Polyhedron 30 (2011) 1502–1506

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Polyhedron

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Synthesis, characterization and antimicrobial studies of mixed ligand silver(I)complexes of triphenylphosphine and heterocyclic thiones: Crystal structure ofbis[{(l2-diazinane-2-thione)(diazinane-2-thione)(triphenylphosphine)silver(I)nitrate}]

Sidra Nawaz a, Anvarhusein A. Isab b, Klaus Merz c, Vera Vasylyeva c, Nils Metzler-Nolte c,Muhammad Saleem d, Saeed Ahmad a,⇑a Department of Chemistry, University of Engineering and Technology, Lahore 54890, Pakistanb Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabiac Lehrstuhl für Anorganische Chemie 1, Bioanorganische Chemie Research Department, Ruhr-Universitaet Bochum, Universitaetsstrasse 150, D-44801 Bochum, Germanyd KINPOE, KNPC, PAEC, P.O. Box # 3183, Karachi 29, Pakistan

a r t i c l e i n f o

Article history:Received 26 January 2011Accepted 25 February 2011Available online 22 March 2011

Keywords:Silver(I) complexesThionesTriphenylphosphineX-ray structureAntimicrobial activity

0277-5387/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.poly.2011.02.054

⇑ Corresponding author. Tel.: +92 333 5248570.E-mail address: saeed_a786@hotmail.com (S. Ahm

a b s t r a c t

Mixed ligand silver(I) complexes of triphenylphosphine and heterocyclic thiones (imidazolidine-2-thione(Imt), diazinane-2-thione (Diaz) and 2-mercaptopyridine (Mpy)) having the general formulae[(Ph3P)Ag(thione)2]NO3 and [(Ph3P)2Ag(thione)]NO3 were prepared and characterized by elemental anal-ysis, IR and NMR (1H, 13C and 31P) spectroscopic methods. The crystal structure of one of the complexes,[Ag(Ph3P)(Diaz)2]2(NO3)2 (1) was determined by X-ray crystallography. The title complex (1) is dinuclear,having each silver atom coordinated to three thione sulfur atoms of Diaz and to one phosphorus atom ofPPh3 in a nearly tetrahedral environment, with an average P–Ag–S bond angle of 108.5�. The spectral dataof the complexes are consistent with sulfur coordination of the thiones to silver(I). Antimicrobial activ-ities of the complexes were evaluated by minimum inhibitory concentrations and the results showed thatthe complexes exhibit a wide range of activity against two gram-negative bacteria (E. coli, P. aeruginosa)and molds (A. niger, P. citrinum), while the activities were poor against yeasts (C. albicans, S. cerevisiae).

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1. Introduction

Our interest in mixed ligand silver(I) complexes with heterocy-clic thiones stems from our current research on the coordinationcompounds of coinage metals with such ligands [1–11] andbecause of the relevance of their binding sites to those in livingsystems [12–15]. A large number of structural reports on silver(I)complexes of phosphines and thiourea type ligands have shownthat silver(I) is quite variable in terms of its geometries, whichcan vary between linear, trigonal planar and tetrahedral [2,3,16–27]. Tertiary phosphines are quite useful for the crystallization ofmixed ligand silver(I) complexes with oxygen, nitrogen and sulfurdonor ligands [11,21–27]. In view of this fact, a number of studieshave been devoted to the preparation and structural characteriza-tion of thione–Ag(I)–phosphine complexes [11,27–34]. The crystalstructures of these complexes have shown that some of them aremononuclear [28–31], while others are binuclear with a tetrahe-dral arrangement around the silver atom [11,32–34]. It also

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appears from these studies that dinuclear complexes are generallyformed with silver halides, while only mononuclear complexeshave been observed in the reactions of heterocyclic thiones withsilver(I) nitrate [28–32]. In this paper, we present the crystal struc-ture of the first example of a dinuclear complex of silver nitratewith a heterocyclic thione (diazinane-2-thione), along with theanalytical data and other spectroscopic results of some other sim-ilar complexes. The structures of the thiones used in this study aregiven in Scheme 1.

