( thus ( , ( ( X-ray ( crystallographic studies ) ) ( ( have ( led ( to ( ( the discovery ) ( of ( ( ( even ( more exotic ) ) types ) ( of ( ( ( bonding ( in ( inorganic chemistry ) ) ) , ) ( such ( as ( ( ( ( ( ( metal-metal ( double bonds ) ) , ) ( metal-metal ( quadruple bonds ) ) ) , ) and ) ( three-center ( , ( two-electron bonds ) ) ) ) ) ) ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (ADVP (RB thus)) (, ,) (NP (NN X-ray) (NN crystallographic) (NNS studies)) (VP (VBP have) (VP (VBN led) (PP (TO to) (NP (NP (DT the) (NN discovery)) (PP (IN of) (NP (NP (ADJP (RB even) (RBR more) (JJ exotic)) (NNS types)) (PP (IN of) (NP (NP (NN bonding)) (PP (IN in) (NP (JJ inorganic) (NN chemistry))) (, ,) (PP (JJ such) (IN as) (NP (NP (JJ metal-metal) (JJ double) (NNS bonds)) (, ,) (NP (JJ metal-metal) (NN quadruple) (NNS bonds)) (, ,) (CC and) (NP (JJ three-center) (, ,) (JJ two-electron) (NNS bonds)))))))))))) (. .)))
thus, X-ray crystallographic studies have led to the discovery of even more exotic types of bonding in inorganic chemistry, such as metal-metal double bonds, metal-metal quadruple bonds, and three-center, two-electron bonds.
(ROOT (S (NP (JJ Metallic) (NNS bonds)) (VP (VBP hold) (PRT (RP together)) (NP (NP (JJ positive) (NN metal) (NNS ions)) (CC and) (NP (PRP$ their) (JJ valence) (NNS electrons)))) (. .)))