Synthesis and Characterization of a New Family of Square-Planar Nickel( ii ) Carbonyl Derivatives Juan Fornie ¬s,* [a] Antonio MartÌn, [a] L. Francisco MartÌn, [a] Babil Menjo ¬n, [a] Heather A. Kalamarides, [b] Larry F. Rhodes, [b, c] Cynthia S. Day, [d] and Victor W. Day [d] Abstract: The reaction of [NBu 4 ] 2 - [Ni(C 6 F 5 ) 4 ](1) with solutions of dry HCl(g) in Et 2 O results in the protonol- ysis of two Ni C 6 F 5 bonds giving [NBu 4 ] 2 [{Ni(C 6 F 5 ) 2 } 2 (m-Cl) 2 ](2a) to- gether with the stoichiometrically re- quired amount of C 6 F 5 H. Compound 2a reacts with AgClO 4 in THF to give cis- [Ni(C 6 F 5 ) 2 (thf) 2 ](3). Reacting 3 with phosphonium halides, [PPh 3 Me]X, gives dinuclear compounds [PPh 3 Me] 2 [{Ni- (C 6 F 5 ) 2 } 2 (m-X) 2 ] (X  Br (2b)orI(2c)). Solutions of compounds 2 in CH 2 Cl 2 at 0 8C do not react with excess CNtBu, but do react with CO (1 atm) to split the bridges and form a series of terminal Ni II carbonyl derivatives with general formula Qcis-[Ni(C 6 F 5 ) 2 X(CO)] (4). The n(CO) stretching frequencies of 4 in CH 2 Cl 2 solution decrease in the order Cl (2090 cm 1 ) > Br (2084 cm 1 ) > I (2073 cm 1 ). Compounds 4 revert to the parent dinuclear species 2 on in- creasing the temperature or under re- duced CO pressure. [NBu 4 ]cis-[Ni- (C 6 F 5 ) 2 Cl(CO)] (4a) reacts with AgC 6 F 5 to give [NBu 4 ][Ni(C 6 F 5 ) 3 (CO)] (5, n CO (CH 2 Cl 2 )  2070 cm 1 ). Compound 5 is also quantitatively formed ( 19 F NMR spectroscopy) by 1:1 reaction of 1 with HCl(Et 2 O) in CO atmosphere. Complex 3 reacts with CO at  78 8C to give cis-[Ni(C 6 F 5 ) 2 (CO) 2 ](6, n CO (CH 2 - Cl 2 )  2156, 2130 cm 1 ), which easily decomposes by reductive elimination of C 6 F 5 C 6 F 5 . Compounds 3 and 6 both react with CNtBu to give trans- [Ni(C 6 F 5 ) 2 (CNtBu) 2 ](7). The solid-state structures of compounds 3, 4b, 6, and 7 have been established by X-ray diffrac- tion methods. Complexes 4 ± 6 are rare examples of square-planar Ni II carbonyl derivatives. Keywords: carbonyl ligands ¥ nickel ¥ pentafluorophenyl ligands ¥ structure elucidation Introduction The study of group trends are among the most useful tools at the chemist×s disposal when trying to rationalize the chemical properties of the elements within the periodic table. Some- times, however, the differences in the properties and chemical behavior of the elements for a given group outweigh their analogies. One clear example is found in the carbonyl chemistry of Group 10 elements. Thus, [Ni(CO) 4 ] is a well- known, stable molecule prepared in 1890 by L. Mond and co- workers under very mild conditions. [1] In contrast, the heavier zerovalent homologues [M(CO) 4 ] (M  Pd, Pt) are very unstable and can only be detected using low-temperature matrix techniques. [2] When moving to the oxidation state II, however, the reverse behavior is observed: the reasonably stable compounds [{PtCl 2 (CO)} 2 (m-Cl) 2 ] and cis-[PtCl 2 (CO) 2 ] were prepared by P. Sch¸tzenberger as early as 1868, [3] while the related species [NiCl 2 (CO)] and [NiCl 2 (CO) 2 ] are detect- able only at very low temperatures. [4] These are just a few of the many differences between the light and heavier elements of Groups 8 ± 10 that have induced, and even justified, the empirical division traditionally adopted in iron group and platinum group metals. [5] Some years ago, we reported the synthesis and character- ization of the series of square-planar dicarbonyl complexes cis-[M(C 6 X 5 ) 2 (CO) 2 ] (M  Pd, Pt; X  F, Cl). [6] Perhaps the most surprising aspect of this work was the high n(CO) frequencies observed; these suggested the existence of ™negligible metal-to-CO p back-bonding∫. In the years following this report, the chemistry of various metal carbonyl [a] Prof. Dr. J. Fornie ¬s, Dr. A. MartÌn, Dipl.-Chem. L. F. MartÌn, Dr. B. Menjo ¬n Departamento de QuÌmica Inorga ¬nica Instituto de Ciencia de Materiales de Arago ¬n Facultad de Ciencias, Universidad de Zaragoza-C.S.I.C. Pza. S. Francisco s/n, 50009 Zaragoza (Spain) Fax: ( 34)976-761187 E-mail: forniesj@posta.unizar.es [b] Dipl.-Chem. H. A. Kalamarides, Dr. L. F. Rhodes The BFGoodrich Company, 9921 Brecksville Road Brecksville, Ohio 44141 (USA) [c] Dr. L. F. Rhodes Present address: Promerus LLC, 9921 Brecksville Road Brecksville, Ohio 44141 (USA) [d] Dr. C. S. Day, Prof. Dr. V. W. Day Crystalytics Company, 1701 Pleasant Hill Road Lincoln, NE 68523 (USA) FULL PAPER Chem. Eur. J. 2002, 8, No. 21 ¹ 2002 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim 0947-6539/02/0821-4925 $ 20.00+.50/0 4925