2. Experimental

2.1. Materials

Silver nitrate (AgNO3) was obtained from Panreac, Spain andtriphenylphosphine (PPh3) from Alfa Aesar, USA. 2-Mercaptopyri-dine was purchased from Acros Organics, Belgium. Amoxil (anantibiotic), prepared by SmithKline Beecham Pharmaceuticals,Worthing, United Kingdom, was purchased from Market in theform of capsules. Each capsule contains 500 mg amoxicillin asthe trihydrate. Imidazolidine-2-thione (Imt) and diazinane-2-

PNHHN

S

NHHN

S

NH

S

Imidazolidine-2-thione Diazinane-2-thione 2-Mercaptopyridine Triphenylphosphine

Scheme 1. The structures of the ligands used in the study.

S. Nawaz et al. / Polyhedron 30 (2011) 1502–1506 1503

thione (Diaz) were synthesized according to the procedure de-scribed in the literature [9].

2.2. Synthesis of the complexes

The complexes were prepared by adding one or two equivalentsof a thione dissolved in 15–20 ml methanol to a 1:1 mixture ofAgNO3 and PPh3 in methanol–acetonitrile medium. The mixturewas stirred for 25–30 min. In the case of the imidazolidine-2-thione complexes, clear solutions were obtained, while fordiazinane-2-thione and 2-mercaptopyridine, white and yellowprecipitates were formed, respectively. The solutions were filteredand the filtrate was kept at low temperature for crystallization. Inthe case of Imt and Diaz, white, while for Mpy, yellow crystallineproducts were obtained (yield: 50–60%).

The elemental analyses of the complexes are given in Table 1.

Table 2Summary of crystal data and details of structure determination for 1.

2.3. IR and NMR measurements

The IR spectra of the complexes were recorded on a Perkin–Elmer FTIR 180 spectrophotometer using KBr pellets over the range4000–400 cm�1. The 1H NMR spectra of the complexes in DMSO-d6

were obtained on a Jeol JNM-LA 500 NMR spectrometer operatingat a frequency of 500.00 MHz at 297 K, using 0.10 M solutions. The13C NMR spectra were obtained on the same instrument at a fre-quency of 125.65 MHz with 1H broadband decoupling at 298 K.The spectral conditions were: 32 K data points, 0.967 s acquisitiontime, 1.00 s pulse delay and 45� pulse angle. The 13C chemicalshifts were measured relative to TMS.

Formula (C26H31PS2AgN5O3)2

Formula weight 1329.04Crystal system monoclinicSpace group P21/ca (Å) 15.256(3)b (Å) 10.3729(17)b (Å) 18.719(3)a (�) 90b (�) 101.046(7)

2.4. X-ray structure determination

Single crystal data collection for complex 1 was performed on aBruker-axs-SMART 1000 CCD diffractometer at 213 K. The struc-ture was solved by direct methods with SHELXS-97 [35,36] andrefined by full-matrix least squares procedures on F2 with all

Table 1Elemental analysis and melting points of the silver(I) complexes of phosphines andthiones [(R3P)xAg(thione)y].

Complex Found (Calc) M.p. (�C)

C, H, N, S

[Ag(PPh3)1]NO3 50.24 3.39 3.87 – 206(50.03) (3.47) (3.24)

[(PPh3)2Ag(Imt)1]NO3 58.75 4.38 4.73 3.04 200(58.80) (4.56) (5.27) (4.02)

[(PPh3)1Ag(Imt)2]NO3 44.34 4.20 11.07 9.57 205(45.29) (4.28) (11.00) (10.08)

[(PPh3)2Ag(Diaz)1]NO3 58.51 4.39 5.60 3.16 196(59.27) (4.70) (5.18) (3.96)

[(PPh3)1Ag(Diaz)2]NO3 47.19 4.60 8.54 7.08 218(46.99) (4.67) (10.54) (9.64)

[(PPh3)1Ag(Mpy)2]NO3 52.25 3.99 6.05 8.13 170(51.38) (3.85) (6.42) (9.80)

measured reflections using SHELXL-97 [35,36]. Crystal data and de-tails of the data collection are summarized in Table 2.

