Infrared spectra of the complexeswere recorded in KBr using a Perkin-Elmer-62Ispectrometer. Reflectance spectra were recorded usingCarey-14 spectrometer. Magnetic susceptibilities weremeasured using the Gouy method. ESR spectra of thecomplexes were recorded at room temperature on aVarian-4502 spectrometer using powdered samples.

All these compounds are yellow solids. They areexceptionally stable in air and remain almostunattacked in dilute acids and alkalies and decomposeonly on warming. They are thermally stable and do notshow any melting or decomposition up to 250°C. Allthe compounds are slightly soluble in acetonitrile butare insoluble in other common solvents. The molarconductance values (8-15 ohm -I ern? rnol ""] of thecomplexes in 10 -4 M methanol solution suggest theirnon-electrolytic nature.

The analytical and important infrared spectral dataof the complexes are given in Table I. Strong bandsobserved in the regions 1692-1708 and 2150-2157

Complex(Colour)

Table. I-Analytical Data and Important IR Spectra BandsFound (Calc.),% v(NO)+ v(CN)

Cr N C HKJ[CrNO(CN)s]H20t 1645vs 2137s

2095sh[CrNO(CN)idipy)] 18.1 24.2 49.6 2.8 1694vs 2157s(Greenish-yellow) (I7.9) (24.1) (49.7) (2.76) 2133sh[CrNO(CN),(o-phen)] 16.7 22.3 53.5 2.7 1696vs 2156s(Greenish-yellow) (16.6) (22.3) (53.5) (2.5) 2125sh[CrNO(CN),(pY)2] 17.8 24.1 48.9 3.6 1700vs 2156s(Yellow) (17.8) (24.0) (49.3) (3.4) 2125sh[CrNO(CN),(Qu),] 13.1 17.7 61.0 3.6 1708vs 2150s(Brownish-yellow) (I3.3) (17.9) (61.2) (3.6) 2125sh

[ref. 9

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INDIAN 1. CHEM., VOL. 22A, 1UNE 1983

Table 2- Magnetic and Spectral Data of the Complexes

Complex j1,,, at g I.m•• (nmj ;303 K(BM)

K3[CrNO(CN)s]t 724.0, 447.0335.0, 270.0

[CrNO(CNh(dipy)] 1.60 1.980 595.2, 450.0325.2, 244.9

[CrNO(CNh(o-phen) 1.67 1.980 591.0, 450.0325.7, 246.1

[CrNO(CN),(py),] 1.62 1.981 598.0, 450.0327.3, 252.8

[CrNO(CN)iQu),] 1.66 1.981 632.9, 449.6

tin aq. solution", ireflectance spectra.327.8, 248.7

em -\ with a shoulder around 2125-2133 em -I areassigned to v(NO) + and v(CN) modes respectivelywhich are in accordance with the assignment made forthe parent complex I. Coordination of the ligands inthese complexes is indicated by shifts in the positionsof free ligand IR bands 7.8.

The magnetic moment and spectral data of thecomplexes are given in Table 2. All the complexes showfour well-resolved electronic spectral peaks around591.0-632.9,449.6-450.0,325.2-327.8 and 244.9-252.8nm which are comparable to those reported for theparent complex", The magnetic moment values of1.60-1.67 B.M. at 303°K for these complexes and gvalues of 1.980-1.981 of undiluted powderedcompounds which are comparable to the observationsmade by Manoharan and Gray"" and Meriwether andcoworkers II are consistent with a low-spin dS

configuration of Cr(I).All the results described above suggest the

formulation of these complexes as [CrNO(CN)2L2] (L= py or Qu) and [CrNO(CNhL] (L = bipy or o-phen).The insolubility of the complexes in common solvents,resistance to substitution reactions, unusually low-magnetic moments and broadening of ESR signalssuggest the possibility that these complexes exist inpolymeric form where the metal has a coordinationnumber six. To achieve hexa-coordination, either thenitrosyl group or one of the cyanide groups of the twounits should form a bridge. The wide variation oforganic ligands (in conformation, bulkiness and donorcapacity) suggests that the polymerisation is not onaccount of the bridging by these ligands. The positionof v(NO) + in the range 1692-1708 em -I in thesecomplexes indicates the terminal NO group?" 2.15,thus ruling out the possibility of nitrosyl group actingas a bridging group. Apparently, hexa-coordination isachieved through the cyano group acting as a bridgingmoiety. All the complexes show two v(CN) modesappearing at 2150-2157 and 2125-2133 cm -I. The

530

lower band appears as a shoulder in all the cases. Thev(CN) at 2150-2157 em -I is assigned in the presentcomplexes to the bridging cyano group and the otherband to terminal cyano group. Bustin I4, on the basis ofelectrochemical studies on the reaction of[Cr(CNXH20)4NO] with Ag ' and Hg2+ ions,proposed that a bridge structure like Cr - N - C - M(M = Ag + or Hg2 +) is formed. The eN bridge linkageisomer, Cr-C - N - M (M =Hg'2+) has also beencharacterised. Thus, in accordance with the abovelinear cyano bridge structure and considering the transdirecting effect of the NO group", a tetrameric cyanobridging structure(I) is proposed for the presentcomplexes.

NO

CJ CN I CN/ //1' -N = CN/CDJ' NO NO ,#'

/N I/COr_c_N-D/1 /NC NCNO

Oipy. ~- Phen or0= two molecutes ofPy or au

The author is grateful to Prof. Wahid U. Malik,Vice-Chancellor, The University of Kashmir,Srinagai ; Dr. B. Singh, principal and Mr. JagpatSingh, president of Atarra P.G. College forencouragement and facilities. Thanks are also due toDr. S. Sarkar, Reader, Department of Chemistry,I.I.T. Kanpur.

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