New bulky diphosphino-ferrocene 4 and diphosphino-zirconocene 8, modified with a number of tert-butyl groups, were synthesized and characterized by single crystal X-Ray and/or NMR analyses. Complex [NiCl 2 (4)] was prepared and its molecular structure was established by a single crystal X-Ray analysis. Buried percent volume (% V Bur ) of novel bulky diphosphino-ferrocene 4 was calculated to be greater than that of dppf. Because of this [NiCl 2 (4)] catalyst demonstrated 2.5 times higher activity compared to [NiCl 2 (dppf)] in the model ethylene polymerization reaction.Among phosphine ligands, dppf is the one of the most widely used, in particular in cross-coupling reactions. Though, being coordinatively saturated, iron does not participate in catalytic cycle, but ferrocene itself creates steric bulk around the metal center. Synthetic protocols for the phosphine moieties incorporation into ferrocene molecules are established quite well, which allows for preparation of various phosphines in a high yield. Bulky ferrocenyldiphosphines are widely used in different catalytic processes. [1] In order to evaluate the bulkiness of the particular ligand, buried percent volume (%V bur ), a technique complementary to Tolman cone angle, was developed. [2,3] In the literature there are numerous examples where steric congestion at the metal center of the catalyst should be adjusted carefully in order to gain high selectivity, for example, in isomerizing methoxycarbonylation of the internal double bond of methyl oleate. [4] Ethylene polymerization is the processes in which polymer structure is largely affected by many factors. Especially, the bulkiness of the used catalyst [5] and solvent are important. [6] Aliphatic solvents could be of choice when inert medium is prerequisite. [7] However, catalyst low solubility could be an issue in this case. Having all abovementioned in mind, in current paper we suggest to use 3,5-di-tert-butylphenyl substituents at phosphorus atoms in order to create more steric bulk compared to 1,1'-bis(diphenylphosphino)ferrocene (dppf), but not to blind completely the metal center. Due to the presence of numerous tert-butyl groups in the catalyst, its solubility in non-polar solvents, such as toluene or hexane, was improved.Arylbromide 1 was prepared according to the reported procedure. [8] Bromide 1 lithiation with nBuLi followed by addition of Et 2 NPCl 2 and subsequent treatment with PCl 3 gave chlorophosphine 2 in 77 % yield (Scheme 1).Ligand 4 was obtained by the reaction of ferrocene dilithium salt 3 with two equivalents of chlorophosphine 2 (Scheme 2). The structure of 4 was proved by NMR spectra and a single-crystal X-Ray analysis (Table S1).In order to check the influence of bulky 3,5-di-tertbutylphenyl groups on ligand 4 geometry, its crystal structure was compared with the structure of dppf. [9] The molecule is [a] Dr.