2.5. Antimicrobial studies of the complexes

Antimicrobial activities of the compounds prepared here wereestimated by their minimum inhibitory concentrations (MIC;lg mL�1) [37]. Standard culture media of bacteria, Escherichia coli,(ATCC 13706) and Pseudomonas aeruginosa (MTCC 424), and molds,Aspergillus niger (MTCC 1349) and Penicillium citrinum (MTCC 5215)and yeast Candida albicans (MTCC 183) and Saccharomyces cerevisi-ae (MTCC 463) were obtained from Qingdao Yijia Huuyi Co., China.Bacteria were inoculated into 5 mL of liquid SCD medium(soybean, casein and digest) and cultured for 24 h at 35.5 �C. Thecultured fluids were diluted, adjusted to a concentration of 105–106 microorganisms per mL and used for inoculation in the MICtest. In the case of the mold cultures, the agar slant (potato anddextrose) medium for one week cultivation at 27 �C was gentlywashed with saline containing 0.05% Tween 80. The spore suspen-sion obtained was adjusted to a concentration of 105 microorgan-isms per mL and used for inoculation in the MIC test. Yeast wereinoculated into 5 mL of glucose polypeptone (GP) medium and cul-tured for 48 h at 30 �C. The cultured fluids were diluted, adjusted toa concentration of 106–107 mL�1 and used for inoculation in theMIC test. The test materials (Ag(I) complexes) were suspended inwater, and solutions were then diluted with SCD medium for bac-

c (�) 90V (Å3) 2907.4(9)Z 2qcalc (g cm�3) 1.518l(Mo Ka) (mm�1) 0.928F(0 0 0) 1360Crystal size (mm) 0.30 � 0.30 � 0.20Temperature (K) 213(2)k Mo Ka (Å) 0.710732h Range (�) 2.22–25.00h, k, l limits �17:18

�11:12�13:22

Reflections; collected/unique 10201/4445(Rint = 0.0650)

Tmin, Tmax 0.7681, 0.8361Nref, Npar 4445, 344R1, wR2, S[I > 2r(I)]

w = [r2(Fo2) + (0.0242P)2 + 1.6454P]�1 where

P = (Fo2 + 2Fc

2)/3

0.0854, 0.2372,1.066

Largest differences in peak, hole (e �3) 1.480, �2.778

Table 41H and 13C NMR chemical shifts of the free ligands and their silver(I) complexes inDMSO-d6.

Species dN–H dC@S dC–N dC-5

Imt 7.98 183.44 43.97[(PPh3)1Ag(Imt)1]NO3 8.65 179.22 44.71[(PPh3)2Ag(Imt)2]NO3 8.58 180.03 44.66Diaz 7.81 175.62 39.76 19.19[(PPh3)2Ag(Diaz)1]NO3 8.68 170.77 40.00 18.43[(PPh3)1Ag(Diaz)2]NO3 8.82 172.72 40.70 18.08Mpy – 177.69 137.91 137.49

(133.00)a (112.78)b

1504 S. Nawaz et al. / Polyhedron 30 (2011) 1502–1506

teria and with GP medium (Glucose and Polypeptone) for mold andyeast. Using these, twofold diluted solutions with concentrationsof 1000 mg mL�1 to 10 mg mL�1 were prepared. Each 1 mL of cul-ture medium containing various concentrations of test materialswas inoculated with 0.1 mL of the microorganism suspension pre-pared above. Bacteria were cultured for one day at 35.5 �C, mold for7 days at 25 �C and yeast for 2 days at 30 �C. The growth of themicroorganisms was monitored during this period. When nogrowth of microorganism was observed in the medium containingthe lowest concentration of test materials, the MIC of the testmaterial was defined at this point of dilution.

[(PPh3)1Ag(Mpy)2]NO3 7.87, 7.88 172.72 139.73 139.24(132.22)a (116.13)b

Where a,b are the values for C-3 and C-4, respectively.

3. Results and discussion

The reactions of silver nitrate, triphenylphosphine and thionesin molar ratios of 1:1:1 or 1:1:2 in methanol–acetonitrile mediaresulted in products of the compositions [(Ph3P)Ag(thione)]NO3

and [(Ph3P)Ag(thione)2]NO3 respectively. The elemental analysisof the starting Ag–PPh3 complex corresponds to the formula,[Ag(PPh3)NO3]. The crystal structure of the expected [(Ph3P)Ag(Mpy)]NO3 complex was also determined, but the measurementsshowed that the resulting complex was a bis(phosphine) complex,[Ag(PPh3)2NO3] rather than a mixed ligand complex.

