I use nexus opensource version maven 2.2.1

When I type "mvn help:effective-settings", i got the error below

[INFO] Scanning for projects...
[INFO] Searching repository for plugin with prefix: 'help'.
[INFO] ------------------------------------------------------------------------
[ERROR] BUILD ERROR
[INFO] ------------------------------------------------------------------------
[INFO] Error building POM (may not be this project's POM).

Project ID: org.apache.maven.plugins:maven-help-plugin

Reason: Error getting POM for 'org.apache.maven.plugins:maven-help-plugin' from the   repository: Failed to resolve artifact, possibly due to a repository list that is not appropriately equipped for this artifact's metadata.
 org.apache.maven.plugins:maven-help-plugin:pom:2.2-SNAPSHOT

from the specified remote repositories:
 Nexus (http://192.168.56.191:8081/nexus/content/groups/public)

for project org.apache.maven.plugins:maven-help-plugin

When I check the local repository under ~.m2\repository\org\apache\maven\plugins\maven-help-plugin

It has a file maven-metadata-central.xml

<?xml version="1.0" encoding="UTF-8"?>
<metadata>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-help-plugin</artifactId>
  <versioning>
    <latest>2.2-SNAPSHOT</latest>
    <release>2.1.1</release>
    <versions>
      <version>2.0</version>
      <version>2.0.1</version>
      <version>2.0.2</version>
      <version>2.1</version>
      <version>2.1.1</version>
      <version>2.2-SNAPSHOT</version>
    </versions>
   <lastUpdated>20100519065440</lastUpdated>
  </versioning>
</metadata>

And I can't find any jar files under directory, what's wrong with nexus server ? I can't easily find support information from nexus.

Any hints