Metall-Ν,N′-bis(trimethylsilyl)benzamidinate: Synthese und Kristallstruktur von Bis[N,N′-bis(trimethylsilyl)benzamidinato]chrom(II), [PhC(NSiMe 3) 2] 2Cr / Metal-N,N′-bis(trimethylsilyl)benzamidinates: synthesis and crystal structure of bis[N,N′-bis(trimethylsilyl)benzamidinato]chromium(II), [PhC(NSiMe 3) 2] 2Cr