3.1. Spectroscopic studies

Selected IR spectroscopic vibration bands for the free ligandsand their silver(I) complexes are given in Table 3. A low frequencyshift in the m(C@S) band and a high frequency shift in the m(N–H)band in the complexes compared to the free ligands indicate thecoordination of the ligands to metal ion [38]. A characteristic peakdue to m(P–CPh) around 1090 cm�1 indicates the presence of PPh3

in the complexes [28]. A sharp band around 1300 cm�1 for NO3�

bending was observed for all of the complexes, indicating the pres-ence of non-coordinated NO3

� [30,31].The 1H and 13C NMR chemical shifts of the complexes in

DMSO-d6 are summarized in Table 4. In the 1H NMR spectra ofthe complexes, the N–H signal of the thiones became less intenseupon coordination and shifted downfield by 0.5–1.0 ppm from itspositions in the free ligands. The deshielding of the N–H protonis related to an increase in the p electron density of the C–N bondupon complexation [4,9].

In 13C NMR spectra of all the complexes, the C@S resonance ap-pears upfield compared to that of the free ligands, in accordancewith the data observed for other complexes of Cu(I), Ag(I) andAu(I) with thiones [4–11,39]. The upfield shift is attributed to alowering of the >C@S bond order upon coordination and a shiftof the N ? C electron density producing partial double bond char-acter in the C–N bond [4–6,9]. The upfield shift decreases as thenumber of thione ligands attached to silver(I) increases from onein [(Ph3P)2Ag(thione)]NO3 to two in [(Ph3P)Ag(thione)2]NO3. Thisis because of the increase in number of electronegative groups(sulfur of thione) attached to silver(I). A small shift of �1 ppm is

Table 3Selected IR absorptions (cm�1) for the free ligands and their silver(I) complexes.

Species m(C@S) m(NH2,NH) m(C–N) m(NO)3

Imt 510 3200 1457[(PPh3)2Ag(Imt)1]NO3 496 3245 1432 1379[(PPh3)1Ag(Imt)2]NO3 498 3245 1432 1379Diaz 510 3200 1450[(PPh3)2Ag(Diaz)1]NO3 518 3203 1433 1385[(PPh3)1Ag(Diaz)2]NO3 515 3246 1432 1361Mpy 613 3176 1487[(PPh3)1Ag(Mpy)2]NO3 595 – 1478 1382

observed for the other carbon atoms, which shows that the nitro-gen atoms are not involved in coordination. The difference inshielding at C@S is related to the strength of the metal–sulfurbond, which arises from back donation from silver(I) to sulfur[4]. It appears from Table 4 that the Mpy complex, with the mostsignificant shift in C@S, is found to be the most stable product.

The resonances for the triphenylphosphine ring carbons of thecomplexes were observed in the regions 127–134 ppm. The carbonatoms C(-P), C-2, C-3 and C-4 of free triphenylphosphine resonateat 137.2, 133.6, 128.4 and 128.5 ppm respectively. In the com-plexes, the C–P resonance shifted upfield (Table 5), while the otherresonances remained almost unshifted. Upfield shifts of about4–5 ppm in the C–P resonance compared to free Ph3P are consis-tent with its coordination to the metal center [40]. The upfield shiftis attributed to the shift of electron density from the metal ion to-wards C(-P) because of the p-accepting nature of PPh3. The C(-P),C-2 and C-3 resonances were in the form of doublets, while C-4appeared as a singlet.

The 31P NMR chemical shifts are given in Table 5. In the 31P NMRspectra of the complexes, a sharp singlet was observed for Ph3P, ex-cept for [Ag(Ph3P)NO3] where a doublet was observed (probablydue to 31P–109/107Ag coupling). The P–Ag coupling could not be de-tected for the other complexes and only singlets were observeddue to rapid exchange of PPh3 at various positions in solution.The 31P resonance in the complexes is significantly downfieldshifted compared to free Ph3P. A downfield shift in the 31P reso-nance of the complexes is related to the donation of the electronpair on phosphorus to the metal, which reduces the shielding atthe phosphorus nucleus. The shifts are found to be smaller forbis(thione) complexes than for mono(thione) complexes.

3.2. X-ray structure description

The molecular structure of 1, along with the crystallographicnumbering scheme, is shown in Fig. 1. Selected bond distancesand bond angles are given in Table 6. The complex is dinuclear,consisting of [Ag(Ph3P)(Diaz)2]+ cationic units and nitrate counterions. The two cationic units are bridged by sulfur atoms of thione

Table 531P and 13C (C–P) chemical shifts of the Ph3P–Ag–thione complexes.

