Si-containing film forming compositions comprising Si—C free and volatile silazane-containing precursors are disclosed. The compositions may be used to deposit high purity thin films. The Si—C free and volatile silazane-containing precursors have the formulae: [(SiR3)2NSiH2]m—NH2-m—C≡N, with m=1 or 2;  (a) [(SiR3)2NSiH2]n—NL3-n,with n=2 or 3;  (b) (SiH3)2NSiH2—O—SiH2N(SiH3)2; and  (c) (SiR′3)2N—SiH2—N(SiR′3)2;  (d) with each R independently selected from H, a dialkylamino group, or an amidinate; each R′ independently selected from H, a dialkylamino group, or an amidinate, with the provision that all R′ are not H; and L is selected from H or a C1-C6 hydrocarbyl group.