Guanidinate complexes of niobium and tantalum.

This work is concerned with the preparation and examination of guanidinate complexes. One method of synthesis of such ligands, as presented in Chapter 2, is the insertion of carbodiimide into a metal-amido group. The direct reactions of $\rm M(NMe\sb2)\sb5$ with either dicyclohexylcarbodiimide (CyN=C=NCy) or diisopropylcarbodiimide $\rm(\sp{i}PrN{=}C{=}N\sp{i}Pr)$ proceeded smoothly at room temperature under nitrogen to yield $\rm\lbrack RNC(NMe\sb2)NR\rbrack M(NMe\sb2)\sb4$ (M = Ta, Nb; R = Cy, $\rm\sp{i}Pr).$ The spectroscopic characterization of these materials is consistent with a symmetrical chelating bidentate guanidinate anion bonded to a pseudo-octahedral metal center. In chapter 3, we prepared $\rm N,N\sp\prime,N\sp{\prime\prime}$-tricyclohexylguanidine and triisopropylguanidine. With this guanidine ligand, we focus on the elimination of amido ligands from homoleptic Ta(V) and Nb(V) dimethylamido complexes. The complex $\rm TaCl(NMe\sb2)\sb3\{\lbrack CH\sb3)\sb2CHN\rbrack\sb2CN(H)CH(CH\sb3)\sb2\}$ was also isolated and crystallographically characterized. In Chapter 4, the direct reaction of Ta and Nb alkyl complexes $\rm M(Me)\sb{x}Cl\sb{y}$ (M = Ta, x = 3, y = 2; M = Nb, x = 2, y = 3) with $\rm N,N\sp\prime,N\sp{\prime\prime}$-tricyclohexylguanidine and $\rm N,N\sp\prime,N\sp{\prime\prime}$-triisopropylguanidine was investigated. The preliminary results of efforts to get a second alkyl elimination are also reported. (Abstract shortened by UMI.