Species d31P d13C(C–P)

PPh3 �5.5 137.20[Ag(PPh3)NO3] 10.20, 13.55 131.52[(PPh3)2Ag(Imt)1]NO3 9.52 131.60[(PPh3)1Ag(Imt)2]NO3 8.58 131.97[(PPh3)2Ag(Diaz)1]NO3 9.89 131.51[(PPh3)1Ag(Diaz)2]NO3 8.08 132.20[(PPh3)1Ag(Mpy)2]NO3 6.80 132.45

Fig. 1. A view of the molecular structure of 1 with the atomic numbering scheme. Displacement ellipsoids are drawn at the 50% probability level.

Table 6Bond lengths (Å) and bond angles (�) for 1.

Bond lengths (Å)Ag1–S1 2.537(2) P1–C15 1.812(9)Ag1–P1 2.430(3) P1–C9 1.816(9)Ag1–S2 2.537(3) P1–C21 1.822(10)Ag1–S2#1 2.764(2) N1–C2 1.308(11)S2–Ag1#1 2.764(2) N2–C2 1.430(16)S2–C20 1.728(9) N1–O1 1.261(10)S1–C1 1.731(12) N1–O2 1.268(10)Ag1–Ag1#1 3.684 N1–O3 1.226(11)

Bond angles (�)P1–Ag1–S1 123.33(10) C(15)–P(1)–Ag(1) 115.4(3)P1–Ag1–S2 123.88(8) C(21)–P(1)–Ag(1) 120.2(3)S1–Ag1–S2 107.44(9) C15–P1–C9 103.3(4)P1–Ag1–S2#1 96.57(7) N2–C1–S1 119.9(9)S1–Ag1–S2#1 104.50(7) N4–C5–S2 121.3(7)S2–Ag1–S2#1 92.04(7) N3–C1–N2 121.9(11)C5–S2–Ag1 102.1(4) N4–C5–N5 119.3(8)C5–S2–Ag1#1 111.5(3) O1–N1–O3 121.3(9)Ag1–S2–Ag1#1 87.96(7) O2–N1–O3 121.3(8)C9–P1–Ag1 109.1(3) O1–N1–O2 117.4(8)

Symmetry transformations used to generate equivalent atoms: #1 �x + 1, �y + 1,�z.

S. Nawaz et al. / Polyhedron 30 (2011) 1502–1506 1505

to form a four-membered Ag–S–Ag–S ring. The Ag–Ag distance inthe four-membered Ag–S–Ag–S rings is 3.684 Å. This value issomewhat larger than the sum of the van der Waals radii of two

Table 7Antimicrobial activities of the Ph3P–Ag–thione complexes, evaluated by their minimum in

Complexes Microbial activity (in terms of (MIC: lg mL�1)

Escherichia coli Pseudomonas aeruginosa Aspergillus

Amoxil 8 12 890[(PPh3)1Ag(Imt)1]NO3 390 430 920[(PPh3)1Ag(Imt)2]NO3 220 70 780[(PPh3)Ag(Diaz)2]NO3 300 360 760[(PPh3)Ag(Mpy)2]NO3 360 410 210

Ag(I) centers (3.44 Å), which is considered to be the upper limitof the distance for viable argentophilic interactions [41–44]. Thesilver atom is tetrahedrally coordinated to one P atom of PPh3,one sulfur atom of a terminal thione and two S atoms of bridgingthione groups, with S–Ag–X (X = P, S) angles varying from92.04(7)� to 123.88(8)�. Four of the six bond angles show distor-tions from tetrahedral geometry. This tetrahedral distortion isdue to steric interactions between the bulky ligands, as observedpreviously in the other analogous silver(I) complexes [32,33]. TheAg–P, Ag–S and other bond lengths observed in the title complexare comparable to those in other reported complexes [27–34].

The nitrate ions are nearly planar, but exhibit low symmetryowing to rather strong hydrogen bonding interactions with theNH group of the thione ligand. Within the crystal packing,[Ag(Ph3P)(Diaz)2]+ units and NO3

� ions are connected throughhydrogen bonds.

3.3. Antimicrobial activities

The antimicrobial activities (average of three measurements) ofthe Ph3P–Ag–thione complexes and Amoxil (as a standard drug),estimated by minimum inhibitory concentrations (MIC; lg mL�1),are listed in Table 7. The commercially available antibiotic Amoxilis highly effective against the studied bacteria. However, moderateactivity of Amoxil is found for molds (A. niger, P. citrinum) and yeast

hibitory concentration (MIC: lg mL�1).

niger Penicillium citrinum Candida albicans Saccharomyces cerevisiae

870 660 580900 >1000 >1000690 >1000 >1000720 >1000 >1000180 780 620

1506 S. Nawaz et al. / Polyhedron 30 (2011) 1502–1506

(C. albicans, S. cerevisiae). It can be seen from Table 7 that one of thefour tested complexes showed significant activity against two thegram-negative bacteria (E. coli, P. aeruginosa), while anotherone displayed moderate activity against the molds (A. niger, P.citrinum). The complexes are almost ineffective towards the yeasts(C. albicans, S. cerevisiae); the MIC values of three of the complexesbeing >1000 lg mL�1.

The activities of the complexes, as given in Table 7, are associ-ated with the ease of displacement of ligands from a complex withbiological ligands, such as proteins, enzymes and membranes[45,46]. The poor activities of the complexes in this study suggestthat they are highly stable, due to which the ligands could not bedisplaced easily by biological ligands.

4. Conclusion

The present report describes that thiones in the presence ofPPh3 react with AgNO3 to form complexes of the types[(Ph3P)2Ag(thione)]NO3 and [(Ph3P)Ag(thione)2]NO3, in which thethione ligands coordinate through the sulfur atom. The crystalstructure of a novel dinuclear silver nitrate complex is also pre-sented. Antimicrobial activities of the complexes have been stud-ied and it has been pointed out that only one of these complexesexhibits significant antibacterial activity.

Appendix A. Supplementary data

CCDC 750411 contains the supplementary crystallographic datafor 1. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the CambridgeCrystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.

References

[1] P. Zoufalá, T. Rüffer, H. Lang, S. Ahmad, M. Mufakkar, Anal Sci.: X-ray Struct.Anal. Online 23 (2007) x219.

[2] M. Hanif, S. Ahmad, M. Altaf, H. Stoeckli-Evans, Acta Crystallogr., Sect. E 63(2007) m2594.

[3] Habib-ur-Rehman, S. Ahmad, M. Hanif, H. Ajaz, M. Altaf, H. Stoeckli-Evans, J.Struct. Chem. 52 (2011) 160.

[4] S. Ahmad, A.A. Isab, M. Arab, Polyhedron 21 (2002) 1267.[5] S. Ahmad, A.A. Isab, W. Ashraf, Inorg. Chem. Commun. 5 (2002) 816.[6] W. Ashraf, S. Ahmad, A.A. Isab, Transition Met. Chem. 29 (2002) 400.[7] S. Ahmad, A.A. Isab, H.P. Perzanowski, Transition Met. Chem. 27 (2002) 782.[8] S. Ahmad, A.A. Isab, Inorg. Chem. Commun. 4 (2001) 362.[9] S. Ahmad, A.A. Isab, H.P. Perzanowski, Can. J. Chem. 80 (2002) 1279.

[10] A.A. Isab, M.B. Fettouhi, S. Ahmad, L. Ouahab, Polyhedron 22 (2003) 1349.[11] A.A. Isab, S. Nawaz, M. Saleem, M. Altaf, M. Monim-ul-Mehboob, S. Ahmad, H.

Stoeckli-Evans, Polyhedron 29 (2010) 1251.[12] E.S. Raper, Coord. Chem. Rev. 153 (1996) 199.[13] P.D. Akrivos, Coord. Chem. Rev. 213 (2001) 181.[14] T.S. Lobana, S. Khanna, R.J. Butcher, A.D. Hunter, M. Zeller, Polyhedron 25

(2006) 2755.[15] S.J. Berners-Price, R.J. Bowen, P. Galettis, P.C. Healy, M.J. McKeage, Coord.

Chem. Rev. 185–186 (1999) 823.[16] T.C. Lee, E.L. Amma, J. Chem. Crystallogr. 2 (1972) 125.[17] C. Pakawatchai, K. Sivakumar, H.K. Fun, Acta Crystallogr., Sect. C 52 (1996)

1954.[18] H.K. Fun, I.A. Razak, C. Pakawatchai, S. Chantrapromma, S. Saithong, Acta

Crystallogr., Sect. C 54 (1998) 453.[19] F.B. Stocker, D. Britton, V.G. Young, Inorg. Chem. 39 (2000) 3479.[20] G.A. Bowmaker, C. Pakawatchai, S. Saithong, B.W. Skelton, A.H. White, J. Chem.

Soc., Dalton Trans. (2010) 4391.[21] R. Meijboom, R.J. Bowen, S.J. Berners-Price, Coord. Chem. Rev. 253 (2009) 325.[22] M. Altaf, H. Stoeckli-Evans, Polyhedron 29 (2010) 701.[23] R. Noguchi, A. Sugie, A. Hara, K. Nomiya, Inorg. Chem. Commun. 9 (2006) 107.[24] G.A. Bowmaker, Effendy, P.J. Harvey, P.C. Healy, B.W. Skelton, A.H. White, J.

Chem. Soc., Dalton Trans. (1996) 2449.[25] G.A. Bowmaker, Effendy, P.J. Harvey, P.C. Healy, B.W. Skelton, A.H. White, J.

Chem. Soc., Dalton Trans. (1996) 2459.[26] G.A. Bowmaker, Effendy, D. Martini, C. Pettinari, B.W. Skelton, A.H. White,

Inorg. Chim. Acta 359 (2006) 2183.[27] M.B. Ferrari, F. Bisceglie, E. Cavalli, G. Pelosi, P. Tarasconi, V. Verdolino, Inorg.

Chim. Acta 360 (2007) 3233.[28] P. Karagiannidis, P. Aslanidis, S. Kokkou, C.J. Cheer, Inorg. Chim. Acta 172

(1990) 247.[29] P. Aslanidis, P. Karagiannidis, P.D. Akrivos, B. Krebs, M. Lage, Inorg. Chim. Acta

254 (1997) 277.[30] W. McFarlane, P.D. Akrivos, P. Aslanidis, P. Karagiannidis, C. Hatzisymeon, M.

Numan, S. Kokkou, Inorg. Chim. Acta 281 (1998) 121.[31] T.S. Lobana, R. Sharma, R.J. Butcher, Polyhedron 27 (2008) 1375.[32] P.J. Cox, P. Aslanidis, P. Karagiannidis, S. Hadjikakou, Inorg. Chim. Acta 310

(2000) 268.[33] P. Aslanidis, P.J. Cox, S. Divandis, P. Karagiannidis, Inorg. Chim. Acta 357 (2004)

2677.[34] P. Aslanidis, S. Divanidis, P.J. Cox, P. Karagiannidis, Polyhedron 24 (2005) 853.[35] G.M. Sheldrick, SHELXTL-97, University of Göttingen, Germany, 1997.[36] G.M. Sheldrick, Acta Crystallogr., Sect. A 64 (2008) 112.[37] K. Nomiya, K. Tsuda, N.C. Kasuga, J. Chem. Soc., Dalton Trans. (1998) 1653.[38] J.A. Garcia-Vazquez, A. Sousa-Pedrares, M. Carabel, J. Romero, A. Sousa,

Polyhedron 24 (2005) 2043.[39] A.A. Isab, M.S. Hussain, Transition Met. Chem. 11 (1986) 298.[40] T. Ahmad, T. Rüffer, H. Lang, A.A. Isab, S. Ahmad, Inorg. Chim. Acta 362 (2009)

2609.[41] P. Pyykko, Chem. Rev. 97 (1997) 597.[42] J. Cernak, K.A. Abboud, J. Chomic, M.W. Meisal, M. Orendac, A. Orendacova, A.

Feher, Inorg. Chim. Acta 311 (2000) 126.[43] K. Nomiya, S. Takahashi, R. Noguchi, J. Chem. Soc., Dalton Trans. (2000) 2091.[44] P.C. Zachariadis, S.K. Hadjikakou, N. Hadjiliadis, A. Michaelides, S. Skoulika, Y.

Ming, Y. Xiaolin, Inorg. Chim. Acta 343 (2003) 361.[45] S. Ahmad, A.A. Isab, S. Ali, A.R. Al-Arfaj, Polyhedron 25 (2006) 1633.[46] R.P. John, A. Sreekanth, V. Rajakannan, T.A. Ajith, M.R.P. Kurup, Polyhedron 23

(2004) 2549.